Fluorescence of transition (A) over tilde (1)A(u)-(X) over tilde (1)A(g) of trans-glyoxal in a supersonic jet is recorded with laser excitation in the wavelength region 393-417 nm. Full rotational analysis is performed for nine bands in a region 395-410 nm. Fluorescence decays with quantum beats from several rovibrational levels in (A) over tilde (1)A(u) of glyoxal are detected. As lines in Fourier-transform spectra of those decay curves split on application of an external magnetic field, this oscillatory behavior results from coherent excitation of mixed S-1 and T-1 states. On excitation with high energy, those lines become broadened as the lifetime of the T-1 state decreases because of coupling to a dissociation continuum. An abrupt increase of linewidth in transformed spectra at 25 350 cm(-1) (394.5 nm) indicates that this position corresponds to a threshold for dissociation. For excitation energy greater than 25 450 cm(-1), most observed decay curves of fluorescence emission display biexponential behavior. The decay of the fast component of the biexponential curve corresponds to intersystem crossing and the slow component to decay of those eigenstates. For energies exceeding 25 550 cm(-1), the slow component diminishes, to yield nearly single exponential decay, indicating that the rate of dissociation is greater than that of intersystem crossing. Results of these quantum-beat experiments imply a new dissociation pathway from the triplet surface.