SIGNATURES OF HYDROGEN ATOM QUANTUM DIFFUSION: H + N₂O REACTION IN SOLID PARAHYDROGEN

Abstract

In 1969 A. F. Andreev and I. M. Lifshitz radically changed the way we think about diffusion in cryocrystals by predicting that at sufficiently low temperatures the probability of exchange tunneling of neighboring particles in quantum crystals becomes noticeable such that impurities can move freely through the crystal as narrow-band quasiparticles. a The term "quantum crystal" was introduced by de Boer in 1948 for substances in which the energy of the zero-point vibrations of the particles is comparable to the total energy of the crystal. b The main idea put forth by Andreev and Liftshitz is that the rate of quantum diffusion should increase with falling temperatures and should show an inverse dependence on the concentration of impurities. As we will show, the hydrogen atom (H-atom) trapped in a parahydrogen crystal is an ideal candidate for quantum diffusion owing to its small mass and neutral charge. In 2013 our group published a communication c on the kinetics of the H + N 2 O reaction in solid parahydrogen that showed an anomalous temperature dependence. In these studies we generate the H-atoms as byproducts of the in situ photodissociation of N 2 O and monitor the subsequent reaction kinetics using rapid scan FTIR. Specifically, if we photolyze N 2 O doped parahydrogen solids with 193 nm UV radiation at 4.3 K, we observe little to no reaction; however, if we then slowly reduce the temperature of the sample, we observe an abrupt onset to the reaction at temperatures below 2.4 K. In a number of studies conducted since this original work we have come to a better understanding of the effect of temperature on the reaction and will show data that the rate constant for the H + N 2 O reaction shows an inverse dependence on the N 2 O concentration. These findings support previous ESR measurements of H-atom quantum diffusion in solid parahydrogen d and more importantly illustrate how H-atom quantum diffusion impacts the kinetics of these anomalous low temperature, condensed phase reactions.