Name: direct imaging of laser-driven ultrafast molecular rotation
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This work was supported in part by grants-in-aid KAKENHI from the Japan Society for the Promotion of Science (JSPS) and the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) Japan (#26104539, #26620020, #26810011, #15H03766, #15KT0060, #16H00826, and #16K13927); the Konica Minolta Science and Technology Foundation; the "Planting Seeds for Research" program of TokyoTech; the Imaging Science Project of the Center for Novel Science Initiatives (CNSI) at the National Institutes of Natural Sciences (NINS) (#IS261006); the RIKEN-IMS joint program on "Extreme Photonics;" and the Consortium for Photon Science and Technology (CPhoST).

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The authors have nothing to disclose.

The present procedure enables us to capture a real-time movie of molecular rotation with a slit-based 2D imaging setup. Because the observed ions pass through the slit, step 1.5 is one of the critical steps. The edges of the slit blades must be sharp. When there is a small defect, such as a 0.3 mm dent in the slit, a scratch is observed in the ion image (Figure 6). In such a case, the slit blade should be polished with 2,000-grit wet sandpaper.
Apart from the unique camera angle shown in Figure 1, this method has several advantages over the 3D imaging detector, which was previously the only solution for rotational wave packet imaging.
First, in the present procedure, optical beam alignment can be carried out easily by monitoring raw ion images, as in steps 4.1-4.2. Figure 7 shows the raw camera image at the optimized pump-probe overlap condition. When the pump-probe beam overlap is lost, anisotropic or enhanced image signatures cannot be seen, as in Figure 4A. This fact emphasizes the importance of steps 4.1-4.2 in the present method. Because the spot sizes of the pump and probe beams are on the order of 10 &#181;m, it is generally difficult to find an optimal overlap condition without monitoring real-time images. In the case of a 3D imaging detector, several seconds are required to form an image with sufficient data points (at least 1,000 ions) when a 1,000 Hz or lower repetition-rate laser is employed, because the count rate is limited to a few events per laser shot in the 3D detector. In the present method, on the other hand, the count rate is essentially unlimited, and the number of ions per frame can be increased simply by extending the exposure time. In the present case, more than 1,000 ions are detected within the 50 ms exposure time.
The high count rate of the present method also leads to a shorter data acquisition time. Because the frame rate of the camera is 250 fps, it takes only ~40 s to take one snapshot of the molecular motion at a particular time. For the measurement over the one molecular rotational revival time (~8.4 ps) with a ~33-fs step, the measurement time is only a few hours. This is another advantage, because experimental data would be degraded by the limited long-term stability of the lasers and the entire experimental setup. In our setup, for example, time duration changes with time, partly due to the temperature change in the fs amplifier. A 3-K change within 6 h resulted in the thermal expansion of the amplifier, including the elongation of the distance between the pulse compressor gratings, leading to the elongation of the pulse duration31. Laser-beam drift, which degrades the signal of the pump-induced dynamics, was also detected within ~8 h, although the origin of this drift was not identified.
The present technique is a type of 2D imaging, limiting the information in 3D. In the case of a Coulomb explosion, only the fragment of ions ejected in the detection plane contribute to the image. This implies that it is difficult to apply the present method directly to complicated fragmentation processes, such as those involved in coincidence imaging studies25,32,33. We note that the sum of the signal intensity with our method is proportional to the probability in the detection plane. This represents indirect information on the dimension not included in the imaging plane11,12.
While we focus on Coulomb explosion imaging in this paper, the present approach can be, in principle, applied to general charged-particle imaging, such as that involved in photodissociation studies14. In the existing imaging procedure, to obtain a 2D tomogram of a 3D Newton sphere of charged particles, the polarization of light should be parallel to the detector surface. In other words, the camera angle is limited to particular conditions. Also, in the present 2D imaging technique, a 3D ion cloud is spatially sliced to a 2D cut and is then imaged. With this slice imaging, the freedom of the camera angle will open a way to obtain hitherto-unobserved information that sometimes appears in the laser propagation direction26,34.

For a deeper understanding and better use of the dynamic nature of molecules, it is essential to clearly visualize molecular motions of interest. Time-resolved Coulomb explosion imaging is one of the powerful approaches to achieve this objective1,2,3. In this approach, the molecular dynamics of interest are initiated by a pump ultrashort laser field and are then probed by a time-delayed probe pulse. Upon probe irradiation, molecules are multiply ionized and broken into fragment ions due to the Coulomb repulsion. The spatial distribution of the ejected ions is a measure of the molecular structure and spatial orientation at the probe irradiation. A sequence of measurement scanning the pump-probe delay time leads to the creation of a molecular movie. It is noteworthy that, for the simplest case &#8211; diatomic molecules &#8211; the angular distribution of the ejected ions directly reflects the molecular axis distribution (i.e., the squared rotational wave function).
With regard to the pump process, recent progress in the coherent control of molecular motion using ultrashort laser fields has led to the creation of highly controlled rotational wave packets4,5. Furthermore, the direction of rotation can be actively controlled by using a polarization-controlled laser field6,7,8. It has therefore been expected that a detailed picture of molecular rotation, including wave natures, could be visualized when the Coulomb explosion imaging technique is combined with such a pump process9,10,11,12,13. However, we sometimes encounter experimental difficulties associated with the existing imaging methods, as mentioned below. The purpose of this paper is to present a new way of overcoming these difficulties and of creating a high-quality movie of molecular rotational wave packets. The first experimental movie of molecular rotation taken with the present method, along with its physical implications, were presented in our previous paper11. The background of development, the detailed theoretical aspect of the present imaging technique, and a comparison with other existing techniques will be given in a forthcoming paper. Here, we will mainly focus on the practical and technical aspects of the procedure, including the combination of the typical pump-probe optical setup and the new imaging apparatus. As in the previous paper, the target system is unidirectionally rotating nitrogen molecules11.
The main experimental difficulty of the existing imaging setup, schematically shown in Figure 1, has to do with the position of the detector, or the camera angle. Because the rotational axis coincides with the laser propagation axis6,7,8 in laser-field-induced molecular rotation, it is not practical to install a detector along the rotational axis. When the detector is installed so as to avoid laser irradiation, the camera angle corresponds to a side observation of rotation. In this case, it is impossible to reconstruct the original orientation of molecules from the projected (2D) ion image14. A 3D imaging detector14,15,16,17,18,19, with which the arrival time to the top detector and the ion impact positions can be measured, offered a unique way to directly observe molecular rotation using Coulomb explosion imaging10,12. However, the acceptable ion counts per laser shot are low (typically &#60; 10 ions) in the 3D detector, meaning that it is difficult to create a long movie of molecular motion with high image quality14. The dead time of the detectors (typically ns) also affects the image resolution and imaging efficiency. It is also not a simple task to make a good pump-probe beam overlap by monitoring a real-time ion image with a laser repetition rate of &#60;~1 kHz. Although several groups have observed rotational wave packets using the 3D technique, the spatial information was limited and/or direct, and a detailed visualization of wave nature, including complicated nodal structures, was not achieved10,12.
The essence of the new imaging technique is the use of the &#34;new camera angle&#34; in Figure 1. In this configuration, laser beam exposure to a detector is avoided while the 2D detector is parallel to the rotational plane, leading to the observation from the rotational axis direction. The slit allows only the ion in the rotational plane (the polarization plane of the laser pulses) to contribute to an image. A 2D detector, which offers a higher count rate (typically ~100 ions) than a 3D detector, can be used. The setup of the electronics is simpler than in the case of 3D detection, while the measurement efficiency is higher. Time-consuming mathematical reconstruction, such as Abel inversion14, is also not needed to extract angular information. These features lead to the easy optimization of the measurement system and to the production of high-quality movies. A standard 2D/3D charged-particle imaging apparatus can be easily modified to the present setup without the use of expensive equipment.

<table><tbody><tr><td>CMOS camera</td><td>Toshiba TELI</td><td>BU-238M-ES</td><td>equipped with SONY IMX174 sensor</td></tr><tr><td>High voltage switch</td><td>Behlke</td><td>HTS-41-03-GSM</td><td></td></tr><tr><td>High voltage switch</td><td>Behlke</td><td>HTS-80-03</td><td></td></tr><tr><td>Digital delay generator</td><td>Stanford research systems</td><td>DG535</td><td></td></tr><tr><td>Digital delay generator</td><td>Stanford research systems</td><td>DG645</td><td></td></tr><tr><td>Microchannel plate</td><td>Photonis</td><td>3075</td><td></td></tr><tr><td>Pulsed valve</td><td>LAMID LTD</td><td>Even-Lavie valve&amp;nbsp;</td><td>High repetition, room temperature model</td></tr><tr><td>Molecular beam skimmers</td><td>Institute for Molecular Science</td><td>13C11</td><td>3 and 1.5 mm center hole, 25 degrees full inner angle, and ~50 mm length</td></tr><tr><td>Optical Comparator</td><td>Nikon</td><td>V-24B</td><td></td></tr><tr><td>DPSS laser</td><td>Lighthouse Photonics</td><td>Sprout</td><td></td></tr><tr><td>Femtosecond Ti:Sapphire oscillator</td><td>KMLabs</td><td>Halcyon</td><td></td></tr><tr><td>Femtosecond Ti:Sapphire amplifier</td><td>Quantronix</td><td>Odin-II HE</td><td></td></tr><tr><td>Motorized linear stage</td><td>Sigma Koki</td><td>KST(GS)-100X</td><td></td></tr><tr><td>Manual X-stage</td><td>Sigma Koki</td><td>TSD-601S</td><td></td></tr><tr><td>High resolution mirror mount</td><td>Newport</td><td>Suprema SX100-F2KN-254</td><td></td></tr><tr><td>High resolution mirror mount</td><td>LIOP-TEC GmbH</td><td>SR100-100R-2-HS</td><td></td></tr><tr><td>Polarization checker</td><td>Paradigm Devices, Inc.</td><td>O-tool VIS</td><td></td></tr><tr><td>Instrument communication interface</td><td>National Instruments</td><td>NI-MAX</td><td></td></tr><tr><td>Graphical development environment for measurement programs</td><td>National Instruments</td><td>LabVIEW 2014</td><td></td></tr><tr><td>Laser line dielectric mirror</td><td>CVI/LEO</td><td>TLM2-400/800-45UNP</td><td></td></tr><tr><td>Laser line dielectric mirror</td><td>Altechna</td><td>Low GDD Ultrafast mirror</td><td></td></tr><tr><td>Laser line dielectric mirror</td><td>Altechna</td><td>Low GDD Ultrafast mirror</td><td></td></tr><tr><td>Femtosecond polarizer</td><td>Advanced Thin Films</td><td>PBS-GVD</td><td></td></tr></tbody></table>

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.

Figure 4A shows a probe-only raw image of the N2+ ion ejected upon probe irradiation (Coulomb explosion), taken for one probe laser shot. Each bright spot corresponds to one ion. Figure 4B shows a summed image of 10,000 binarized raw camera images. These images show that our imaging setup can monitor the molecules of all the orientation angles in the polarization plane. Figure 4C shows the normalized polar plot corresponding to that of Figure 4B. Because the rotational control (pump) pulse was absent, the distribution is isotropic (Figure 4C shows a circle).


In Figure 4B, a small defect due to detector inhomogeneity can be seen in the bottom of the ellipse. Such a defect always appears in the same position of the image. Therefore, it can be compensated for by normalizing the observed images with a probe-only image (step 4.5.3.7).
Figure 5 shows selected snapshots taken after the irradiation of the two pump pulses. So as to improve understanding, not only the observed ion images, but also the corresponding polar plots and &#34;dumbbell&#34; model pictures are shown as a function of probe time. The polar plots are created in step 4.5.3.5. The dumbbell picture is an overlapped image of dumbbells of various orientation angles, and their weights (opacity) are the observed angular probabilities. The sequence of images forms a clear movie of unidirectional molecular rotation. The wave nature of motion can be seen as the complicated nodal structures and the dispersion, including an &#34;X&#34;-shape formation.
Figure 6 shows an ion image taken with a damaged slit and a photograph of the slit edge with a dent. A small defect largely affects the observed image. In such a case, repeating step 1.5 is required. This fact is also discussed in the discussion section.
Figure 7 shows the raw camera image at the optimized pump-probe overlap condition. By monitoring such a beam overlap signal, the optical paths can be optimized. This leads to a clear movie, as in Figure 5.
<img alt="Figure 1" src="/files/ftp_upload/54917/54917fig1.jpg" /> 
Figure 1: Conceptual diagram of the camera angles in the impractical, typical, and new configurations. In the typical camera angle, a detector is installed to avoid laser exposure, but the ejection angles of ions cannot be reconstructed from the 2D projected image. In the present, new camera angle, the rotational plane (laser polarization plane) is parallel to the detector surface and is therefore suitable for visualizing the rotational motion. Typical bias voltages are 2,500 V, 1,799 V, 1,846 V, 253 V, 0 V, 3,500 V, -800 V, and 4,500 V for ion optics 1, 2, 3, 4, and 5, the pulsed repeller, the microchannel plates, and the phosphor screen, respectively. The ion optics numbering starts at the bottom electrode. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54917/54917fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/54917/54917fig2.jpg" /> 
Figure 2: Schematic diagrams of the 2D imaging unit. (A) Schematic diagram of the detector assembly. A circle plate colored in orange is a baseplate to which the other parts are mounted with bolts. (B) Schematic diagram of the slit assembly. The right picture explains the motion of the slit. The size values are in mm. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54917/54917fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/54917/54917fig3.jpg" /> 
Figure 3: Schematic diagram of the present pump-probe optical setup. The optical paths of the pump pulses for rotational excitation are illustrated by the red lines, while that of the probe (imaging) pulse is shown by the blue line. NLC, nonlinear crystal for second-harmonic generation; HWP, half-waveplate; QWP, quarter-waveplate; DM, dichroic mirror; BS, 50:50 beam splitter; HRM: high-resolution mirror mount. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54917/54917fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" src="/files/ftp_upload/54917/54917fig4.jpg" /> 
Figure 4: Raw and analyzed Coulomb exploded ion images. (A) A typical raw image of N2+ taken for one probe shot. (B) Summed image for 10,000 binarized camera images. The size of the camera image is 1,200 x 750 pixels. The corresponding real-space size is 80 mm x 50 mm. (C) The normalized polar plot constructed from the summed image. In the raw and summed images, false color was added to show the signal intensity. The polar angles in degrees are shown along the circumference. The radial value is an angle-dependent probability (arbitrary unit). <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54917/54917fig4large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" src="/files/ftp_upload/54917/54917fig5.jpg" /> 
Figure 5: Selected snapshots of the laser-induced rotational wave packet dynamics. In each time delay, the upper panel shows the ion image in which the elliptical shape has been converted to a circle. The middle panel shows the corresponding polar plot. The bottom panel shows a dumbbell model of the angular distribution. This dumbbell picture is an overlapped image of dumbbells from various orientation angles, and their weights (opacity) are the observed angular probabilities. The polar plot uses the same unit and scale as in Figure 4. The ion image employs transformed coordinates, as in step 4.5.3.4. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54917/54917fig5large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 6" src="/files/ftp_upload/54917/54917fig6.jpg" /> 
Figure 6: Effect of the slit defect on the experimental ion image. (A) Observed probe-only N2+ ion image taken with a damaged slit. (B) Photograph of the slit edge having a sub-mm dent. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54917/54917fig6large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 7" src="/files/ftp_upload/54917/54917fig7.jpg" /> 
Figure 7: Raw camera image at the optimized pump-probe overlap condition. The probe time is set at t = 4.0 ps after the first pump pulse irradiation. At this time, the maximum degree of molecular alignment is achieved. The size of the camera image is 1,200 x 750 pixels. The corresponding real-space size is 80 mm x 50 mm. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54917/54917fig7large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Direct Imaging of Laser-driven Ultrafast Molecular Rotation at JoVE.com

Direct Imaging of Laser-driven Ultrafast Molecular Rotation

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 ...

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Nanyang Technological University

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Chemistry

Chemical Bonding: Molecular Geometry and Bonding Theories

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9.6K Views.  Tokyo Institute of Technology. We present a protocol for creating a real-time movie of a molecular rotational wave packet using a high-resolution Coulomb explosion imaging setup.

Video: Direct Imaging of Laser-driven Ultrafast Molecular Rotation

Fluorescence Lifetime Imaging of Molecular Rotors in Living Cells

Diffusion is often an important rate-determining step in chemical reactions or biological processes and plays a role in a wide range of intracellular events. Viscosity is one of the key parameters affecting the diffusion of molecules and proteins, and changes in viscosity have been linked to disease and malfunction at the cellular level.1-3 While methods to measure the bulk viscosity are well developed, imaging microviscosity remains a challenge. Viscosity maps of microscopic objects, such as single cells, have until recently been hard to obtain. Mapping viscosity with fluorescence techniques is advantageous because, similar to other optical techniques, it is minimally invasive, non-destructive and can be applied to living cells and tissues.
Fluorescent molecular rotors exhibit fluorescence lifetimes and quantum yields which are a function of the viscosity of their microenvironment.4,5 Intramolecular twisting or rotation leads to non-radiative decay from the excited state back to the ground state. A viscous environment slows this rotation or twisting, restricting access to this non-radiative decay pathway. This leads to an increase in the fluorescence quantum yield and the fluorescence lifetime. Fluorescence Lifetime Imaging (FLIM) of modified hydrophobic BODIPY dyes that act as fluorescent molecular rotors show that the fluorescence lifetime of these probes is a function of the microviscosity of their environment.6-8 A logarithmic plot of the fluorescence lifetime versus the solvent viscosity yields a straight line that obeys the F&ouml;rster Hoffman equation.9 This plot also serves as a calibration graph to convert fluorescence lifetime into viscosity.
Following incubation of living cells with the modified BODIPY fluorescent molecular rotor, a punctate dye distribution is observed in the fluorescence images. The viscosity value obtained in the puncta in live cells is around 100 times higher than that of water and of cellular cytoplasm.6,7 Time-resolved fluorescence anisotropy measurements yield rotational correlation times in agreement with these large microviscosity values. Mapping the fluorescence lifetime is independent of the fluorescence intensity, and thus allows the separation of probe concentration and viscosity effects. 
In summary, we have developed a practical and versatile approach to map the microviscosity in cells based on FLIM of fluorescent molecular rotors.

Fluorescence Lifetime Imaging (FLIM) has emerged as a key technique to image the environment and interaction of specific proteins and dyes in living cells. FLIM of fluorescent molecular rotors allows mapping of viscosity in living cells.

Diffusion is often an important rate-determining step in chemical reactions or biological processes and plays a role in a wide range of intracellular ...

Bioengineering

Spatial Separation of Molecular Conformers and Clusters

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold molecular beams. However, these beams often contain multiple conformers and clusters, even at low rotational temperatures. We present an experimental methodology that allows the spatial separation of these constituent parts of a molecular beam expansion. Using an electric deflector the beam is separated by its mass-to-dipole moment ratio, analogous to a bender or an electric sector mass spectrometer spatially dispersing charged molecules on the basis of their mass-to-charge ratio. This deflector exploits the Stark effect in an inhomogeneous electric field and allows the separation of individual species of polar neutral molecules and clusters. It furthermore allows the selection of the coldest part of a molecular beam, as low-energy rotational quantum states generally experience the largest deflection. Different structural isomers (conformers) of a species can be separated due to the different arrangement of functional groups, which leads to distinct dipole moments. These are exploited by the electrostatic deflector for the production of a conformationally pure sample from a molecular beam. Similarly, specific cluster stoichiometries can be selected, as the mass and dipole moment of a given cluster depends on the degree of solvation around the parent molecule. This allows experiments on specific cluster sizes and structures, enabling the systematic study of solvation of neutral molecules.

We present a technique that allows the spatial separation of different conformers or clusters present in a molecular beam. An electrostatic deflector is used to separate species by their mass-to-dipole moment ratio, leading to the production of gas-phase ensembles of a single conformer or cluster stoichiometry.

Gas-phase molecular physics and physical chemistry experiments commonly use supersonic expansions through pulsed valves for the production of cold ...

Engineering

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 ...

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 ...

Cooling an Optically Trapped Ultracold Fermi Gas by Periodical Driving

We present a cooling method for a cold Fermi gas by parametrically driving atomic motions in a crossed-beam optical dipole trap (ODT). Our method employs the anharmonicity of the ODT, in which the hotter atoms at the edge of the trap feel the anharmonic components of the trapping potential, while the colder atoms in the center of the trap feel the harmonic one. By modulating the trap depth with frequencies that are resonant with the anharmonic components, we selectively excite the hotter atoms out of the trap while keeping the colder atoms in the trap, generating parametric cooling. This experimental protocol starts with a magneto-optical trap (MOT) that is loaded by a Zeeman slower. The precooled atoms in the MOT are then transferred to an ODT, and a bias magnetic field is applied to create an interacting Fermi gas. We then lower the trapping potential to prepare a cold Fermi gas near the degenerate temperature. After that, we sweep the magnetic field to the noninteracting regime of the Fermi gas, in which the parametric cooling can be manifested by modulating the intensity of the optical trapping beams. We find that the parametric cooling effect strongly depends on the modulation frequencies and amplitudes. With the optimized frequency and amplitude, we measure the dependence of the cloud energy on the modulation time. We observe that the cloud energy is changed in an anisotropic way, where the energy of the axial direction is significantly reduced by parametric driving. The cooling effect is limited to the axial direction because the dominant anharmonicity of the crossed-beam ODT is along the axial direction. Finally, we propose to extend this protocol for the trapping potentials of large anharmonicity in all directions, which provides a promising scheme for cooling quantum gases using external driving.

We present a parametric driving method to cool an ultracold Fermi gas in a crossed-beam optical dipole trap. This method selectively removes high-energy atoms from the trap by periodically modulating the trap depth with frequencies that are resonant with the anharmonic components of the trapping potential.

We present a cooling method for a cold Fermi gas by parametrically driving atomic motions in a crossed-beam optical dipole trap (ODT). Our method employs ...

Novel Techniques for Observing Structural Dynamics of Photoresponsive Liquid Crystals

We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational spectroscopy and time-resolved electron diffraction. Liquid crystal phase is an important state of matter that exists between the solid and liquid phases and it is common in natural systems as well as in organic electronics. Liquid crystals are orientationally ordered but loosely packed, and therefore, the internal conformations and alignments of the molecular components of LCs can be modified by external stimuli. Although advanced time-resolved diffraction techniques have revealed picosecond-scale molecular dynamics of single crystals and polycrystals, direct observations of packing structures and ultrafast dynamics of soft materials have been hampered by blurry diffraction patterns. Here, we report time-resolved IR vibrational spectroscopy and electron diffractometry to acquire ultrafast snapshots of a columnar LC material bearing a photoactive core moiety. Differential-detection analyses of the combination of time-resolved IR vibrational spectroscopy and electron diffraction are powerful tools for characterizing structures and photoinduced dynamics of soft materials.

Here, we present the protocols of differential-detection analyses of time-resolved infrared vibrational spectroscopy and electron diffraction which enable observations of the deformations of local structures around photoexcited molecules in a columnar liquid crystal, giving an atomic perspective on the relationship between the structure and the dynamics of this photoactive material.

We discuss in this article the experimental measurements of the molecules in liquid crystal (LC) phase using the time-resolved infrared (IR) vibrational ...

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 ...

Construction and Operation of a Light-driven Gold Nanorod Rotary Motor System

The possibility to generate and measure rotation and torque at the nanoscale is of fundamental interest to the study and application of biological and artificial nanomotors and may provide new routes towards single cell analysis, studies of non-equilibrium thermodynamics, and mechanical actuation of nanoscale systems. A facile way to drive rotation is to use focused circularly polarized laser light in optical tweezers. Using this approach, metallic nanoparticles can be operated as highly efficient scattering-driven rotary motors spinning at unprecedented rotation frequencies in water.
In this protocol, we outline the construction and operation of circularly-polarized optical tweezers for nanoparticle rotation and describe the instrumentation needed for recording the Brownian dynamics and Rayleigh scattering of the trapped particle. The rotational motion and the scattering spectra provides independent information on the properties of the nanoparticle and its immediate environment. The experimental platform has proven useful as a nanoscopic gauge of viscosity and local temperature, for tracking morphological changes of nanorods and molecular coatings, and as a transducer and probe of photothermal and thermodynamic processes.

Plasmonic gold nanorods can be trapped in liquids and rotated at kHz frequencies using circularly-polarized optical tweezers. Introducing tools for Brownian dynamics analysis and light scatteringspectroscopy leads to a powerful system for research and application in numerous fields of science.

The possibility to generate and measure rotation and torque at the nanoscale is of fundamental interest to the study and application of biological and ...

Light-driven Molecular Motors on Surfaces for Single Molecular Imaging

The design and synthesis of a synthetic system that aims for the direct visualization of a synthetic rotary motor at the single molecule level on surfaces are demonstrated. This work requires careful design, considerable synthetic effort, and proper analysis. The rotary motion of the molecular motor in solution is shown by 1H NMR and UV-vis absorption spectroscopy techniques. In addition, the method to graft the motor onto an amine-coated quartz is described. This method helps to gain more insight into molecular machines.

The manuscript describes how to synthesize and graft a molecular motor on surfaces for single molecular imaging.

The design and synthesis of a synthetic system that aims for the direct visualization of a synthetic rotary motor at the single molecule level on surfaces ...

Infrared Degenerate Four-wave Mixing with Upconversion Detection for Quantitative Gas Sensing

We present a protocol for performing gas spectroscopy using infrared degenerated four-wave mixing (IR-DFWM), for the quantitative detection of gas species in the ppm-to-single-percent range. The main purpose of the method is the spatially resolved detection of low-concentration species, which have no transitions in the visible or near-IR spectral range that could be used for detection. IR-DFWM is a nonintrusive method, which is a great advantage in combustion research, as inserting a probe into a flame can change it drastically. The IR-DFWM is combined with upconversion detection. This detection scheme uses sum-frequency generation to move the IR-DFWM signal from the mid-IR to the near-IR region, to take advantage of the superior noise characteristics of silicon-based detectors. This process also rejects most of the thermal background radiation. The focus of the protocol presented here is on the proper alignment of the IR-DFWM optics and on how to align an intracavity upconversion detection system.

Here, we present a protocol to perform sensitive, spatially resolved gas spectroscopy in the mid-infrared region, using degenerate four-wave mixing combined with upconversion detection.

We present a protocol for performing gas spectroscopy using infrared degenerated four-wave mixing (IR-DFWM), for the quantitative detection of gas species ...