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 000 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 higher than that of the LIF spectrum recorded with the same machine. The depletion nature of the signals unequivocally establishes that both molecules dissociate before reaching the detector (i.e., within ∼0.5 ms) following excitation to the S1 state. The s-tetrazine spectrum is fit to an asymmetric rotor Hamiltonian that includes the observed interchange of the a and b inertial axis in the excited state. The 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 for the m=0 free-rotor states only. Its rotational lines require a Lorentzian component of 34(2) MHz, corresponding to an excited state lifetime of 4.7 ns. The relative efficiency of the dissociative and radiative decay channels is evaluated. Despite the fact that we believe we have sufficient sensitivity, we failed to observe the s-tetrazine T1 state at 735 nm, which may be evidence of its nondissociative character.