THE ROTATIONAL STATE DEPENDENCE OF THE DYNAMICS OF PYRIMIDINE IN THE S-1(B-1(1)) STATE STUDIED BY TIME-RESOLVED SPECTROSCOPY

The dynamics of pyrimidine in three vibronic levels of the S1 (1B1) state with excess energies of 1100-1300 cm -1 above the 0° level has been studied by time resolved spectroscopy. The fluorescence decays from 40 to 50 single rovibronic levels in each vibronic band were measured and were found to be biexponential due to coupling with background triplet states. Analysis of the decays yielded the amplitude ratio of the fast (coherent) and slow (incoherent) component of the fluorescence intensity, A + /A-, and the lifetime of the slow component, being τ ≈ 700-900 ns. Both data show a clear dependence on the S1 rotational quantum number J. The A + /A - ratio, which reflects the number of coupled states, takes the form of a linear dependence on (2J+ 1) implying full rovibronic state mixing in the background 3B1 state. More states were found to be coupled than could be accounted for by the calculated background state density, suggesting further state mixing, possible mechanisms for which are discussed. The lifetime results were found to converge to limiting values at high J, the lifetime of the 3B1 state at the given energy. A small but significant difference (∼10%) in the lifetimes of the asymmetry doublets with K-c= 1 was observed. Finally, a comparison of the rotational state dependent dynamics in pyrimidine with those in propynal and butynal is presented, pointing to a role for nuclear hyperfine interactions in dynamics. © 1995 American Institute of Physics.