The ultraviolet photodissociation of HN3: The H+N3 product channel

Abstract

The near ultraviolet photolysis of hydrazoic acid, HN₃, has been investigated at a range of wavelengths, λ240 nm, using the technique of H Rydberg atom photofragment translational spectroscopy (PTS). Analysis of the total kinetic energy release (TKER) spectra so obtained reveals that the resulting N₃(X) fragments are formed in a progression of levels involving both the symmetric stretch mode (v₁00) and the symmetric stretch mode combined with one and two quanta of bending motion. The deduced N₃(X) product rotational energy disposal can be reproduced by an impulsive model, which yields both an impact parameter, b=1.26±0.05 Å, and a refined measure of the H–N₃ bond strength, D₀(H–N₃)=30970±50 cm^-1. The N₃(X) product vibrational progressions observed at each photolysis wavelength ‘break off’ at a low kinetic energy release of ∽4600±800 cm^-1. This is interpreted by invoking a barrier to dissociation via H–N bond fission on the ùA″ state potential energy surface, with the exoergicity associated with this barrier released mainly as product translation. The observed energy disposals and the deduced impact parameter are consistent with a fragmentation in which the most probable recoil velocity of the departing H atom is directed at ∽90° to the near linear N₃ chain. DN₃ photolysis has been investigated to a lesser extent. The H/D+N₃ spectra obtained following ultraviolet excitation of HN₃ and, especially, DN₃ also show signal at high TKER that is most readily accommodated in terms of a two photon dissociation process. The general shape of the PTS arising from the latter process is suggestive of a statistical energy disposal amongst the available H/D+N₃(X) product states, such as might arise if the two photon excited molecules undergo radiationless transfer to a lower energy potential energy surface and then fragment. There is some evidence that a statistical fragmentation process also plays a role at the energy of one absorbed photon. Both would also contribute to the observed yield of H/D atoms at low TKER; in the case of HN₃, the ‘slow’ H atom yield is augmented by H atoms resulting from photolysis of NH(a; v=3) products arising via the competing NH(a)+N₂(X) dissociation channel.