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(ArN2n– 6) and ArN+2(N2n– 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.