Laser-induced gratings in free jets. III. Saturation-induced anharmonic gratings

Abstract

We apply two-color laser-induced grating spectroscopy (LIGS) to obtain excitation spectra of the S1(Ã 1B1)–S0(X̃ 1A1) system of jet-cooled azulene. The ground-state depletion grating created by excitation with crossed beams is probed via either the S2(B̃ 1A1)–S0(X̃ 1A1) or S4(D̃ 1A1)–S0(X̃ 1A1) transitions. First-order LIGS spectra of the S1–S0 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 S1–S0 origin absorption band, which is homogeneously broadened due to rapid internal conversion (τ∼1 ps) in the S1 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.