Knowledge Bank

Transient $S2O$ molecules were entrained in a pulsed slit-jet expansion and interrogated through use of sub-Doppler Degenerate Four-Wave Mixing (DFWM) spectroscopy. High-resolution scans have been acquired for the $2ov(\nu =3-10)$ vibronic bands of the intense $C~\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A\prime \leftarrow X~\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}A\prime (\pi \ast \leftarrow \pi )$ absorption system, where increasing excitation of the $v2S-S$ stretching mode is known to promote predissociation of the $C~$ state. For bands involving moderately-predissociated states (e.g., $204$ where $\tau \ast \ast \ast 63ps)$, 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., $203$ where $\tau \ast \ast \ast \simeq 22ns)$. 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 $20v$ progression (i.e., $v\ge 5$), 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.