ABSTRACT
As discussed in more detail elsewhere in this encyclopaedia, many optical spectroscopic methods have been developed over the last century for the characterization of bulk materials. In general, optical spectroscopies make use of the interaction of electromagnetic radiation with matter to extract molecular parameters from the substances being studied. The methods employed usually rely on the examination of the radiation absorbed, emitted or scattered by a system, and may be based on simple linear optical processes, resonance transitions and/or nonlinear processes. Molecular spectroscopy probes energy transitions at all levels, from the excitation of spins in the radiofrequency range (NMR), to rotational (microwave), vibrational (infrared) and electronic valence (visible-UV) and core (x-rays) excitations. Additional diffraction-and polarization-based techniques provide structural information and laser-based pump-probe methods allow for the study of molecular dynamics down to the femtosecond time scale.
