Cis-glyoxal in effusive and supersonic beams. Spectroscopy, selective pumping, and cis-trans interconversion

S1-S0 fluorescence excitation and dispersed fluorescence spectra from molecular beams containing both trans- and cis-glyoxal have been used to extend the characterization of the 1A1 (S0) and 1B1 (S1) states of cis-glyoxal. Explorations using both effusive and supersonic beams with rotational temperatures ranging from 350 to 30 K have revealed no conditions where cis can be pumped (S1←S0) without simultaneous excitation of trans. Selective cis excitation at low beam temperatures is hampered by highly efficient cis→trans conformational interconversion in the molecular beam expansions. Under conditions of optimal cis contrast (cool expansions with Ar carrier gas), four new S1-S0 cis absorption bands (501,501,6 01, and 702) are identified, yielding cis frequencies ν′5=303 cm-1, ν′6=713 cm-1, and 2ν′7=688 cm-1. Single vibronic level fluorescence spectra have been obtained from the levels 00, 51, and 61 of cis-glyoxal, from which values of two cis S0 fundamentals are newly established: ν″4=826 cm-1 and ν″6=1049 cm-1. Previous assignments of ν″4 and ν″8 are shown to be incorrect and ν″8 now joins the list of unknown frequencies. The 1B1- 1A1 system of cis-glyoxal contains forbidden transitions, vibronically induced by Δv=±1 changes in the a2 mode ν6. A remeasurement of the cis-trans energy separation in the ground electronic state gives ΔH=1350±200 cm-1, matching to within experimental uncertainty a previous experimental determination. As an aside, the trans-glyoxal fundamental ν″3=1352 cm -1 has been obtained from observations of the trans 3 10 and 310501 transitions. With this addition, all trans S0 fundamentals have now been directly measured. © 1987 American Institute of Physics.