Laboratory spectroscopic studies of the collisions between slow H⁺, H₂⁺, H₃⁺ ions and molecular nitrogen

Institute of Experimental Physics, University of Gdansk, ul. Wita Stwosza 57, 80-952 Gdansk, Poland
e-mail: dokws@univ.gda.pl

Received: 20 April 2011
Accepted: 7 August 2011

Abstract

Luminescence in the 275–470 nm spectral region was observed in the collisions of H⁺, H₂⁺, and H₃⁺ with N₂ in the 10–1000 eV projectile energy range. For all the systems, a luminescent charge exchange leads to the electronically excited states of N₂⁺(B²Σ_u) and N₂(C²Π_u, G²Δ_g). Computer simulation of the spectra was used to estimate relative cross sections for the reactions, as well as the temperatures corresponding to population distributions of vibrational and rotational levels of the products. The vibrational temperatures of the products for the H₂⁺+N₂ and H₃⁺+N₂ collision systems are found to be changing significantly with the projectile velocity, increasing from 1500 K at the lowest projectile velocities (≤50 km s^-1) and going through a maximum (15 000 K) at about 80 km s^-1, to decrease rapidly with increasing velocity, down to 3000 K. The H⁺+N₂ collision system, however, does not show a similar increase in temperature. The increase in vibrational temperature indicates that, for velocities in the 50–1000 km s^-1 range, the charge transfer process is the non-Franck-Condon type, a model that assumes a change in the internuclear distance of target molecule by the incident ion before electron capture occurs. This vibrational temperature dependency could be used to infer the ion velocity producing the observed emission of N₂⁺. Also we found no evidence of any formation of excited NH molecules in these reactions.