Variation of the transition dipole moment across the OH stretching band of water

Abstract

Hydrogen bonding increases the magnitude of the OH stretching transition dipole moment. We investigate the significance of this non-Condon effect on the linear and nonlinear infrared spectroscopy of the OH stretching vibration of HOD in D 2O. The frequency dependence of the transition dipole across the line shape is tested by comparing temperature-dependent amplitudes of IR and Raman spectra and IR photon echoes. Since the Raman polarizability is largely independent of hydrogen bonding, differences between IR and Raman spectra reflect the changes in the IR transition dipole moment induced by hydrogen bonding. Increasing temperature shifts the OH density of states to higher frequencies consistent with a weakening of hydrogen bond strength and therefore a decrease in the transition dipole. Photon echo intensity is found to decrease significantly with temperature. Using an empirical transition dipole variation determined from the square root of the ratio of the IR and Raman intensity, we are able to model the IR spectra and the IR photon echo across a broad temperature range. The variation with frequency of this empirical transition dipole is nonlinear, changes by a factor of 1.7 across the OH line shape, and on resonance has a slope similar to recent theoretical predictions. The role of water dynamics and motional narrowing on the non-Condon effect is evaluated with the help of molecular dynamics simulations of the fluctuating OH frequency and transition dipole moment. Simulated linear and two-dimensional infrared line shapes that use the empirical model are red shifted compared to line shapes calculated within the Condon approximation, and give slightly better agreement with measured data.