Doppler-free saturation spectroscopy of polyatomic molecules: Photochemical hole burning of gas phase s-tetrazine

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Abstract
Doppler‐free spectra of two vibronic bands in the à (1B3u)←X̃ (1Ag) transition of the photochemically instable s‐tetrazine (H2C2N4) are presented. For the first time saturation spectroscopy is successfully applied to a large polyatomic molecule. Photochemical decomposition of s‐tetrazine molecules takes place after excitation and prevents the molecule from returning to the ground state by radiation and nonradiative processes. This represents a particular type of hole burning in the ground state velocity distribution. The elimination of the inhomogeneous Doppler broadening enables us to determine collisionless homogeneous linewidths. For the 0–0 band a value of γ=190 MHz is found whereas the 16a11 band at higher excess energy (ν’16a =256 cm−1) shows a sharper linewidth of γ=140 MHz. No rotational dependence of the linewidth is found in the 0–0 band up to J levels higher than 70. It is argued that internal conversion to the X̃ (1Ag) state is the process responsible for the observed linewidths and that dissociation takes place on the electronic ground state potential surface.