TIME-RESOLVED STUDIES OF PHOTOINITIATED REACTIONS IN BINARY AND LARGER (N2O)(M)(HI)(N) (M-GREATER-THAN-OR-EQUAL-TO-1, N-GREATER-THAN-OR-EQUAL-TO-1) COMPLEXES

Under gas-phase conditions, reactions of fast hydrogen atoms with N2O lead mainly to OH(X(2) Pi)+N-2(X(1) Sigma) products via a 1,3 hydrogen -shift mechanism involving the HNNO intermediate. In complexes formed by using supersonic expansions, the reaction mechanism is less clear. In this paper, OH build-up times are reported for photoinitiated react ions in complexes of the form (N2O)(m)(HI)(n) with m greater than or e qual to 1 and n greater than or equal to 1. The build-up times vary fr om many hundreds of fs under conditions that encourage the formation o f larger complexes (i.e. the highest stagnation pressures) down to les s than or equal to 100 fs for the lowest stagnation pressures. We conc lude that in binary complexes OH is produced on timescales below 100 f s. This rules out the participation of a long-lived intermediate such as HNNO. Comparisons are made with the analogous CO2-HI system, in whi ch reactions in larger complexes either yield the same lifetimes as do binary complexes or are inhibited, presumably because of the relative ly large H + CO2 entrance channel barrier.