Molecular Motion in Infrared and Raman Spectra

It is suggested that Fourier transformation of infrared and Raman band shapes reveals the meaning of the spectrum in terms of molecular rotation much more clearly than does the usual frequency shape. By looking at the time dependence directly, one may separately examine the molecular motion at short and long times The motion at short times may be analyzed directly in terms of the molecular dynamics, by the use of a power series in the time, whereas the behavior at long times is best examined by statistical arguments. This kind of analysis is illustrated by several examples, including the spectra of liquid CO and CH4.