C i s-glyoxal in effusive and supersonic beams. Spectroscopy, selective pumping, and c i s–t r a n s interconversion

Abstract

S 1–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:trans contrast (cool expansions with Ar carrier gas), four new S1–S0 cis absorption bands (510,520,610, and 720 ) are identified, yielding cis frequencies ν′5 =303 cm−1, ν6 =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: ν4 =826 cm−1 and ν′′6 =1049 cm−1. Previous assignments of ν4 and ν′′8 are shown to be incorrect and ν8 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 301 and 301510 transitions. With this addition, all trans S0 fundamentals have now been directly measured.