LASER-INDUCED GRATINGS IN FREE JETS .1. SPECTROSCOPY OF PREDISSOCIATING NO2

We use two-color laser-induced grating spectroscopy (LIGS) to obtain double-resonance, absorptionlike spectra of jet-cooled NO2 below and above the threshold for predissociation at 398 nm. The grating-laser frequency is tuned through the dissociation threshold while the probe-laser frequency is fixed to a specific rotational line of an isolated, cold vibronic band well below threshold. The signal beam diffracts off the ground state depletion grating formed when the grating laser excites a transition out of the rotational level selected by the probe laser. The above-threshold spectra observed for NO2 indicate that, under collision-free conditions in the free jet, the LIGS spectrum maps the square of the absorption spectrum. We determine predissociation lifetimes from LIGS linewidths and discuss the temporal decay of the induced grating as a function of its orientation with respect to the jet axis. Finally, we consider the effects of phase matching on two-color LIGS in the forward geometry and derive expressions for the variation in signal-beam angle and for the phase matching bandwidths for experiments in which the frequency of either the grating or probe laser is scanned.