Single vibronic level fluorescence and relaxation spectra of s-tetrazine-d0 and d2 vapors

The third strongest cold band in the 5500Å absorption spectrum of s-tetrazine vapor is assigned to 6b₀² by analysis of single-vibronic-level (SVL) fluorescence spectra obtained with a tunable cw dye laser. Fermi resonance model calculations for the doublet 6a₀¹…6b₀² simulate the perturbations of relative intensities in ν_6a^″ fluorescence progressions which arise fron interference between the two terms of the Franck-Condon factors. Addition of a quartic anharmonic term to the quadratic ν_6b^′ potential accounts for the observed spacing of the observed 6b¹ and 6b² levels and suggests a likely candidate for the heretofore missing 6b₀⁴ member of the Fermi resonance triad. Weak bands with in-plane polarization are revealed in fluorescence and contribute to a list of newly measured frequencies of the ground state. Experiments with added n-pentane show that rotational relaxation proceeds at the hard-sphere rate while vibrational relaxation is about five times slower. Vibrational relaxation exhibits non-statistical distribution in the early stages; it is rather distinctive when starting from the zero-level of the excited state, but is reminiscent of earlier work (on benzene) when starting from the 16b¹ level. Identification of the level 6b¹ is supported by these relaxation experiments.