Non-exponential decays of the S1 vibronic levels of acetaldehyde Original Research Article

The oscillatory decay of fluorescence from numerous vibronic levels in the S1 (n-π∗) 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 S1 and T1 states. From Fourier-transform spectra of quantum-beat data, the coupling matrix element vs of the S1 and T1 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 S1 states and the coupled T1 states of acetaldehyde. Above the excitation wavenumber ≈ 31500 cm−1, the lines in the Fourier-transform spectra become broadened because the lifetime of T1 state is decreased from coupling to a dissociation continuum. For the same reason, the density of T1 states increases; consequently most observed decay curves of fluorescence emission display biexponential behavior for excitation wavelength shorter than ∼ 320 nm. The decay of the fast component in the biexponential curve is explained to be from the intersystem crossing S1 → T1 and the slow component results mainly from decay of the coupled triplet 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.