VIBRATIONAL AND ROTATIONAL EFFECTS ON THE INTERSYSTEM CROSSING IN PYRAZINE AND PYRIMIDINE

We have measured the laser induced fluorescence spectra of the 6a0(1) and the 6b0(2) vibronic bands of the S1 <-- S0 electronic transition in pyrimidine as well as the 6a(0)1 vibronic band of the S1 <-- S0 electronic transition in pyrazine. The instrumental line width of 12 MHz allowed the recording of rotationally resolved spectra. Each rotational transition consists of a group of lines due to the coupling of the excited singlet state to background triplet states. The rotational constants of the S1 6a1 and the S1 6b2 states in pyrimidine were determined. Using the standard deconvolution method the singlet-triplet coupling constants and the density of coupled states are derived for several excited rotational states in both bands. The result shows that the intersystem crossing in pyrimidine is independent of the rotational quantum numbers up to J' = 4. Comparison with the 0(0)0 vibronic band shows an absence of a strong vibrational effect on the intersystem crossing. We have also applied the deconvolution method on the P(1)-branch of the 6a(0)1 vibronic band of pyrazine. The density of background triplet states is increased with a factor of 2.4, in good agreement with the calculated increase in the density of states. Except for a minor diminution of the average coupling matrix element no vibrational effect on the intersystem crossing was found.