LASER-INDUCED GRATINGS IN FREE JETS .3. SATURATION-INDUCED ANHARMONIC GRATINGS

We apply two-color laser-induced grating spectroscopy (LIGS) to obtain excitation spectra of the S-1(A(1)B(1))-S-0(($) over tilde X(1)A(1)) system of jet-cooled azulene. The ground-state depletion grating created by excitation with crossed beams is probed via either the S-2(($) over tilde B(1)A(1))-S-0(($) over tilde X(1)A(1)) or S-4(($) over tilde D(1)A(1))-S-0(($) over tilde X(1)A(1)) transitions. First-order LIGS spectra of the S-1-S-0 origin band at several grating-laser intensities reveal saturation-induced line broadening and significant saturation dips at line center. We develop a simple model, based on the anharmonic grating created in the saturated regime, and use it to fit the first-order data. From these fits we determine a saturation parameter and the linewidth of the S-1-S-0 origin absorption band, which is homogeneously broadened hue to rapid internal conversion (tau similar to 1 ps) in the S-1 state. We also observe LIGS spectra of the azulene origin taken at the second Bragg order of the induced grating; the observed second-order lineshapes are also fit with the anharmonic-grating model.