Vibrational energy redistribution in glyoxal following internal conversion

The vibrational redistribution of energy following internal conversion in glyoxal has been studied in a molecular beam using the "pump-and-probe" technique. A beam of glyoxal is initially pumped from So to a selected vibronic level of Sl. After internal conversion occurs, a tunable dye laser is used to probe the hot band absorption of the molecules by re-excitation into Sl. The total fluorescence as a function of probe laser wavelength is observed. The excitation spectrum is broad and featureless in the red but shows sharp structure in the vicinity of the pump laser wavelength. However, the sharp structure depends on the portion of the rotational contour excited. The experimental results show that the redistribution of energy is nonstatistical among the isoenergetic levels. This is explained by assuming that combination bands of low quanta of vibrations are the accepting levels in the internal conversion process and that these accepting levels undergo slow energy redistribution.