Electronic state, infrared energy

Far-infrared and mid-infrared spectroscopy usually provide the most detailed picture of the vibration-rotation energy levels in the ground electronic state. However, they are not always possible and other spectroscopic methods are also important. 

Transitions between different electronic states result in absorption of energy in the ultraviolet, visible and, for many transition metal complexes, the near infrared region of the electromagnetic spectrum. Spectroscopic methods that probe these electronic transitions can, in favourable conditions, provide detailed information on the electronic and magnetic properties of both the metal ion and its ligands. 

The observations of complex dynamics associated with electron-stimulated desorption or desorption driven by resonant excitation to repulsive electronic states are not unexpected. Their similarity to the dynamics observed in the visible and near-infrared LID illustrate the need for a closer investigation of the physical relaxation mechanisms of low energy electron/hole pairs in metals. When the time frame for reaction has been compressed to that of the 10 s laser pulse, many thermal processes will not effectively compete with the effects of transient low energy electrons or nonthermal phonons. It is these relaxation channels which might both play an important role in the physical or chemical processes driven by laser irradiation of surfaces, and provide dramatic insight into subtle details of molecule-surface dynamics. 

Infrared spectroscopical data encode a lot of structural information and can be analyzed with the help of computational methods (vide supra) aiding in the identification of the observed species. Sometimes, two different electronic states may lie very close in energy and have similar geometries. In such cases (e.g., the quinonoid radicals to be described in Section II.B.), the predicted differences in the IR spectra are too small to allow an unambiguous assignment of the ground-state multiplicity. In this respect, ESR spectroscopy provides valuable comple-... 

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