The production of electronically-excited species from the photolysis of N₂H₄ and N₂D₄ at 193 nm

Abstract

Emission in the spectral region 300–900 nm has been observed following 193 nm excimer laser photolysis of N₂H₄ and N₂D₄. Electronically-excited NH(A) and ND(A) are formed by a two photon process with rotational and vibrational distributions that can be characterized by a single temperature of ∼ 4000 K and ∼ 3800 K, respectively. The visible spectrum in the region 420–900 nm shows a broad and largely unstructured continuum emission upon which is superimposed a large number of well-resolved spectral features corresponding to NH₂(Ã) (or ND₂(Ã)). This emission is formed by a single photon process that is essentially synchronous with the laser pulse and it is suggested that NH*₂ is formed directly by cleavage of the N-N bond in hydrazine. The visible emission is found to show two lifetimes; one of ∼ 25 μs and the other in excess of 100 μs, depending whether the measurement is performed on discrete spectral features or on the total emission. It is suggested that the longer component is associated with the continuum emission and possibly results from a Renner-Teller coupling of the excited NH₂(Ã, ² A ₁) state with high vibrational levels of the ground state, NH₂([Xtilde], ² B ₁). Experiments with various buffer gases show that for the high vibrational levels of the NH*₂ populated, vibrational relaxation of the NH*₂ takes place on a timescale that is comparable to electronic quenching.