Study of dark states in naphthalene, pyrimidine and pyrazine by detection of phosphorescence after UV laser excitation

Abstract

The high-resolution fluorescence and phosphorescence detected spectra after excitation of the (3a g) 1 and (4a g) 1 vibronic states of the S 1 ( 1B 3u) in naphthalene have been studied in a molecular beam. The residual Doppler linewidth of 20 MHz allowed rotational resolution. All lines were assigned and the rotational constants of the (3a g) 1 and (4a g) 1 states were determined. It has been shown that the intersystem crossing rates are independent of the rotational quantum numbers J and K up to J=10 in both excited vibronic states. The strong ISC rate of the (4a g) 1 state is confirmed. No evidence was found for a promoting second triplet T 2 state. Detection of phosphorescence after excitation of the vibrationless S 1 ( 1B 3u) state in pyrazine and the S 1 ( 1B 1) s pyrimidine yielded no signal. This observation confirms the assumption that after excitation of the S 1 state the energy is transferred to high vibrational states of the S 0 state.