J4. REMPI-LIF Studies of Ion-molecule Association Reactions

Department of Chemistry
Heriot-Watt University
Riccarton, Edinburgh EH14 4AS
United Kingdom

E-mail: M.J.Frost@hw.ac.uk

Whilst laser-based techniques have been routinely applied to kinetic measurements in neutral chemistry, they have not been widely used to study ion-molecule reactions. In ion chemistry, kinetic measurements are commonly performed using flow tube techniques coupled with mass spectrometric detection. However, the use of flow tubes restricts the range of total pressure over which measurements can be conveniently made owing to the requirement for laminar flow in the tube. As a result, studies of simple ion-molecule association reactions have largely been restricted to kinetics in the low-pressure limit. Here we will report the use of a laser technique based upon Resonance Enhanced Multiphoton Ionisation (REMPI) for ion production and laser-induced fluorescence (LIF) for ion detection which allows us to extend significantly the range of pressures over which ion-molecule association reactions can be studied.

Our work focuses on the pressure-dependent kinetics of the archetypal ion-molecule association reaction:

N₂⁺ + N₂ + M ® N₄⁺ + M [1]

This reaction is of key importance in a number of high pressure plasma environments. For example, in the terrestrial atmosphere, N₂⁺ is readily produced by ionising radiation or electrical phenomena. At altitudes where pressures are significant (>1 Torr), Reaction [1] is a major route to removal of N₂⁺ ions.

In this work, we study Reaction [1] under conditions of 1-13 Torr for M=N₂ and 10-950 Torr for M=He. In both pressure ranges, the kinetics of the reaction are found to deviate significantly from the low-pressure limiting kinetics. Indeed for reaction in He, a significant portion of the fall-off range is covered. Detailed information on both the bond energy¹ and structure² of the N₄⁺ ion is available which allows us to apply statistical theories to this particular reaction. A discussion of the results of these calculations in reproducing both the fall-off behaviour observed here and the temperature-dependence of the low pressure limiting rate constant³ will be presented.

1. R.H. Schultz and P.B. Armentrout, Int. J. Mass Spectrom. Ion Proc. 107, 29 (1991).
2. C. Léonard, P. Rosmus, S. Carter and N.C. Handy, J. Phys. Chem. A 103, 1846 (1999).
3. (a) L.K. Randeniya, X.K. Zeng, R.S. Smith and M.A. Smith, J. Phys. Chem. 93, 8031 (1989).