Diffraction mechanisms in gas-phase laser induced grating spectroscopy of vibrational overtone transitions

We have investigated the grating formation mechanism in laser-induced grating spectroscopy by preparing transient gratings via excitation of O–H vibrational overtones in water vapor. In principle, our experiments are sensitive to three different mechanisms of diffraction: a density-based phase grating originating from local thermalization of the vibrational overtone state, a population-based phase grating caused by differences in ground and excited state polarizability volumes, and a population-based amplitude grating that appears when the probe laser is coincident with an electronic transition of the overtone-excited molecules. For water saturated air at 50–760 Torr, bulk acoustic responses originating from both collisional thermalization and from electrostriction dominate the diffraction efficiency. Measurement of the probe-wavelength dependence of the diffraction efficiency at pressures of 18 Torr and below shows that the dominant signal originates from a phase grating and that contributions from the amplitude grating are relatively unimportant at all pressures. Analysis of the temporal evolution of the diffraction efficiency suggests that the thermalization mechanism dominates at 18 Torr, despite there being only one hard-sphere collision on the time scale of the measurement. At 10 Torr, the temporal evolution of the diffraction signal begins to show evidence of a population based phase grating contribution as well. These conclusions are consistent with our measured thermalization rate constants for the vibrational overtone states and also with calculations of the effects of vibrational excitation on the molecular polarizability volume.