A study of the S 1 6 1 ( 1A″ 2) vibronically excited state of sym-triazine by high-resolution UV laser spectroscopy

Abstract

We have measured the laser induced fluorescence spectrum of the S 1 6 1 ( 1A″ 2)←S 0( 1A′ 1) vibronic transition in sym-triazine at 317 nm. An instrumental line width of 12 MHz was achieved by the use of a molecular beam in combination with a cw single frequency laser. In this way rotationally resolved spectra were obtained. Furthermore, we have measured the lifetime of some selected ensembles of rovibronic states with a narrow band pulsed laser system. It is shown that the excited S 1 6 1 (A″ 2) vibronic state is strongly perturbed by a singlet-triplet interaction. This interaction results in additional lines in the high-resolution spectrum. A nearly complete rotational assignment is given for the transitions to the upper state with rotational quantum number J up to J = 4 and the rotational constants are derived. No evidence is found for a Jahn-Teller distortion of the rotational states in the S 1 6 1 (A″ 2) vibronic state. With some assumptions the singlet-triplet coupling matrix elements were deduced as well as the energy separation between the singlet state and the perturbing triplet state. A singlet-triplet gap of 4800 ± 600 cm −1 is estimated.