Vibrational predissociation and intramolecular vibrational relaxation in electronically excited s -tetrazine–argon van der Waals complex

The van der Waals complex tetrazine–argon formed in a supersonic expansion has been investigated by means of laser-induced excitation and fluorescence spectroscopy. Excitation spectra reveal the band shift D′′0−D′0=−23 cm−1, the tetrazine–argon stretch frequency νw=44 cm−1, and a lower limit to the dissociation energy D′′0≳254 cm−1. The band shift is vibrationally dependent in the case of two low frequency out-of-plane modes of tetrazine. The absence of additional broadening of the excitation linewidths due to the presence of argon shows that the dynamical processes in the complex are slower than the photodissociation of tetrazine itself (0.5–1.4 ns) which serves as an internal timing mechanism. At five single vibronic levels of tetrazine–argon above the S1 zero-point level, the rates of vibrational predissociation and intramolecular vibrational relaxation are ∼108 s−1. The V→V propensities in dissociation and relaxation are selective and different from collision-induced propensities. Among the V→V channels in dissociation, we find the lowest upper limit to D′0 is 381 cm−1. Considerable rotational excitation of the tetrazine fragment is observed following dissociation which may reduce the estimate of D′0 to as low as 306 cm−1. Although redistribution of vibrational energy during dissociation is selective, no mode selective effects could be attributed to the initial level of excitation, despite variations in the energy gap, number of quanta, total energy, or the dynamics of the vibrational motion. The van der Waals potential is virtually identical in the two electronic states.