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

Doppler-free two-photon absorption spectra of the A(1)A(u)(upsilon(7)=1)<--X(1)A(g)(upsilon=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 T-0 is determined to be 22 206.749 43 cm(-1). From the analysis of the perturbed A(1)A(u), and perturbing a(3)A(u) levels, the hyperfine constant of the deperturbed a(3)A(u), level is determined to be 88 MHz. The magnetic moment of the a(3)A(u), level is determined to be 0.86 mu(B) 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 A(1)A(u) state and where no appreciable energy shifts are observed, the 1(3)B(u) state is shown to be slightly mixed into the 1(1)A(u) state: \A(1)A(u)] similar or equal to \1(1)A(u)] + 0.02\1(3)B(u)]. The perturbation between the A(1)A(u)(upsilon(7)= 1(a(u))) and a(3)A(u)(upsilon'(a(u))) levels is shown to occur through the vibronic interaction between the mixed 1(3)B(u)(nu "(a(u))) and the a(3)A(u)(upsilon'(a(u))) states. The perturbation is appreciable if levels of the A(1)A(u) and a(3)A(u) states are accidentally close in energy. The energy spacing between levels of the A(1)A(u) and a(3)A(u) 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 A(1)A(u)(upsilon(7)= 1(a(u)),K,J) is clarified in this study. (C) 1998 American Institute of Physics.