12.3 : 分子光谱：吸收和发射

In a molecule, discrete energy states called quantum states exist. Each quantum state—be it electronic, vibrational, or rotational—is unique, with a definite energy value, and separated from others of its type by energy gaps.

On absorbing a photon of electromagnetic radiation, a molecule can be excited to a higher energy level. As a result, it undergoes relaxation to a lower energy level by emitting a photon.

The molecule can only absorb or emit photons of specific energies that match the energy gaps between these energy levels. So, each transition is frequency- or wavelength-dependent.

Accordingly, different kinds of excitation occur in a molecule, depending on the radiation wavelength.

For example, photons in the microwave region are absorbed to alter internal bond rotation, while those in the infrared region possess the energy required to change the frequency of bond vibrations.

Photons in the UV–visible range can excite electrons to higher electronic energy states.