ENERGY AND ROTATIONAL STATE DEPENDENCE OF RADIATIONLESS PROCESSES IN PROPYNAL

The dynamics of propynal, HCCCHO and HCCCDO, have been investigated in an excess energy range between 2900 and 4500 cm-1 of the S1 state using quantum beat spectroscopy. A rotational state dependence of S1-T1 interaction has been established. The number of interacting triplet states, determined by quantum beat counting, is shown to increase with the rotational quantum number N, and with the average eigenstate lifetime. This effect is attributed to K state mixing in the triplet state which is proposed to be induced by hyperfine interaction via the dipole-dipole term. Furthermore, the nonradiative decay rates exhibit a weak excess energy dependence up to a sudden fluorescence breakdown at almost-equal-to 4000 cm-1 where they increase dramatically. The breakdown, located at 86.6 +/- 0.3 kcal/mol, is attributed to the onset of the predissociation due to the radical dissociation HCCCHO --> HCCCO+H which proceeds on the S0 potential energy surface in competition with the concerted reaction HCCCHO --> HC = CH+CO already accessible at zero S1 vibrational excess energy.