Pronounced non-Condon effects in the ultrafast infrared spectroscopy of water.

In the context of vibrational spectroscopy in liquids, non-Condon effects refer to the dependence of the vibrational transition dipole moment of a particular molecule on the rotational and translational coordinates of all the molecules in the liquid. For strongly hydrogen-bonded systems, such as liquid water, non-Condon effects are large. That is, the bond dipole derivative of an OH stretch depends strongly on its hydrogen-bonding environment. Previous calculations of nonlinear vibrational spectroscopy in liquids have not included these non-Condon effects. We find that for water, inclusion of these effects is important for an accurate calculation of, for example, homodyned and heterodyned three-pulse echoes. Such echo experiments have been "inverted" to obtain the OH stretch frequency time-correlation function, but by necessity the Condon and other approximations are made in this inversion procedure. Our conclusion is that for water, primarily because of strong non-Condon effects, this inversion may not lead to the correct frequency time-correlation function. Nevertheless, one can still make comparison between theory and experiment by calculating the experimental echo observables themselves.