Internal state distributions of fragment HCO via S0 and T1 pathways of glyoxal after photolysis in the ultraviolet region.

The dynamics of photodissociation of glyoxal (HOC-COH) near the dissociation threshold on the triplet manifold are studied through measurement of distributions of nascent fragment HCO in various internal states. Three rotational levels 1(01) (), 4(13) (), and 3(21) ()+3(22) () of vibrational state U (excitation wavelength approximately 394.4 nm, origin at 25,331.865 cm(-1)) of glyoxal in state A (1)A(u) and two other vibrational states at excitation wavelengths 390.33 and 382.65 nm are selected to produce fragment HCO. By means of fluorescence in the transition B (2)A(')-X (2)A(') of HCO, we determined the relative populations of internal states of that fragment. Rotational states of product HCO up to N=26 and K=2 are populated, and bimodal distributions of these rotational states are observed for the photolysis wavelengths used in this work. The high rotational part of the distribution with average energy near values calculated on the basis of the statistical model-phase-space theory is assigned to arise from glyoxal on its S(0) surface, and the low rotational part from the T(1) surface with an exit barrier. After photolysis near the threshold region on the triplet surface, HCO arising from the T(1) state appears to be a major component of products because these rotational levels 1(01) (), 4(13) (), and 3(2) (*) of U state selected are gateway states with an enhanced rate of intersystem crossing.