15min:

THOMAS MÜLLER, PATRICK DUPRÉ, QINGGUO ZHANG AND PATRICK H. VACCARO, Department of Chemistry, Yale University, 225 Prospect Street, New Haven, CT 06520.

Transient S₂O molecules were entrained in a pulsed slit-jet expansion (T_rot < 10 K) and interrogated through use of sub-Doppler \underlineDegenerate \underlineFour-\underlineWave \underlineMixing (DFWM) spectroscopy. High-resolution scans have been acquired for the 2^v_o ( v =3-10) vibronic bands of the intense C ¹\!\textrmA' <- X ¹\!\textrmA' (^*\!<- \!) absorption system, where increasing excitation of the ₂ S-S stretching mode is known to promote predissociation of the C state.

For bands involving moderately-predissociated states ( e.g. , 2⁴₀ where _c~eq63\,ps), the recorded pattern of rovibronic line intensities exhibits a pronounced dependence upon DFWM polarization geometry, a situation not encountered in analogous studies performed for features terminating on long-lived levels of the C manifold ( e.g. , 2³₀ where _c~eq22\,ns). This behavior can be reproduced quantitatively through detailed weak-field analyses of the resonant DFWM response, however, a qualitative understanding follows from the selective dissipation of optically-induced transient gratings as incurred by unimolecular relaxation pathways. In strongly-predissociated members of the 2^v₀ progression ( \textrmi.e., v \geq 5 ), additional polarization specificity is introduced by the presence of strong depopulation pumping processes which lead to the creation of net orientation and/or alignment of the molecular ensemble on a timescale commensurate with that of the pulsed four-wave mixing experiment

Owing to its absorption-based response and laser-limited spectral resolution, DFWM is often applied to target species where rapid nonraditive relaxation channels preclude successful exploitation of detection techniques based upon secondary matter-field interactions (e.g., fluorescence or ionization). Therefore, a detailed understanding of the role which molecular lifetime plays as a mediator for resonant nonlinear response is of central importance for the quantitative application of this optical scheme.