Non-exponential decays of the S-1 vibronic levels of acetaldehyde

The oscillatory decay of fluorescence from numerous vibronic levels in the S-1 (n-pi*) state of acetaldehyde are investigated in a supersonic jet. The quantum-beat pattern, showing no specificity of vibrational mode, is explained to result from coherent excitation of the mixed S-1 and T-1 states, From Fourier-transform spectra of quantum-beat data, the coupling matrix element v(sl) of the S-1 and T-1 states is estimated to be 0.5-20 MHz. Limited by the multimode structure of the laser, these data set the lower bounds for the coupling matrix elements of the rovibronic states of S-1 states and the coupled T-1 states of acetaldehyde. Above the excitation wavenumber approximate to 31500 cm(-1), the lines in the Fourier-transform spectra become broadened because the lifetime of T-1 state is decreased from coupling to a dissociation continuum. For the same reason, the density of T-1 states increases; consequently mast observed decay curves of fluorescence emission display biexponential behavior for excitation wavelength shorter than similar to 320 nm, The decay of the fast component in the biexponential curve is explained to be from the intersystem crossing S-1-->T-1 and the slow component results mainly from decay of the coupled tripler states, The rate coefficients fitted to the slow components of the measured decay curves agree with the appearance rate of HCO measured previously at the same excitation wavelength. According to the combined experimental results, the data indicate that product HCO is dissociated from the triplet channel of acetaldehyde excited in the UV region. (C) 1997 Elsevier Science B.V.