Metastable, collision-induced and laser-induced decomposition of Ar m N + 2n (m= 0, 1) cluster ions in the gas phase

Abstract

Triple-quadrupole mass spectrometry has been used to investigate the N⁺ _2n(n= 1–27) and ArN⁺ _{2n– 2}(n= 1–23) cluster ions produced by sputtering of solid nitrogen–argon mixtures with fast argon atoms. Laser-induced, collision-induced and metastable decomposition of the cluster ions were examined. Four kinds of metastable energy storage were identified: thermal, vibrational [N₂(v= 1)], electronic and chemical. The argon-to-nitrogen loss-intensity ratio is a function of n and of the loss size, and also depends on the source of activation; it deviates strongly from the statistically expected values. The data lead to the conclusion that the ArN⁺ _{2n– 2} cluster ions are present as a mixture of N⁺ ₄(ArN_{2n– 6}) and ArN⁺ ₂(N_{2n– 4}) isomers, with distinct metastable electronic states and distinct but similar absorption spectra which are nearly independent of n, and have definite solid-like structures. The chemical metastability is due to the reaction Ar + N⁺ ₄→ N₂+ ArN⁺ ₂ and is absent in the larger cluster ions, presumably because the Ar atom is separated from the N⁺ ₄ core by a solid shell of N₂ molecules. Delivery of energy by collision causes melting, conversion of N⁺ ₄ to ArN⁺ ₂ by chemical reaction, vibrational relaxation and evaporation. The interpretation of the data has been tested by measurements on cluster ions that were sputtered and then collisionally pre-heated prior to examination.