Isotope Effects in the Predissociation of Excited States of N2 + Produced by Photoionization of 14N2 and 15N2 at Energies Between 24.2 and 25.6 eV

Abstract

Photoelectron/photoion imaging spectrometry employing dispersed VUV radiation from the SOLEIL synchrotron has been used to study the predissociation of N₂ ⁺ states located up to 1.3 eV above the ion's first dissociation limit. Branching ratios for unimolecular decay into either N₂ ⁺ or N⁺ were obtained by measuring coincidences between threshold electrons and mass-selected product ions, using a supersonic beam of either ¹⁴N₂ or ¹⁵N₂ as photoionization target. The results confirm that predissociation of the ${\text{C}}^2{\text{Σ}}_{\text{u}}^{+}$ state of ¹⁴N₂ ⁺ is faster than emission to the electronic ground-state by a factor 10 or more for all vibrational levels v' ≥ 3, while for ¹⁵N₂ ⁺ the two decay modes have comparable probabilities for the levels v' = 3, 4, and 5. In contrast, no significant isotope effect could be observed for the other states of N₂ ⁺ identified in the photoelectron spectrum. For both ¹⁴N₂ ⁺ and ¹⁵N₂ ⁺ isotopologues all vibrational levels of these other states decay to an extent of at least 95% by predissociation.

Keywords: TPEPICO; charge exchange; helium; nitrogen; photoionization; predissociation.