Knowledge Bank

High pressure laser-induced grating spectroscopy experiments universally show a background that is independent of the wavelength of the excitation beams. In experiments on water vapor at atmospheric total pressures, we excite a vibrational overtone transiton in water, detecting absorption by monitoring diffraction of a probe beam via a thermal phase grating, and observe the time-dependence of the signal, both on and off resonance. The diffracted signal due to absorptive heating shows recurrences at a characteristic frequency determined entirely by experimental conditions, whereas the time-dependence of the signal when the laser is tuned off-resonance exhibits recurrences at twice the frequency of those due to absorption. Analagous condensedphase experiments have shown this off-resonance signal to be due to $electrostriction.1$ Quantitative calculations of the electrostriction effect reproduce the time-dependence and magnitude of the diffracted signal observed in our experiments.