Communication: H-atom reactivity as a function of temperature in solid parahydrogen: The H + N2O reaction.

We present low temperature kinetic measurements for the H + N2O association reaction in solid parahydrogen (pH2) at liquid helium temperatures (1-5 K). We synthesize (15)N2(18)O doped pH2 solids via rapid vapor deposition onto an optical substrate attached to the cold tip of a liquid helium bath cryostat. We then subject the solids to short duration 193 nm irradiations to generate H-atoms produced as byproducts of the in situ N2O photodissociation, and monitor the subsequent reaction kinetics using rapid scan FTIR. For reactions initiated in solid pH2 at 4.3 K we observe little to no reaction; however, if we then slowly reduce the temperature of the solid we observe an abrupt onset to the H + N2O → cis-HNNO reaction at temperatures below 2.4 K. This abrupt change in the reaction kinetics is fully reversible as the temperature of the solid pH2 is repeatedly cycled. We speculate that the observed non-Arrhenius behavior (negative activation energy) is related to the stability of the pre-reactive complex between the H-atom and (15)N2(18)O reagents.