Electronic spectroscopy of size-selected ionic complexes

Abstract

This article describes spectroscopic investigations of small ionic complexes consisting of a diatomic (N₂⁺) or triatomic (N₂O⁺) core bound to one or more rare-gas atoms (He, Ne, Ar). The experiments involve detecting ionic fragments arising from resonance-enhanced, tuneable laser photodissociation in a tandem mass spectrometer. On the basis of progressions in the low-frequency intracluster vibrations conclusions are drawn concerning ion–neutral potential-energy surfaces in the equilibrium region. Thus the smaller N₂⁺–He_n complexes consist of an almost freely rotating N₂⁺ surrounded by delocalised He atoms, each bound by around 100 cm^–1. In contrast N₂⁺–Ne_n complexes appear to be more stable and at lower internal energies have reasonably localised structures. For the N₂O–Ar⁺ complex, charge–transfer bands exhibiting long, strong progressions in the two low-frequency intermolecular vibrations are apparent, proving that considerable structural rearrangement accompanies the transition. Other experiments concern the relaxation of a vibrationally excited N₂⁺ molecule that is surrounded by various numbers of He ‘solvent’ atoms. Quenching occurs on the µs–ms timescale with a rate that increases with the degree of solvation.