Sub-Doppler nonlinear spectroscopy as a probe of photodissociation dynamics

Abstract

The nonlinear optical technique of Jegenerate Four-Wave Mixing (DFWM) is shown to be an effective tool for the interrogation of nascent product molecules generated during a photodissociation event. By combining an absorption-based, beam-like response with the sensitivity afforded by full resonant enhancement, this scheme provides an attractive alternative to the ubiquitous laser-induced fluorescence (LIF) methodology. DFWM spectroscopy has been used to probe the unrelaxed hydroxyl (OH) radicals formed upon 266 nm photolysis of hydrogen peroxide (HOOH), with a sub-Doppler experimental configuration enabling extraction of both scalar and vector properties. In particular, the rovibrational population distribution of the ground electronic state fragments, as well as the spatial alignment of their rotational angular momenta, have been measured. These results are compared with those obtained in previous LIE studies, thereby demonstrating the utility of four-wave mixing techniques for the quantum state-specific characterization of nascent reaction products.

Photodissociation Dynamics

“…110 Here, H 2 O 2 is photodissociated within a gas cell, and the OH product is detected through a degenerate four-wave mixing scheme. The technique is inherently subDoppler and allows one to extract both product state distributions and alignment information.…”

“…Resonant Four-Wave Mixing Processes . Recent experiments have demonstrated that resonant four-wave mixing provides an elegant means to study the spectroscopy , and photodissociation dynamics , of molecules. In a study of NO 2 photodissociation in a free jet, two laser beams at the same frequency cross at a small angle θ inside the jet.…”

“…A somewhat different approach has been used to study H 2 O 2 photodissociation . Here, H 2 O 2 is photodissociated within agas cell, and the OH product is detected through a degenerate four-wave mixing scheme.…”

“…Experiments on CH 3 O yielded the vibrational distribution in the umbrella (ν 2 ) mode of the CH 3 fragment at several excitation energies . Results have also been obtained for the N 3 , NCO, CH 2 NO 2 , and CH 2 CHO (vinoxy) radicals. These experiments are, in many cases, the first studies of the photodissociation dynamics of these radicals and represent an expansion of this field to include more electronically complex species than are typically investigated.…”

Photo‐fragment excitation spectroscopy (PHOFEX) by DFWM and LIF: propensities for H2CO → HCO + H near the So threshold

“…8,9 Also, the potential of the technique for the investigation of reaction dynamics has been demonstrated. 10,11 The collimated signal beam emerging from the process allows efficient rejection of stray light and yields a backgroundfree spectrum. Furthermore, the resonant character of the method offers a considerable spectral selectivity.…”