Chlorine atom diffusion and reactivity in solid parahydrogen at 1.6–4.3 K

Abstract

One of the surprising properties of quantum solids is that chemical impurities are expected to become delocalized when they are embedded in a quantum solid even at T = 0 K due to the large amount of zero-point energy in these systems. For example, hydrogen atoms, HD, ortho-H2, HF, H2O and oxygen atoms are all known to be mobile in solid parahydrogen (pH2) at liquid helium temperatures; however, each species diffuses by a different mechanism, and the exact mechanism is still unclear for HF, H2O, and oxygen atoms. In this work we studied the diffusion and reactivity of Cl atoms trapped in solid pH2 using Fourier transform infrared (FTIR) spectroscopy by studying the reaction kinetics of the diffusion-controlled Cl + CO → ClCO reaction. We first present and assign the infrared spectra of ClCO and OC-HCl isolated in solid pH2, which are two species that are observed at different stages of these experiments. Eight separate kinetic experiments were performed on annealed Cl2/CO/pH2 samples at 4.0 K with varying initial CO concentrations which showed that the rate constant for the Cl + CO reaction is inversely proportional to the CO concentration, which is a predicted feature of quantum diffusion. One important finding in this work has to do with the effect of exposing the pH2 sample to near-infrared radiation (NIR) between 4200 and 4700 cm−1 from the FTIR source; we show that irradiating the sample with NIR radiation speeds up the diffusion of Cl and CO within solid pH2.