OPTOTHERMAL SPECTROSCOPY OF THE DISSOCIATING LOWEST ELECTRONIC SINGLET-STATES OF S-TETRAZINE AND DIMETHYL-S-TETRAZINE IN A MOLECULAR-BEAM

We report the spectra of the 0(0)(0) bands of s-tetrazine and dimethyl -s-tetrazine in a seeded molecular beam, using optothermal detection. The S/N of the optothermal s-tetrazine spectrum is about 1000 times hi gher than that of the LIF spectrum recorded with the same machine, The depletion nature of the signals unequivocally establishes that both m olecules dissociate before reaching the detector (i.e., within similar to 0.5 ms) following excitation to the S-1 state. The s-tetrazine spe ctrum is fit to an asymmetric rotor Hamiltonian that includes the obse rved interchange of the a and b inertial axis in the excited state. Th e rotational constants and the homogeneous line broadening of 215(10) MHz (FWHM) observed here are in good agreement with, but more accurate than, those obtained in earlier sub-Doppler (saturation) gas-cell and free-jet spectra. The spectrum of dimethyl-s-tetrazine is analyzed fo r the m=0 free-rotor states only. Its rotational lines require a Loren tzian component of 34(2) MHz, corresponding to an excited state lifeti me of 4.7 ns. The relative efficiency of the dissociative and radiativ e decay channels is evaluated. Despite the fact that we believe we hav e sufficient sensitivity, we failed to observe the s-tetrazine T-1 sta te at 735 nm, which may be evidence of its nondissociative character. (C) 1997 American Institute of Physics.