Degenerate four-wave mixing spectroscopy of the (glyoxal)2 van der Waals complex

Glyoxal dimer generated in a supersonic expansion can be detected through the relatively weak glyoxal A→X transition by using degenerate four-wave mixing spectroscopy. A detection limit on the order of 1011 molecules/cm3 is obtained. The spectrum can be simulated with a simple N20σn (n=2−4) intensity dependence, where N0 is the ground-state population and σ is the absorption cross section. Inclusion of nondegenerate grating scattering effects due to congestion of transitions within the laser bandwidth improves the spectral simulation. The rotational constants obtained from a rotational band contour analysis are reported for the dimer.