The authors thank Mr. J. O. Hornkohl for interest and discussion on computation of diatomic molecular line strengths. This work is in part supported by the Center for Laser Applications at the University of Tennessee Space Institute.

1. Optical System Setup
<ol>
	<li>Place a beam splitter at the exit of the laser, allowing the 1,064 nm wavelength light to pass through and to reflect all other transient laser radiation into a beam dump.</li>
	<li>Place a high-speed PIN photodiode detector to record a portion of the laser radiation reflected off the beam splitter. Connect this detector to the oscilloscope with coaxial cable to monitor the optical pulse with respect to triggering by the function generator and occurrence of the Q-switching in the Nd:YAG laser device.</li>
	<li>Align three IR reflective mirrors to position the beam parallel to the slit of the spectrometer.</li>
	<li>Position a lens above translational stage to focus the beam in order to generate optical breakdown plasma parallel to the spectrometer slit. Align two quartz lenses for the purpose of imaging the plasma onto the slit. The two focusing lenses optimally match the spectrometer design, meaning the final lens has an aperture to accomplish a f# identical with the f# of the spectrometer&#39;s internal optics.</li>
	<li>For measurements above 380 nm, position a cut-on filter between the two lenses for the purpose of blocking radiation below 380 nm. The cut-on filter suppresses possible UV contributions (due to 2nd order of grating) to the measured spectra.</li>
</ol>
2. Data Acquisition Setup
<ol>
	<li>Connect a wave-form function generator providing a triangular wave at 50 Hz to a custom built divide-by-five circuit to obtain 10 Hz. The optical multichannel analyzer (OMA) is operated at 50 Hz and the flash lamps of the Nd:YAG laser are synchronously operated at 10 Hz. One can use an ICCD in place of the OMA, operating&#160;synchronously at the rate of the pulsed laser radiation&#160;as well.</li>
	<li>Connect one of the outputs of the custom built divide-by-five circuit to a digital delay generator. Use one output to synchronize the Nd:YAG flash lamps and another output to control the triggers of the linear diode array intensifier and optical multichannel analyzer. Again, instead of the intensified linear diode array and OMA one can use an ICCD.</li>
	<li>Relay the adjustable trigger output of the laser device to an oscilloscope and to a pulse generator. The oscilloscope will be used to monitor when the pulsed laser radiation will be available for optical breakdown generation or laser ablation.</li>
	<li>Connect the high voltage output of the digital pulse generator to the intensified linear diode array.</li>
	<li>Connect the other output of the pulse generator to the oscilloscope.</li>
	<li>Connect the intensified linear diode array output to the OMA.</li>
</ol>
3. Synchronization and Measurement
<ol>
	<li>Set the wave-form function generator to output a triangle pulse operating at 50&#177;1 Hz. This function generator provides the master frequency. A custom built divide-by-five circuit and a digital delay generator are used for accurate synchronization.</li>
	<li>Initiate water cooling system and power supply for laser device. Activate laser.</li>
	<li>Determine the time for laser radiation to travel from the exit aperture of the Nd:YAG laser to the area in front of the spectrometer slit as follows: Measure the distance of the light path and compute the&#160;transit time using the speed-of-light. Account for this transit time in setting the gate delay time in next step.</li>
	<li>On the digital pulse generator, set the gate width for the measurement and the delay time from optical breakdown or laser ablation pulse,&#160;and use the oscilloscope to monitor the delay time. The delay time will determine how long to wait for data collection after breakdown occurs. The gate width determines how long the diode array is exposed to plasma radiation.</li>
	<li>Generate optical breakdown in air and/or place a sample on the translational stage such that ablation will occur. Image the microplasma onto the spectrometer slit.</li>
	<li>Begin measurements and record data with the intensified linear diode array and optical multichannel analyzer (or with an ICCD).</li>
</ol>
4. Wavelength Calibration
<ol>
	<li>Record spectra from standard calibration lamps: neon, mercury, and hydrogen lamps. Use the experimental arrangement with lamps put at the place where plasma was generated.</li>
	<li>Using the known wavelengths from lamps, perform a linear or cubic fit to obtain the pixel-wavelength correspondence. The purpose of an accurate calibration is to correct for nonlinearities that are usually associated with measurement of spectra.</li>
	<li>Repeat calibrations for H, C2, CN, and TiO spectral regions of interest.</li>
</ol>
5. Intensity Calibration
<ol>
	<li>Turn on a tungsten calibration lamp and wait for it to warm up.</li>
	<li>Use an optical pyrometer to measure the temperature of the active lamp.</li>
	<li>Use the experimental arrangement to record the spectrum of the active lamp.</li>
	<li>Compute a blackbody curve from Plank&#39;s radiation law using the measured temperature as an input parameter.</li>
	<li>Fit the computed curve to the spectrum of the active lamp. Determine the factors by which the recorded intensities from the computed curve. Apply those factors to correct recorded spectra for wavelength-dependent sensitivity of the detector.</li>
	<li>Repeat this for each region the spectrometer was used.</li>
</ol>
6. Data Transfer
<ol>
	<li>Prepare medium for file transfers.</li>
	<li>For each data measurement, record it onto the medium.</li>
	<li>Take this medium and upload the files on it to a work computer.</li>
</ol>
7. File Preparation
<ol>
	<li>For each file, parse it into sections, one containing recorded data, and the others specifying starting wavelength and average wavelength shift per data point.</li>
	<li>Use these sections to create a new file to match wavelengths with recorded data.</li>
</ol>
8. Diatomic Molecular Analysis
<ol>
	<li>Select the wavelength file and corresponding line strength file.</li>
	<li>Select the baseline offset.
	<ol>
		<li>Set whether the offset is constant, linear, or quadratic.</li>
		<li>Set the corresponding coefficients to either fixed or variable values.</li>
	</ol>
	</li>
	<li>Set the resolution and temperature, both of which can be either fixed or varied.</li>
	<li>Set the tolerance of fit for the synthetic spectra to be fit to the measured spectra.</li>
	<li>Fit the computed to the experimental spectra using a Nelder-Mead algorithm.</li>
	<li>Using the best fitting parameters of the computed spectra for each measurement, infer the microplasma parameters observed at the various time delays and gate widths used.</li>
</ol>

<ol>
	<li>Miziolek, A. W., Palleschi, V., Schechter, I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Laser Induced Breakdown Spectroscopy. , (2006).</li><li>Cremers, D. E., Radziemski, L. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Handbook of laser-induced Breakdown Spectroscopy. , (2006).</li><li>Singh, J. P., Thakur, S. N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Laser Induced Breakdown Spectroscopy. , (2007).</li><li>Hahn, D. W., Omenetto, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Laser-induced+breakdown+spectroscopy+(LIBS),+Part+I:+review+of+basic+diagnostics+and+plasma-particle+iterations:+still-challenging+issues+within+the+analytical+plasma+community.">Laser-induced breakdown spectroscopy (LIBS), Part I: review of basic diagnostics and plasma-particle iterations: still-challenging issues within the analytical plasma community.</a> Appl. Spectrosc. 64, (2010).</li><li>Hahn, D. W., Omenetto, N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Laser-induced+breakdown+spectroscopy+(LIBS),+Part+II:+review+of+instrumental+and+methodological+approaches+to+material+analysis+and+applications+to+different+fields.">Laser-induced breakdown spectroscopy (LIBS), Part II: review of instrumental and methodological approaches to material analysis and applications to different fields.</a> Appl. Spectrosc. 66, 347 (2012).</li><li>Parigger, C. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Atomic+and+molecular+emissions+in+laser-induced+breakdown+spectroscopy.">Atomic and molecular emissions in laser-induced breakdown spectroscopy.</a> Spectrochim. Acta Part B. 79, 4-16 (2013).</li><li>Konjevi&#263;, N., Lesage, A., Fuhr, J. R., Wiese, W. L. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Experimental+Stark+widths+and+shifts+for+spectral+lines+of+neutral+and+ionized+atoms.">Experimental Stark widths and shifts for spectral lines of neutral and ionized atoms.</a> J. Phys. Chem. Ref. Data. 31, 819-927 (2002).</li><li>Oks, E. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Stark+broadening+of+hydrogen+and+hydrogen-like+spectral+lines+in+plasmas:+the+physical+insight.">Stark broadening of hydrogen and hydrogen-like spectral lines in plasmas: the physical insight.</a> Alpha Science Int. , (2006).</li><li>Parigger, C. G., Dackman, M., Hornkohl, J. 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J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=New+challenges+and+insights+in+the+detection+and+spectral+identification+of+organic+explosives+by+laser+induced+breakdown+spectroscopy.">New challenges and insights in the detection and spectral identification of organic explosives by laser induced breakdown spectroscopy.</a> Spectrochim. Acta Part B. 66 (1), 12-20 (2011).</li><li>Swafford, L. D., Parigger, C. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurement+of+hydrogen+Balmer+Series+lines+following+laser-induced+optical+breakdown+in+laboratory+air.">Measurement of hydrogen Balmer Series lines following laser-induced optical breakdown in laboratory air.</a> Accepted, Int. Rev. Atom. Mol. Phys. 4 (1), (2013).</li><li>Hornkohl, J. O., Nemes, L., Parigger, C. G., Nemes, L., Irle, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Spectroscopy+of+Carbon+Containing+Diatomic+Molecules.">Spectroscopy of Carbon Containing Diatomic Molecules.</a> Spectroscopy, Dynamics and Molecular Theory of Carbon Plasmas and Vapor. , 113-165 (2011).</li><li>Parigger, C., Hornkohl, J. O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diatomic+molecular+spectroscopy+with+standard+and+anomalous+commutators.">Diatomic molecular spectroscopy with standard and anomalous commutators.</a> Int. Rev. Atom. Mol. Phys. 1, 25-43 (2010).</li><li>Parigger, C. G., Hornkohl, J. O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Computation+of+AlO+emission+spectra.">Computation of AlO emission spectra.</a> Spectrochim. Acta Part A. 81, 404-411 (2011).</li><li>Hermann, J., Peronne, A., Dutouquet, C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Analysis+of+the+TiO-&#947;+System+for+temperature+measurements+in+laser-induced+plasma.">Analysis of the TiO-&#947; System for temperature measurements in laser-induced plasma.</a> J. Phys. B: At. Mol. Opt. Phys. 34, 153-164 (2001).</li><li>Woods, A. C., Parigger, C. G., Hornkohl, J. O. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurements+and+analysis+of+titanium+monoxide+spectra+in+laser-induced+plasma.">Measurements and analysis of titanium monoxide spectra in laser-induced plasma.</a> Opt. Lett. 37, 5139-5141 (2012).</li><li>Witte, M. J., Parigger, C. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Measurement+and+analysis+of+carbon+Swan+spectra+following+laser-induced+optical+breakdown+in+air.">Measurement and analysis of carbon Swan spectra following laser-induced optical breakdown in air.</a> Accepted, Int. Rev. Atom. Mol. Phys. 4 (1), (2013).</li><li>Surmick, D. M., Parigger, C. G., Woods, A. C., Donaldson, A. B., Height, J. L., Gill, W. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Analysis+of+emission+Spectra+of+Aluminum+Monoxide+in+a+Solid+Propellant+Flame.">Analysis of emission Spectra of Aluminum Monoxide in a Solid Propellant Flame.</a> Int. Rev. Atom. Mol. Phys. 3 (2), 2-137 (2012).</li><li>Woods, A. C., Parigger, C. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Time-resolved+Temperature+Inferences+Utilizing+the+TiO+A3&#966;&#8594;X3&#916;+Band+in+Laser-induced+Plasma.">Time-resolved Temperature Inferences Utilizing the TiO A3&#966;&#8594;X3&#916; Band in Laser-induced Plasma.</a> Int. Rev. Atom. Mol. Phys. 3 (2), 103-111 (2012).</li></ol>

All authors declare that they have no competing financial interests.

The time resolved measurement protocol and representative results are further discussed here. It is important to synchronize the laser pulses, generated at a rate of 10 Hz, with the 50 Hz operating frequency of the intensified linear diode array and OMA (or ICCD). Furthermore, accurate timing of laser pulses and opening of the gate of the intensified linear diode array (or alternatively ICCD) is essential. The wave generator,&#160;indicated in the experimental schematic, is used to synchronize the laser pulses and intensified linear diode array readout that is accomplished with the optical multichannel analyzer. A pulse generator is utilized to adjust the time delay for the measurements.
Temporal overlap of the laser pulse centered in the intensifier gate-open window corresponds to the initial zero time delay, tdelay = 0. In the effort to determine the temporal evolution of the emission spectra, the time delay from optical breakdown is varied. For example, the hydrogen alpha line cannot be seen in air breakdown events until 0.4 &#181;sec after the initial plasma is generated. Measurements of emission spectra also require wavelength and intensity calibrations. These calibrations are necessary for analysis.
For LIBS measurements, observing plasma evolution by ablating metallic samples such as Ti, there are often many atomic lines present in the wavelength range of the diatomic transitions of interest. These lines can be persistent in the gathered spectra, even at later delay times. However, some of these lines occur in second order of the dispersive grating in the spectrometer. The use of a low-pass filter is beneficial to block such second-order contributions.
While the procedures of data collection are important, the methods used for fitting hold an equal merit. Temperature inferences are accomplished by fitting synthetic spectra of varying microplasma parameters to the gathered spectra utilizing a Nelder-Mead algorithm. The reliability of this procedure is greatly enhanced when the background of the spectra can be accurately identified. In this context, the term background refers to everything in the recorded spectra except for the atomic or molecular transition of interest. To quantify the effect of this background, the recorded data are investigated by analyzing signal to noise ratios with respect to their effect on the inferred temperature inferred from diatomic molecular spectra. This investigation is facilitated by the use of Monte Carlo type simulations.
The significance of the short-gate time-resolved measurement certainly includes the ability to capture highly excited atomic and molecular spectra. For the hydrogen Balmer alpha line, use of 6 nsec gate resulted in successful determination of electron density up to almost 1025/m3. For diatomic molecular emission spectra, excitation temperatures in excess of 6,000 K is typical for LIBS with nanosecond Nd:YAG laser radiation. Without an intensifier, measurements would constitute an average over time recorded during the transient events. The illustrated snapshots in time allow one to investigate generation and decay of laser induced plasma. One can, in principle, record highly excited molecular spectra slightly displaced from the bright laser induced plasma, such as in the study of plasma torches. However, one would not capture the early dynamics of the plasma decay, including measurements of highly excited atomic species.
Future applications of laboratory measurement of atomic and molecular spectra following laser-induced optical breakdown continue to be of both fundamental and application-oriented interest. Measurements of atomic line widths early in the plasma decay allow one to determine the plasma conditions/parameters. Ongoing research efforts include accurate modeling of Stark broadening and/or line shapes. Molecular spectra, along with accurate knowledge of the transitions of interest, show increased numbers of applications especially in areas of identification, characterization, and determination of, for example, explosives, combustion plumes, stellar atmospheres, or chemical composition. The methods and applications of time-resolved spectroscopy are important in establishing the dynamics following generation of plasma with laser radiation. These dynamics include plasma oscillations, changes of line shapes due to collisions, emissions due recombination radiation and/or combustion processes. Temporal separation of the processes that occur following optical breakdown is important for determining the transient phenomena. We understand that the initial LIBS application of elemental analysis due to radiation from atomic species will be strongly augmented by measurements of molecular species that includes analysis of radiation from different, individual atomic isotopes and isotopomers.

Laser-induced breakdown spectroscopy (LIBS) techniques1-5&#160;have applications in atomic6-12 and molecular studies of plasma13-20 generated with laser radiation. Time-resolved spectroscopy is essential for determination of the transient characteristics of the plasma. Temperature and electron density, to name but two plasma parameters, can be measured provided a reasonable theoretical model of the plasma breakdown is available. Separation of free-electron radiation from atomic and molecular emissions&#160;allows us to accurately explore transient phenomena. By focusing on a specific temporal window, one can &#8220;freeze&#8221; plasma decay and thereby obtain accurate spectroscopic fingerprints. LIBS has a variety of applications&#160;and recently interest in LIBS-diagnostics shows a considerable increase when measured by the number of researchers publishing in the field. Pico- and femtosecond generated microplasma is of ongoing research interest, however, historically experimental LIBS arrangements utilize nanosecond laser radiation.
Figure 1 displays a typical experimental arrangement for laser-induced breakdown spectroscopy. For this protocol, the functional breakdown energy for the initial beam is on the order of 75 mJ pulse, at the infrared wavelength of 1,064 nm. This pulse energy can be adjusted as needed. The plasma is dispersed by the spectrometer and measured with an intensified linear diode array and OMA or, alternatively, imaged onto an Intensified 2-dimensional Charge&#160;Coupled&#160;Device (ICCD). Figure 2 illustrates the timing diagram for time-resolved experiments: synchronization of pulsed laser radiation with readout, laser pulse trigger, laser fire, and gate-open delay.
Successful time-resolved spectroscopy requires various calibration procedures. These procedures include wavelength calibration, background correction, and most importantly, sensitivity correction of the detector. Sensitivity corrected data are important for the comparison of measured and modeled spectra. For an increase of signal-to-noise ratio, multiple laser-induced breakdown events are recorded.

<table><tbody><tr><td>Custom Box</td><td>UTSI</td><td>None</td><td>Signal divider and conditioner. An oscilloscope can be used in place of this.</td></tr><tr><td>Four Channel Digital Delay/Pulse Generator</td><td>Stanford Research Systems, Inc.</td><td>Model DG535</td><td>Companies: Tequipment, diyAudio</td></tr><tr><td>Four Channel Color Digital Phosphor Oscilloscope</td><td>Tektronix</td><td>TDS 3054</td><td>500 MHz - 5 GS/sec, Companies: Amazon, Tektronix, Fluke, Agilent Technologies, Pico Technology</td></tr><tr><td>Wavetek FG3C Function Generator</td><td>Wavetek</td><td>FG3C</td><td>Companies: Tequipment, Stanford Research Systems, BK Precision</td></tr><tr><td>Nd:YAG Laser</td><td>Quanta-Ray</td><td>DCR-2A(10) PS</td><td>Laser radiation, Class IV.&amp;nbsp; Companies: Lambda Photometrics, Continuum, Ellipse, Newport</td></tr><tr><td>Si Biased Detector</td><td>Thorlabs</td><td>DET10A/M</td><td>200-1,100 nm, with ND10A reflective filter. Companies: Canberra, Edmund Optics</td></tr><tr><td>Nd:YAG Laser Line Mirror, 1,064 nm</td><td>Thorlabs</td><td>NB1-K13</td><td>Companies: Edmund Optics, Newport</td></tr><tr><td>1 in Fused Silica Bi-Convex Lens, uncoated</td><td>Newport</td><td>SBX031</td><td>Companies: Edmund Optics, Thorlabs</td></tr><tr><td>2 in Fused Silica Plano-Convex lens, uncoated</td><td>Newport</td><td>SPX049</td><td>Convex lens, f/4.&amp;nbsp; Companies: Edmund Optics, Thorlabs</td></tr><tr><td>Spectrograph</td><td>Instruments S.A. division Jobin-Yvon</td><td>HR 640</td><td>Companies: Andor, Newport, Horiba</td></tr><tr><td>Manual and electronic controller for Spectrograph</td><td>Instruments S.A. division Jobin-Yvon</td><td>Model 980028</td><td>Manual and electronic controller for Spectrograph</td></tr><tr><td>Mega 4000</td><td>Mega</td><td>Model 129709</td><td>Computer interface for Spectrograph</td></tr><tr><td>Gateway 2000 Crystal Scan 1024 monitor</td><td>Gateway</td><td>PMV14AC</td><td>Monitor for computer interface</td></tr><tr><td>20 MHz Oscilloscope</td><td>BK Precision</td><td>Model 2125</td><td>Companies: Amazon, Tektronix, Fluke, Agilent Technologies, Pico Technology</td></tr><tr><td>6040 Universal Pulse Generator</td><td>Berkeley Nucleonics Corporation</td><td>Model 6040</td><td>Companies: Agilent Technologies, Tektronix, Quantum Composers</td></tr><tr><td>Separate component to 6040 Universal Pulse Generator</td><td>Berkeley Nucleonics Corporation</td><td>Model 202 H</td><td>Separate component to 6040 Universal Pulse Generator</td></tr><tr><td>ICCD Camera</td><td>EG&amp;amp;G Parc</td><td>Model 46113</td><td>Companies: Andor, Standford Computer Optics, LaVision, Hamamatsu</td></tr><tr><td>OMA III</td><td>EG&amp;amp;G Parc</td><td>Model 1460</td><td>Spectral data acquisition and analysis. Unit discontinued, replaced by software installed on computers.</td></tr></tbody></table>

measurement and analysis of atomic hydrogen and diatomic molecular alo, c2, cn, and tio spectra following laser-induced optical breakdown

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement is used. Here we operate a Nd:YAG laser at a frequency of 10 Hz at the fundamental wavelength of 1,064 nm. The 14 nsec pulses with anenergy of 190 mJ/pulse are focused to a 50 &#181;m spot size to generate a plasma from optical breakdown or laser ablation in air. The microplasma is imaged onto the entrance slit of a 0.6 m spectrometer, and spectra are recorded using an 1,800 grooves/mm grating an intensified linear diode array and optical multichannel analyzer (OMA) or an ICCD. Of interest are Stark-broadened atomic lines of the hydrogen Balmer series to infer electron density. We also elaborate on temperature measurements from diatomic emission spectra of aluminum monoxide (AlO), carbon (C2), cyanogen (CN), and titanium monoxide (TiO).
The experimental procedures include wavelength and sensitivity calibrations. Analysis of the recorded molecular spectra is accomplished by the fitting of data with tabulated line strengths. Furthermore, Monte-Carlo type simulations are performed to estimate the error margins. Time-resolved measurements are essential for the transient plasma commonly encountered in LIBS.

LIBS utilizes pulsed laser radiation to sufficiently ionize a sample to form plasma. Laser-induced breakdown of gaseous substances will create plasma that is centered about the focal region of the excitation beam, while laser ablation on solid surfaces will produce plasma above the sample&#39;s surface. The plasma is generated by focusing the optical radiation on the order of 100 GW/cm2 for nanosecond breakdown pulses. To produce laser ablation plasma, typically 1 GW/cm2 is more than sufficient. The induced plasma radiation is dispersed by a spectrometer. Only for specific delay times from optical breakdown, atomic and diatomic spectra are well developed. Figures 3-7 illustrate representative results.
Early in the plasma decay, atomic lines evolve as the free electron background radiation diminishes. Subsequently, primarily due to recombination, molecular spectra can be recorded. In the experiments, the intensifier of the linear diode array must be accurately triggered.&#160;Alternatively, an ICCD must be accurately triggered as well. This triggering (or timing) must occur consistently, so that acquired data are composed from contributions at an accurate delay time from the breakdown event. Subsequently, the OMA digitizes the signal from the intensified linear diode array and stores data. For an ICCD, timing is equally important and data is stored similarly. In the experiments with the intensified linear diode array, the OMA records only the first of five scans, the other four remaining scans are redundant but consecutive scans are ignored. Figure 2 illustrates how the representative intensified linear diode array results are obtained.
For investigations regarding atomic species, atomic lines are studied with respect to the spectral lines&#39; shape and width. The measured atomic line shapes and widths are compared to corresponding theoretical models. This comparison allows one to determine the electron density in the observed region of the plasma. In investigations of diatomic molecular spectra, usually many spectral lines contribute to several molecular bands. For each measurement, the spectra are analyzed with respect to the shapes, widths, and relative intensities of the individual lines contained in the molecular bands. Comparison of the measured and computed spectra utilizes line strengths derived from diatomic quantum theory together with high-resolution spectral data that are available in the literature. This comparison allows one to infer an excitation temperature of the diatomic molecules in the observed region of the plasma. The fitting is accomplished with a Nelder-Mead algorithm, allowing one to perform a multiparameter, nonlinear fit of computed spectra to the measured spectra. In the following, results are presented for selected experiments that address diatomic molecular and atomic line spectra.
Figure 3 illustrates measured and fitted TiO spectra. When imaging certain areas of the TiO plume, the fitted temperature versus time shows a local minimum. This minimum is possibly due to combustion.
The primary analysis objective for the hydrogen spectra is to determine the electron density and temperature from the time-resolved spectra for the hydrogen-alpha and hydrogen-beta lines. Figure 4 shows hydrogen alpha profiles recorded along the slit height. Laboratory air species such as nitrogen and oxygen cause the hydrogen alpha lines to emerge after the initial optical breakdown event. The evolution of the hydrogen-alpha line is investigated for time delays of 0.4 &#181;sec up to 30 &#181;sec after the optical breakdown. The hydrogen alpha figure also indicates a higher electron density in the center of the plasma, displayed in the figure near the middle of the indicated slit axis.
The C2 molecule shows that its lowest rotational levels are sensitive to temperature variation, and higher rotational levels are sensitive to the surrounding gas density and radiation field. Figure 5 displays a typical measured and fitted result following laser ablation in air. Analysis of the carbon Swan spectra includes studying the formation of C2 due to recombination in an effort to understand the formation of C2 and CN following laser ablation of graphene. Figure 6 shows recorded and fitted CN spectra, and the figure also shows excellent agreement with theory. The experimental spectra match well with the computed spectra, and convincing agreement can be seen in the trailing ends and band heads.&#160;Like the C2 spectra, the rotation/vibration spectra of aluminum monoxide show well developed molecular bands for the elevated temperature in the decaying laser-induced microplasma. Figure 7 shows measured and fitted AlO spectra. Since AlO is an early product in the combustion of aluminum, spectroscopic studies AlO has applications in aluminum combustion studies, primarily for temperature determination.
<img alt="Figure 1" fo:content-width="5in" fo:src="/files/ftp_upload/51250/51250fig1highres.jpg" src="/files/ftp_upload/51250/51250fig1.jpg" width="600px" /> 
Figure 1.&#160;The schematic arrangement shows the following experimental components:&#160;(1) Beam splitter to deflect a small portion of 1,064 nm laser radiation; (2) Infrared reflective mirrors to vertically align the beam in front of the spectrometer; (3) lens to focus the beam to a tight spot for optical breakdown; (4) the star symbolizes optical breakdown or laser ablation plasma; and (5) two lenses to match the focusing optics of the spectrometer. The sync box, wave, and delay generator accomplish electronic synchronization of the laser and intensified linear diode array and OMA (or ICCD) operating frequencies. The photodiode, pulse generator, and oscilloscope allow one to accurately control and monitor the measurement window for LIBS.&#160;<a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/51250/51250fig1highres.jpg" target="_blank">Click here to view larger image</a>.
<img alt="Figure 2" fo:content-width="5in" fo:src="/files/ftp_upload/51250/51250fig2highres.jpg" src="/files/ftp_upload/51250/51250fig2.jpg" width="600px" /> 
Figure 2.&#160;Timing diagram for time-resolved LIBS measurements. OMA sync and laser flash lamp sync are provided by a wave and delay generator and custom-built circuitry. Laser pulse trigger and fire pulse is provided by the Nd:YAG laser device together with Q-switch delay control. An extra pretrigger,&#160;electronic pulse (not shown) is also provided by the Nd:YAG laser manufacturer. This pretrigger allows us to accurately and efficiently control the detector gate delay of the OMA/ICCD system. The OMA readout occurs subsequent to exposure of the diode array, including 4 subsequent but ignored readout scans. For single pulse experiments, individual OMA readouts are stored separately in memory. For 100-scan average measurements, the OMA readout scans are added and then stored.&#160;<a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/51250/51250fig2highres.jpg" target="_blank">Click here to view larger image</a>.
<img alt="Figure 3" fo:content-width="5in" fo:src="/files/ftp_upload/51250/51250fig3highres.jpg" src="/files/ftp_upload/51250/51250fig3.jpg" width="600px" /> 
Figure 3.&#160;The measured and synthetic emission spectra show the TiO&#160;A3&#966;&#8594;X3&#916; &#916;v&#160;= 0 transition.&#160;The measured data was collected with the intensified linear diode array at delay time of&#160;tdelay= 95 &#181;sec&#160;with a gate width of 2 &#181;sec. The measured spectra is fit with computed spectra corresponding to a spectral resolution of FWHM = 0.09&#160;nm and an inferred temperature T = 3,300 K.&#160;<a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/51250/51250fig3highres.jpg" target="_blank">Click here to view larger image</a>.
<img alt="Figure 4" fo:content-width="5in" fo:src="/files/ftp_upload/51250/51250fig4highres.jpg" src="/files/ftp_upload/51250/51250fig4.jpg" width="600px" /> 
Figure 4.&#160;Hydrogen Balmer alpha line recorded in 1.07 x&#160;105&#160;Pa gaseous hydrogen.&#160;The emissions of 1,000 consecutive optical breakdown events were dispersed with the spectrometer and collected with an ICCD, 0.042 &#181;sec from optical breakdown and spatially resolved along the spectrometer slit-height. For this time delay, the maximum electron number density is 0.32 x&#160;1025&#160;electrons/m3, and the maximum H&#945; red shift amounts to 1.2 nm. Individual spectra of hydrogen alpha and beta lines are illustrated for example in Parigger6. Recording of hydrogen alpha and beta lines following breakdown in air is part of the documentary video of this JoVE&#160;publication.&#160;<a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/51250/51250fig4highres.jpg" target="_blank">Click here to view larger image</a>.
<img alt="Figure 5" fo:content-width="5in" fo:src="/files/ftp_upload/51250/51250fig5highres.jpg" src="/files/ftp_upload/51250/51250fig5.jpg" width="600px" /> 
Figure 5.&#160;The C2&#160;Swan spectra were collected with the intensified linear diode array at a time delay&#160;tdelay&#160;=&#160;20 &#181;sec&#160;using a gate width of 1 &#181;sec.&#160;&#160;The measured spectrum was fit with computed spectra for a FWHM spectral resolution of 0.07&#160;nm&#160;and a temperature of T = 5,600 K is inferred.&#160;<a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/51250/51250fig5highres.jpg" target="_blank">Click here to view larger image</a>.
<img alt="Figure 6" fo:content-width="5in" fo:src="/files/ftp_upload/51250/51250fig6highres.jpg" src="/files/ftp_upload/51250/51250fig6.jpg" width="600px" /> 
Figure 6.&#160;The CN&#160;B2&#949;+&#8594;X2&#949;+ violet transition was recorded with the intensified linear diode array at time delay of&#160;tdelay&#160;= 70 &#181;sec&#160;with a gate width of 4&#160;&#181;sec.&#160;The measured spectrum is fit with computed spectra using a FWHM spectral resolution of 0.09&#160;nm.&#160;The inferred temperature is T = 6,600 K.&#160;<a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/51250/51250fig6highres.jpg" target="_blank">Click here to view larger image</a>.
<img alt="Figure 7" fo:content-width="5in" fo:src="/files/ftp_upload/51250/51250fig7highres.jpg" src="/files/ftp_upload/51250/51250fig7.jpg" width="600px" /> 
Figure 7.&#160;The AlO&#160;B&#160;2&#949;+&#8594;X2&#949;+&#160;diatomic spectra for the&#160;&#916; = 0&#160;transitions were collected with the intensified linear diode array at a time delay of&#160;tdelayn= 45 &#181;sec using a gate width of 5 &#181;sec. The fitted spectra are computed using a FWHM spectral resolution of 0.09&#160;nm.&#160;The inferred temperature amounts to T = 3,900 K.&#160;<a href="https://www-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/51250/51250fig7highres.jpg" target="_blank">Click here to view larger image</a>.

Watch this Scientific Journal Video about Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown at JoVE.com

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Time-resolved atomic and diatomic molecular species are measured using LIBS. The spectra are collected at various time delays following the generation of optical breakdown plasma with Nd:YAG laser radiation&#160;and are analyzed to infer electron density and temperature.

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

In this work, we present time-resolved measurements of atomic and diatomic spectra following laser-induced optical breakdown. A typical LIBS arrangement ...

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14.1K Views.  University of Tennessee Space Institute. Time-resolved atomic and diatomic molecular species are measured using LIBS. The spectra are collected at various time delays following the generation of optical breakdown plasma with Nd:YAG laser radiation and are analyzed to infer electron density and temperature.

Video: Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C2, CN, and TiO Spectra Following Laser-induced Optical Breakdown

Balmer Series

The Bohr Model
Niels Bohr proposed a model for the hydrogen atom in 1913 that described discrete energy states are associated with a fixed electron orbit around the nucleus. Importantly, an atom cannot discharge energy while its electrons are in stationary states. An electron can only emit energy by changing energy states. To change energy states, an electron must move from one orbit to another by either absorbing or emitting energy. This change can only occur if the absorbed or emitted energy equals the difference between the two states. Electrons cannot exist between orbits.
The quantum number, n, is used to label the different energy states. The lowest energy state is the ground state, which is n equal to one. The excited states are labeled n equal to 2, 3, 4, and so on. When an electron at the ground state absorbs a photon whose energy equals the difference between the ground and second state, the electron becomes excited and transitions from the ground state to the n= 2 excited state. If the energy of the photon equals the difference between the ground and third state, the electron moves to the n=3 state.
According to Bohr&#8217;s model, the potential energy of an electron in the nth level can be calculated using the following equation:
<img src="https://www-jove-com.remotexs.ntu.edu.sg/CDNSource/lm/labs/54/54_Concepts_1.jpg" />
where En is the potential energy, R is the Rydberg constant (1.0974 &#215; 107 m-1), h is Planck&#39;s constant (6.62607004 &#215; 10-34 m2&#183;kg/s), and c is the speed of light (~ 3 &#215; 108 m/s). Electrons can also spontaneously return to the ground state or any other lower excited state. When this happens, the excess energy is released in the form of an emitted photon. The energy of the photon is equal to the energy difference between the higher and lower energy states. That energy corresponds to wavelengths of light. Since each type of atom has different energy levels, the light emitted from each transition varies for each atom. For a sample of mixed molecules, the emitted light contains a range of wavelengths in what is called a continuous spectrum. For a sample containing atoms of a single element, the emitted light contains only certain wavelengths, which can be viewed as discrete lines once separated by a prism.
<h4>The Hydrogen Atom</h4>
Looking specifically at the hydrogen atom, the excitation of its electrons requires the absorption of sufficient energy to split the bond in the diatomic molecule H2. Since more energy is used to split the molecule than needed, the electrons in the hydrogen atom absorb the excess energy and are excited to a higher energy level. When the electrons spontaneously return to a lower energy level, light is emitted, which corresponds to the energy difference between the excited level and lower level.
When discussing the emission of energy, the higher energy level is considered the initial level, or ni, while the lower level is considered the final level, or nf. The wavelengths of emitted light ultimately depend on the energy difference between the two levels.
In a pure sample of hydrogen gas, the emission spectrum appears as distinct lines of discrete wavelengths that are specific to the element hydrogen. Some of these lines are in the visible range of the electromagnetic spectrum, while some are in the ultraviolet or infrared range.
<h4>The Balmer Series</h4>
The series of visible lines in the hydrogen atom spectrum are named the Balmer series. This series of spectral emission lines occur when the electron transitions from a high-energy level to the lower energy level of n=2. Johann Balmer observed these spectral lines at 410.2 nm, 434.1 nm, 486.1 nm, and 656.3 nm, which correspond to transitions from the n=6, n=5, n=4, and n=3 energy levels to the n=2 level, respectively.
Balmer was able to relate these wavelengths of emitted light using the Balmer formula.
<img src="https://www-jove-com.remotexs.ntu.edu.sg/CDNSource/lm/labs/54/54_Concepts_2.jpg" />
Here, &#955; is the observed wavelength, C is a constant (364.50682 nm), n is the lower energy level with a value of 2, and m is the higher energy level, which has a value greater than 3. This observation was then refined by Johannes Rydberg, where R is the Rydberg constant.
<img src="https://www-jove-com.remotexs.ntu.edu.sg/CDNSource/lm/labs/54/54_Concepts_3.jpg" />
Remember that this equation describes emitted light, so the higher energy level is considered the initial level, or ni, while the lower level is considered the final level, or nf. In the case of the Balmer series, nf is equal to 2. This equation was combined with the Bohr model to calculate the energy needed to move an electron between its initial and final energy levels, &#916;E.
<img src="https://www-jove-com.remotexs.ntu.edu.sg/CDNSource/lm/labs/54/54_Concepts_4.jpg" />
Later, other spectral series for the hydrogen atom were discovered. For example, the Lyman series contains emission lines with energies in the ultraviolet region.
<h4>References</h4>
<ol>
	<li style="font-size:17px">Kotz, J.C., Treichel Jr, P.M., Townsend, J.R. (2012). Chemistry and Chemical Reactivity. Belmont, CA: Brooks/Cole, Cengage Learning.</li>
	<li style="font-size:17px">Silderberg, M.S. (2009). Chemistry: The Molecular Nature of Matter and Change. Boston, MA: McGraw Hill.</li>
</ol>

The Hydrogen Atom: Bohr Model and Balmer Series - Concept

The Bohr Model
Niels Bohr proposed a model for the hydrogen atom in 1913 that described discrete energy states are associated with a fixed electron orbit ...

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Dependence of Laser-induced Breakdown Spectroscopy Results on Pulse Energies and Timing Parameters Using Soil Simulants

The dependence of some LIBS detection capabilities on lower pulse energies (&lt;100 mJ) and timing parameters were examined using synthetic silicate samples. These samples were used as simulants for soil and contained minor and trace elements commonly found in soil at a wide range of concentrations. For this study, over 100 calibration curves were prepared using different pulse energies and timing parameters; detection limits and sensitivities were determined from the calibration curves. Plasma temperatures were also measured using Boltzmann plots for the various energies and the timing parameters tested. The electron density of the plasma was calculated using the full-width half maximum (FWHM) of the hydrogen line at 656.5 nm over the energies tested. Overall, the results indicate that the use of lower pulse energies and non-gated detection do not seriously compromise the analytical results. These results are very relevant to the design of field- and person-portable LIBS instruments.

LIBS detection capabilities on soil simulants were tested using a range of pulse energies and timing parameters. Calibration curves were used to determine detection limits and sensitivities for different parameters. Generally, the results showed that there was not a significant reduction in detection capabilities using lower pulse energies and non-gated detection.

The  dependence of some LIBS detection capabilities on lower pulse energies (&lt;100  mJ) and timing parameters were examined using synthetic silicate ...

Laser-induced Breakdown Spectroscopy: A New Approach for Nanoparticle's Mapping and Quantification in Organ Tissue

Emission spectroscopy of laser-induced plasma was applied to elemental analysis of biological samples. Laser-induced breakdown spectroscopy (LIBS) performed on thin sections of rodent tissues: kidneys and tumor, allows the detection of inorganic elements such as (i) Na, Ca, Cu, Mg, P, and Fe, naturally present in the body and (ii) Si and Gd, detected after the injection of gadolinium-based nanoparticles. The animals were euthanized 1&#160;to 24 hr&#160;after intravenous injection of particles. A two-dimensional scan of the sample, performed using a motorized micrometric 3D-stage, allowed the infrared laser beam exploring the surface with a lateral resolution less than 100 &#956;m. Quantitative chemical images of Gd element inside the organ were obtained with sub-mM sensitivity. LIBS offers a simple and robust method to study the distribution of inorganic materials without any specific labeling. Moreover, the compatibility of the setup with standard optical microscopy emphasizes its potential to provide multiple images of the same biological tissue with different types of response:&#160;elemental, molecular, or cellular.

Laser-induced breakdown spectroscopy performed on thin organ and tumor tissue successfully detected natural elements and artificially injected gadolinium (Gd), issued from Gd-based nanoparticles. Images of chemical elements reached a resolution of 100 &#956;m and quantitative sub-mM sensitivity. The compatibility of the setup with standard optical microscopy emphasizes its potential to provide multiple images of a same biological tissue.

Emission spectroscopy of laser-induced plasma was applied to elemental analysis of biological samples. Laser-induced breakdown spectroscopy (LIBS) ...

Characterizing Far-infrared Laser Emissions and the Measurement of Their Frequencies

The generation and subsequent measurement of far-infrared radiation has found numerous applications in high-resolution spectroscopy, radio astronomy, and Terahertz imaging. For about 45 years, the generation of coherent, far-infrared radiation has been accomplished using the optically pumped molecular laser. Once far-infrared laser radiation is detected, the frequencies of these laser emissions are measured using a three-laser heterodyne technique. With this technique, the unknown frequency from the optically pumped molecular laser is mixed with the difference frequency between two stabilized, infrared reference frequencies. These reference frequencies are generated by independent carbon dioxide lasers, each stabilized using the fluorescence signal from an external, low pressure reference cell. The resulting beat between the known and unknown laser frequencies is monitored by a metal-insulator-metal point contact diode detector whose output is observed on a spectrum analyzer. The beat frequency between these laser emissions is subsequently measured and combined with the known reference frequencies to extrapolate the unknown far-infrared laser frequency. The resulting one-sigma fractional uncertainty for laser frequencies measured with this technique is &#177; 5 parts in 107. Accurately determining the frequency of far-infrared laser emissions is critical as they are often used as a reference for other measurements, as in the high-resolution spectroscopic investigations of free radicals using laser magnetic resonance. As part of this investigation, difluoromethane, CH2F2, was used as the far-infrared laser medium. In all, eight far-infrared laser frequencies were measured for the first time with frequencies ranging from 0.359 to 1.273 THz. Three of these laser emissions were discovered during this investigation and are reported with their optimal operating pressure, polarization with respect to the CO2 pump laser, and strength.

We describe the generation of far-infrared radiation using an optically pumped molecular laser along with the measurement of their frequencies with heterodyne techniques. The experimental system and techniques are demonstrated using difluoromethane (CH2F2) as the laser medium whose results include three new laser emissions and eight measured laser frequencies.

The generation and subsequent measurement of far-infrared radiation has found numerous applications in high-resolution spectroscopy, radio astronomy, and ...

Emission Spectroscopic Boundary Layer Investigation during Ablative Material Testing in Plasmatron

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution for future high-speed reentry missions. But despite the advancements made since Apollo, heat flux prediction remains an imperfect science and engineers resort to safety factors to determine the TPS thickness. This goes at the expense of embarked payload, hampering, for example, sample return missions. 
Ground testing in plasma wind-tunnels is currently the only affordable possibility for both material qualification and validation of material response codes. The subsonic 1.2MW Inductively Coupled Plasmatron facility at the von Karman Institute for Fluid Dynamics is able to reproduce a wide range of reentry environments. This protocol describes a procedure for the study of the gas/surface interaction on ablative materials in high enthalpy flows and presents sample results of a non-pyrolyzing, ablating carbon fiber precursor. With this publication, the authors envisage the definition of a standard procedure, facilitating comparison with other laboratories and contributing to ongoing efforts to improve heat shield reliability and reduce design uncertainties.
The described core techniques are non-intrusive methods to track the material recession with a high-speed camera along with the chemistry in the reactive boundary layer, probed by emission spectroscopy. Although optical emission spectroscopy is limited to line-of-sight measurements and is further constrained to electronically excited atoms and molecules, its simplicity and broad applicability still make it the technique of choice for analysis of the reactive boundary layer. Recession of the ablating sample further requires that the distance of the measurement location with respect to the surface is known at all times during the experiment. Calibration of the optical system of the applied three spectrometers allowed quantitative comparison. At the fiber scale, results from a post-test microscopy analysis are presented.

Development of new ablative materials and their numerical modeling requires extensive experimental investigation. This protocol describes procedures for material response characterization in plasma flows with the core techniques being non-intrusive methods to track the material recession along with the chemistry in the reactive boundary layer by emission spectroscopy.

Ablative Thermal Protection Systems (TPS) allowed the first humans to safely return to Earth from the moon and are still considered as the only solution ...

Direct Imaging of Laser-driven Ultrafast Molecular Rotation

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb explosion imaging setup in which a hitherto-impractical camera angle is realized. In our imaging technique, diatomic molecules are irradiated with a circularly polarized strong laser pulse. The ejected atomic ions are accelerated perpendicularly to the laser propagation. The ions lying in the laser polarization plane are selected through the use of a mechanical slit and imaged with a high-throughput, 2-dimensional detector installed parallel to the polarization plane. Because a circularly polarized (isotropic) Coulomb exploding pulse is used, the observed angular distribution of the ejected ions directly corresponds to the squared rotational wave function at the time of the pulse irradiation. To create a real-time movie of molecular rotation, the present imaging technique is combined with a femtosecond pump-probe optical setup in which the pump pulses create unidirectionally rotating molecular ensembles. Due to the high image throughput of our detection system, the pump-probe experimental condition can be easily optimized by monitoring a real-time snapshot. As a result, the quality of the observed movie is sufficiently high for visualizing the detailed wave nature of motion. We also note that the present technique can be implemented in existing standard ion imaging setups, offering a new camera angle or viewpoint for the molecular systems without the need for extensive modification.

We present a protocol for creating a real-time movie of a molecular rotational wave packet using a high-resolution Coulomb explosion imaging setup.

We present a method for visualizing laser-induced, ultrafast molecular rotational wave packet dynamics. We have developed a new 2-dimensional Coulomb ...

Coulomb Explosion Imaging as a Tool to Distinguish Between Stereoisomers

This article shows how the COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) or the "reaction microscope" technique can be used to distinguish between enantiomers (stereoisomers) of simple chiral species on the level of individual molecules. In this approach, a gaseous molecular jet of the sample expands into a vacuum chamber and intersects with femtosecond (fs) laser pulses. The high intensity of the pulses leads to fast multiple ionization, igniting a so-called Coulomb Explosion that produces several cationic (positively charged) fragments. An electrostatic field guides these cations onto time- and position-sensitive detectors. Similar to a time-of-flight mass spectrometer, the arrival time of each ion yields information on its mass. As a surplus, the electrostatic field is adjusted in a way that the emission direction and the kinetic energy after fragmentation lead to variations in the time-of-flight and in the impact position on the detector.
Each ion impact creates an electronic signal in the detector; this signal is treated by high-frequency electronics and recorded event by event by a computer. The registered data correspond to the impact times and positions. With these data, the energy and the emission direction of each fragment can be calculated. These values are related to structural properties of the molecule under investigation, i.e. to the bond lengths and relative positions of the atoms, allowing to determine molecule by molecule the handedness of simple chiral species and other isomeric features.

For small chiral species, Coulomb Explosion Imaging provides a new approach to determine the handedness of individual molecules.

This article shows how the COLTRIMS (Cold Target Recoil Ion Momentum Spectroscopy) or the "reaction microscope" technique can be used to distinguish ...

Measurement of Ultrafast Vibrational Coherences in Polyatomic Radical Cations with Strong-Field Adiabatic Ionization

We present a pump-probe method for preparing vibrational coherences in polyatomic radical cations and probing their ultrafast dynamics. By shifting the wavelength of the strong-field ionizing pump pulse from the commonly used 800 nm into the near-infrared (1200-1600 nm), the contribution of adiabatic electron tunneling to the ionization process increases relative to multiphoton absorption. Adiabatic ionization results in predominant population of the ground electronic state of the ion upon electron removal, which effectively prepares a coherent vibrational state (&#34;wave packet&#34;) amenable to subsequent excitation. In our experiments, the coherent vibrational dynamics are probed with a weak-field 800 nm pulse and the time-dependent yields of dissociation products measured in a time-of-flight mass spectrometer. We present the measurements on the molecule dimethyl methylphosphonate (DMMP) to illustrate how using 1500 nm pulses for excitation enhances the amplitude of coherent oscillations in ion yields by a factor of 10 as compared to 800 nm pulses. This protocol may be implemented in existing pump-probe setups through the incorporation of an optical parametric amplifier (OPA) for wavelength conversion.

We present a protocol for probing ultrafast vibrational coherences in polyatomic radical cations that result in molecular dissociation.

We present a pump-probe method for preparing vibrational coherences in polyatomic radical cations and probing their ultrafast dynamics. By shifting the ...

Quantitative Analysis of Vacuum Induction Melting by Laser-induced Breakdown Spectroscopy

Vacuum induction melting is a popular method for refining high purity metal and alloys. Traditionally, standard process control in metallurgy involves several steps, include drawing samples, cooling, cutting, transport to the laboratory, and analysis. The whole analysis process requires more than 30 minutes, which hinders on-line process control. Laser-induced breakdown spectroscopy is an excellent on-line analysis method that can satisfy the requirements of vacuum induction melting because it is fast and noncontact and does not require sample preparation. The experimental facility uses a lamp-pumped Q-switched laser to ablate melted liquid steel with an output energy of 80 mJ, a frequency of 5 Hz, a FWHM pulse width of 20 ns, and a working wavelength of 1,064 nm. A multi-channel linear charge coupled device (CCD) spectrometer is used to measure the emission spectrum in real time, with a spectral range from 190 to 600 nm and a resolution of 0.06 nm at a wavelength of 200 nm. The protocol includes several steps: standard alloy sample preparation and an ingredient test, smelting of standard samples and determination of the laser breakdown spectrum, and construction of the elements concentration quantitative analysis curve of each element. To realize the concentration analysis of unknown samples, the spectrum of a sample also needs to be measured and disposed with the same process. The composition of all main elements in the melted alloy can be quantitatively analyzed with an internal standard method. The calibration curve shows that the limit of detection of most metal elements ranges from 20-250 ppm. The concentration of elements, such as Ti, Mo, Nb, V, and Cu, can be lower than 100 ppm, and the concentrations of Cr, Al, Co, Fe, Mn, C, and Si range from 100-200 ppm. The R2 of some calibration curves can exceed 0.94.

During vacuum induction melting, laser-induced breakdown spectroscopy is used to perform real-time quantitative analysis of the main-ingredient elements of a molten alloy.

Vacuum induction melting is a popular method for refining high purity metal and alloys. Traditionally, standard process control in metallurgy involves ...

Atomic Spectroscopy: Absorption, Emission, and Fluorescence

Atomic spectroscopy is a vital tool in elemental analysis, both qualitatively and quantitatively. It can be broadly divided into optical spectroscopy, mass spectroscopy, and X-ray spectroscopy methods. The optical spectroscopic methods are atomic absorption spectroscopy (AAS), atomic emission spectroscopy (AES), and atomic fluorescence spectroscopy (AFS). The first step in all three methods is atomization, where the solid, liquid, or solution-phase samples are converted into gas-phase atoms and ions.
In AAS, the gaseous samples interact with electromagnetic radiation and absorb photons with the exact energies that promote electrons of ground-state atoms to their excited states. For example, the unpaired 3s electron of the Na atom gets promoted to the 3p, 4p, or 5p orbital on the absorption of radiation of 285 nm, 330 nm, or 590 nm, respectively. The reduction in transmitted light of certain wavelengths is measured by the detector and visualized with an absorbance or transmittance spectrum.
In AES, the gas-phase atoms in the ground state are electronically excited with heat or electrical discharge energy. These short-lived, high-energy excited gas-phase atoms relax back to the ground state, emitting photons corresponding to the energy gap. The intensity of the emitted light is detected and converted to an electrical signal that gives a fingerprint of the sample. For instance, electronic transitions of excited Na atoms from the 3p, 4p, and 5p orbitals back to the 3s orbital result in emissions around 590 nm, 330 nm, and 285 nm, respectively. The emitted radiation is measured and processed into a spectrum.
In AFS, the ground-state gas-phase atoms are irradiated by a characteristic wavelength and promoted to the electronically excited state. Provided radiationless transition does not occur, the excited-state atoms relax to the ground state by fluorescing at the exact wavelength corresponding to the energy they absorbed. The detector is usually at a right angle to the source beam, where only fluorescence emissions should reach it.
Unlike molecular spectra, atomic spectra have sharp lines due to the absence of various rotational and vibrational energy states that lead to peak broadening in molecular spectra.

Atomic spectroscopy is a vital tool in elemental analysis, both qualitatively and quantitatively. It can be broadly divided into optical spectroscopy, ...

Measurement and Analysis of Atomic Hydrogen and Diatomic Molecular AlO, C₂, CN, and TiO Spectra Following Laser-induced Optical Breakdown

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