DOPPLER-FREE 2-PHOTON ABSORPTION-SPECTROSCOPY OF THE A(1)A(U)[-X(1)A(G) TRANSITION OF TRANS-GLYOXAL

Doppler-free two-photon absorption spectra of the A(1)A(u)<--X(1)A(g)( pi-n) transition of trans-glyoxal have been measured by means of two- photon absorption spectroscopy with counter-propagating light beams of identical photons within an external cavity. The relative energies of transition lines are measured with accuracy better than 0.0001 cm(-1) . Rotational Lines are fully resolved, and 1809 Lines of the A(1)A(u)( v(7) = 1)<--X(1)A(g)(v = 0) transition are assigned for J= 0 - 75,K= 0 - 17. Rotational constants of the A(1)A(u)(v(7)=1) and X(1)A(g)(v = 0 ) states are determined by a least-squares fitting of eigenvalues of t he A-reduced rotational Hamiltonian to energies of the assigned lines. Energy shifts, intensity anomalies, and Line splittings are observed for several lines. When an external magnetic field is applied, remarka ble changes are observed for these Lines. They are identified as origi nating from perturbations between the A (1)A(u) and a(3)A(u) states, w hich become appreciable when perturbing levels are close in energy. Sp littings into three lines are observed for strongly perturbed levels o f I = 1, and these splittings are identified as the hyperfine splittin g caused by mixing of the a(3)A(u) state. (C) 1997 American Institute of Physics.