Collision induced intersystem crossing. The photophysics of glyoxal vapor excited at 4358 angstrom

The yields. lifetimes and spectra of singlet (1)A(u) (S-1) and triplet (3)A(u) (T-1) emissions from glyoxal vapor (0.003 to 10 torr) have been measured after initially pumping levels about 1000 cm(-1) above the S-1 zero-point level with the 4359 angstrom Hg line and with flash excitation centered at 4345 angstrom. Only S-1 emission is observed at the lowest pressures. The singlet fluorescence contains appreciable structure from the zero-point level even when the hard sphere collision interval exceeds the radiative lifetime calculated from the absorption coefficient. Implications of long lifetimes (due to S-1 - T-1 vibronic interactions) axe not confirmed by pulsed excitation studies. Both S-1 and T-1 emissions are observed at pressures above about 0.1 fort and both are self-quenched. However, added gases such as cyclohexane, argon. and helium selectively quench only S-1 emission. This quenching is collision-induced S-1-T-1 intersystem crossing with cross sections of order 0.1 hard sphere for transitions from the S-1 zero-point level. ne triplet yield in 0.2 torr of pure glyoxal is probably near unity, and the subsequent crossing T-1 --> S-o, if it occurs, lies in the statistical limit. Indications of fast nonradiative decay from high triplet vibrational levels are seen in the phosphorescence yields. Self-quenching of the triplet state appears to be associated with the photochemical activity of glyoxal.