Isomeric structures and van der Waals vibrational frequencies of the glyoxal⋅Ar complexes. I. Fluorescence excitation spectroscopy

Abstract

From the band shape and the concentration dependence of the peak intensity in the fluorescence excitation spectrum observed in a supersonic expansion of a He/Ar/glyoxal mixture near the 000Ã←X̃ band of glyoxal, it was found that the spectral features can be assigned to the following van der Waals complexes; (glyoxal)2⋅Ar, glyoxal⋅Ar2, (glyoxal)2, and three different isomers of glyoxal⋅Ar. The approximate structures of the three glyoxal⋅Ar isomers have been determined from rotational band contour analyses. In the electronic ground state, the Ar atom is either on the C2 axis 3.6 Å above the glyoxal plane (the top isomer), in the HCCO gulf of the glyoxal plane 4.3 Å away from the glyoxal center of mass (the side isomer), or in the HCO gulf of the glyoxal plane 4.8 Å away from the glyoxal center of mass (the front isomer). As the glyoxal molecule is electronically excited to the à 1A1 state, the Ar atom moves away from oxygen by more that 1 Å for all three isomers. Excited van der Waals vibrational levels associated with the 0̄0, à state of glyoxal were observed and assigned for all three glyoxal⋅Ar isomers.