THE ISOMERIC STRUCTURES AND VAN DER WAALS VIBRATIONAL FREQUENCIES OF (GLYOXAL)$_{n} \cdot (Ar)_{m}$ COMPLEXES

Author Institution: Department of Chemistry, University of PennsylvaniaUsing laser induced fluorescence (LIF) and stimulated emission pumping (SEP), we have characterized the (glyoxal)$_{n} \cdot (Ar)_{m}$ complexes generated in a supersonic expansion of He (100 psi)/Ar (0.1--10\%)/glyoxal(70--700 mTorr). More than 20 spectral features near the band origin of the glyoxal $\bar{A} \leftarrow \bar{X}$ transition were observed in the LIF spectrum. From 1) the band type, 2) the rotational constants obtained from the band contour analysis, 3) the intensity dependence on Ar and glyoxal concentrations and 4) the fluorescence lifetime, these features are assigned to the van der Waals vibrational series of 5 complexes. These are 1) a glyoxal $\cdot$ Ar complex with the Ar on the $C_{2}$ axis perpendicular to the glyoxal plane, 2) a glyoxal$\cdot$Ar complex with Ar in the HCO gulf of the glyoxal plane, 3) a glyoxal$\cdot Ar_2$ complex with both Ar's on the same side of the molecular plane, 4) a staggered $(glyoxal)_2$ complex, and 5) a (glyoxal)$_{2}\cdot$Ar complex. The Ar position in one of the glyoxal$\cdot$Ar complex shifts from the $C_{2}$ axis position in the $\bar{X}$ state to a position tilted away from the axis in the $\bar{A}$ state. The van der Waals vibrational frequencies of some of the complexes were obtained for the $\bar{A}$ state from LIF and the $\bar{X}$ state from SEP