Study of singlet-triplet coupling in glyoxal by level anticrossing spectroscopy. VII. Complete assignment of the 00, 81, 6171 and 41 anticrossing spectra
Résumé
In paper VI of this series, we have made the statistical analysis of the singlet-triplet coupling matrix elements for ten NS = 0 singlet vibrational levels of glyoxal, without determining the triplet quantum numbers. In this paper we present the complete assignment (triplet rotational quantum numbers and vibrational symmetry) of each anticrossing observed from four singlet vibrational levels: 0 0, 8 1, 6 17 1 and 4 1 (which give respectively 36, 76, 155 and 145 anticrossings by 0 to 7.5 T scans). Therefore we determine the zero-field energy origin of most of the triplet vibrational levels which are located within 7 cm -1 of the four singlet levels studied. Two kinds of selection rules are found: the first stemming from direct vibronic spin-orbit interactions and the second from indirect ones (involving an intermediate triplet state). About half of the anticrossings are due to direct vibronic spin-orbit interactions, the mean value of their matrix elements is more than ten times larger than indirect matrix elements and thus are dominant in ϱ〈 Vst〉. In conclusion, we confirm that ISC in glyoxal is governed by direct vibronic spin-orbit interactions.