Electronic Relaxation Processes from Single Vibronic Levels in the S1(n, π*) State of Pyridine Vapor

Abstract

Abstract The rate constants of radiative decay (kF), internal conversion (kIC), and intersystem crossing (kISC) for single vibronic levels in the S1(n, π*) state of pyridine-d0 and -d5 vapors have been determined from measured fluorescence quantum yields (ΦF), intersystem crossing quantum yields (ΦICS), and picosecond fluorescence lifetimes. The ΦISC values are fairly constant in the region of lower excess vibrational energy (<1000 cm−1), while the ΦF values are particularly large for vibronic levels involving non-totally symmetric vibrations, ν16b and ν10a. The latter observation is attributed to both an increase of kF and a decrease of kIC and kISC (relative to the zero-point level) for the ν16b levels (16b2 and l6b26a1), and to an increase of kF for the ν10a levels. The relative values of kF are discussed using the Herzberg-Teller theory of vibronic coupling. The values of kF for 16b1 and 16b2 are significantly larger than that for the zero-point level, though no absorption band corresponding to 16b01 can be observed. In both pyridine-d0 and -d5, the value of kIC is as large as 1.7×1010s−1 even for the zero-point level, which is more than twice the kISC value for the same level.