Ultrahigh-resolution spectroscopy of the A1Au←X1Ag transition of trans -glyoxal

Doppler-free two-photon absorption spectra of the A1Au(v7=1)←X1Ag(v=0) transition of trans-glyoxal and the effects of magnetic fields up to 14 kG are measured. The absolute energies of transition lines are measured with accuracy better than 0.000 03 cm−1, and T0 is determined to be 22 206.749 43 cm−1. From the analysis of the perturbed A1Au and perturbing a3Au levels, the hyperfine constant of the deperturbed a3Au level is determined to be 88 MHz. The magnetic moment of the a3Au level is determined to be 0.86μB from the magnitude of Zeeman splitting of a hyperfine component. From an analysis of the Zeeman splittings of unperturbed levels, which are transitions to the A1Au state and where no appreciable energy shifts are observed, the 13Bu state is shown to be slightly mixed into the 11Au state: |A1Au〉≃|11Au〉+0.02|13Bu〉. The perturbation between the A1Au(v7=1(au)) and a3Au(v′(au)) levels is shown to occur through the vibronic interaction between the mixed 13Bu(v″(au)) and the a3Au(v′(au)) states. The perturbation is appreciable if levels of the A1Au and a3Au states are accidentally close in energy. The energy spacing between levels of the A1Au and a3Au states changes with the magnetic field, and the resulting changes of the perturbation are observed. The mechanism of the intersystem crossing of a chosen single level A1Au(v7=1(au),K,J) is clarified in this study.