Infrared spectroscopy vibrational line shapes

Vibrational spectroscopy can help us escape from this predicament due to the exquisite sensitivity of vibrational frequencies, particularly of the OH stretch, to local molecular environments. Thus, very roughly, one can think of the infrared or Raman spectrum of liquid water as reflecting the distribution of vibrational frequencies sampled by the ensemble of molecules, which reflects the distribution of local molecular environments. This picture is oversimplified, in part as a result of the phenomenon of motional narrowing The vibrational frequencies fluctuate in time (as local molecular environments rearrange), which causes the line shape to be narrower than the distribution of frequencies [3]. Thus in principle, in addition to information about liquid structure, one can obtain information about molecular dynamics from vibrational line shapes. In practice, however, it is often hard to extract this information. Recent and important advances in ultrafast vibrational spectroscopy provide much more useful methods for probing dynamic frequency fluctuations, a process often referred to as spectral diffusion. Ultrafast vibrational spectroscopy of water has also been used to probe molecular rotation and vibrational energy relaxation. The latter process, while fundamental and important, will not be discussed in this chapter, but instead will be covered in a separate review [4],... 

Nonlinear infrared spectroscopy can in principle provide knowledge of all the relaxation processes of oscillators, including those that do not manifest themselves in the linear spectral line shapes. The v = 0 v = I transition line shape is determined by the overall rotation of the molecule, population relaxation time T and by the vibrational frequency correlation function. The experimental line-shape is not a very useful determinant of this correlation function [40, 49] because it provides experimental data only along one axis, either frequency or time, and the line-shape function is usually too complex to... 

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