Dissociative photoionization of N₂in the 24–32 eV photon energy range

Abstract

Dissociative photoionization of N2 is studied with synchrotron radiation in the 24–32 eV photon energy range. Branching ratios between the different dissociation limits are measured from coincidence time of flight ion spectra threshold photoelectron–photoion coincidence recorded for state-selected N2+ parent ions. In this energy range, N2+ molecular ions are observed to dissociate only towards the three lowest dissociation limits. Dissociations towards the second and third ones, which correspond to the formation of N+(1D) and N(2D) metastable states, respectively, occur right from their thermochemical onsets. From the second dissociation limit energy up to the third one, the N+(1D) + N(4S)/N+(3P) + N(4S) branching ratio is almost constant and equal to 0.6:0.4, except at the energy of the C 2 Σ u+ (v = 12) state, where this branching ratio is found to be equal to 0.5:0.5. From the third dissociation limit onset, N2+ ions fragment only towards this limit. Possible dissociation mechanisms are discussed, involving a spin–orbit coupling between doublet and quartet states of N2+ to produce N+(1D) + N(4S) and a direct dissociation to produce N+(3P) + N(2D). No dissociation towards the other limits has been observed, in particular to produce the N+(1S), N+(5S) and N(2P) metastable states.

CRITICAL REVIEW OF N, N + , N + 2 , N ++ , And N ++ 2 MAIN PRODUCTION PROCESSES AND REACTIONS OF RELEVANCE TO TITAN'S ATMOSPHERE

“…Using this expression, the reaction cross section value extrapolated down to 150 K is 39.6 Å 2 , giving a rate constant of 1.03 × 10 −9 cm 3 s −1 at 150 K. This should be checked experimentally, but until then, we recommend a rate constant value of 1.0 × 10 −9 cm 3 s −1 at 150 K, with an uncertainty of 50% which includes the value at 300 K. Table 8 summarizes the product branching ratio data of the N + 2 + C 2 H 2 reaction. C 2 H + 2 is the only significant product observed by Nicolas (2002), which is in agreement with Anicich et al (2004), Warneck (1972) and with the recent work at low temperature −9 cm 3 s −1 ) which are the only published rate constant data for this reaction, is not explained by the authors (see Table 6). However, the experimental conditions are substantially different, with a higher pressure in the case of Anicich et al (2004) inducing secondary reactions.…”

“…It can be explained by the fact that it is mainly a nonresonant charge transfer reaction (see below). Nicolas et al (Nicolas 2002) measured a lower rate constant at E CM = 0.08 eV (928 K), which indicates that most probably the rate constant decreases with temperature, as expected such a nonresonant charge transfer. This decrease is compatible with the typical variation of the cross section with collision energy for such reactions, i.e., proportional to E −1/2 CM .…”

“…In spite of a slight increase of the rate constant at low temperatures, 1.20 × 10 −9 at 70 K and 1.90 × 10 −9 at 8-15 K, the difference with the value of the rate constant at 300 K is not significant. The rate constant seems effectively unchanged over the temperature range 8-300 K, so the recommended value is the same at 300 K and 150 K. The study of Nicolas et al (2003b) shows, however, a substantial increase of the rate constant with collision energy, and in the thesis report of Nicolas (2002), a variation of less than 20% in the rate constant is observed with vibrational energy (v = 0-4). A small isotopic effect has been observed by Nicolas et al (2003b), who measured, in the 0.1-3 eV E CM range, rate constants equal to (1.53-2.83) × 10 −9 cm 3 s −1…”

“…Adapted from references (Franceschi et al 2007;Gilmore 1965;Hiyama & Iwata 1993;Lewis et al 2005;Nicolas et al 2003a). ions.…”

“…The excited electronic states of N + 2 , at energies higher than 26.3 eV, dissociate mainly into N + ( 3 P) + N ( 2 D) and somewhat into other dissociation channels (Aoto et al 2006;Nicolas et al 2003a). So globally it can be estimated that N atoms coming from the N 2 dissociative ionization are mainly in the 4 S ground state, with less than 10% being in the N ( 2 D) excited state.…”