• four-wave mixing;
  • photo-dissociation;
  • anisotropy

The potential of a non-linear spectroscopic technique – degenerate four-wave mixing (DFWM) spectroscopy – for photo-dissociation studies is investigated. By applying a carefully considered control of the input beam polarizations, the technique is used to measure the line shapes of specific rovibrational transitions in the A2Σ+ − X2Π (0,0) band of nascent OH radicals upon pulsed photo-dissociation of H2O2. The photofragmentation is performed by linearly polarized radiation at 266 nm, and DFWM spectra are observed at various time delays. In contrast to linear spectroscopic methods, isotropic and anisotropic components of the non-linear susceptibility tensor are separately accessible by using appropriate polarization geometries. In particular, four-wave mixing signals can be generated that are exclusively due to the occurrence of the transient anisotropy of the recoil velocity and angular momentum vector distributions of the fragments emerging from the dissociation reaction. The observed line shapes in isotropic and anisotropic DFWM spectra are governed by the Doppler effect and vectorial correlations between the parent H2O2 molecule's transition dipole moment, the OH fragments’ recoil velocity and angular momentum, as well as by the processes of collision-induced rotational and translational relaxation. Copyright © 2013 John Wiley & Sons, Ltd.