Radiationless decay in propynal: Dependence upon vibrational state

Abstract

New time-resolved measurements of luminescence decay rates and intensities for vibronically selected propynal molecules excited by a pulsed tunable dye laser are reported. The observations allow individual determination of the rates for ’’collision-free’’ and collision-induced intersystem crossing and internal conversion from the initially excited 1A″ state. Explicit ’’no parameter’’ calculations of the collision-free decay rates for the vibrationless level based on the Fermi Golden Rule for time-independent perturbations give 0.38×104 sec−1 for internal conversion and 0.59×105 sec−1 for intersystem crossing if the broadening mechanism for the initially prepared levels is primarily radiative. The observed decay rate for intersystem crossing from the vibrationless level is 0.52 (+0.5, −0.25) μsec−1, while that for internal conversion is 0.48 (+0.3, −0.48) μsec−1. Results are also presented for collisional quenching of selected vibronic levels of propynal (1A″) by acetonitrile. The enhancement of quenching rate with vibronic excitation would also appear to agree with theoretical expectations.