INFRARED SPECTROSCOPIC STUDIES OF ORTHO-PARA CONVERSION IN SOLID HYDROGEN CATALYZED BY HYDROGEN ATOMS

Our group has been studying the reactions of hydrogen atoms (H atoms) with various molecules (NO, N$_{2}$O, CH$_{3}$OH) in solid hydrogen for the last several years.\footnote{F.M. Mutunga, S.E. Follett, D.T. Anderson, \textit{J. Chem. Phys. }\textbf{139}, 151104 (2013).}\footnote{M. Ruzi, D.T. Anderson, \textit{J. Phys. Chem. A }\textbf{119}, 12270 (2015).}\footnote{M.E. Balabanoff, M. Ruzi, D.T. Anderson, \textit{Phys. Chem. Chem. Phys. }\textbf{20}, 422 (2018).} One interesting puzzle that we have been unable to solve is how to detect the concentration of H atoms using FTIR spectroscopy. One possibility to estimate the H atom concentration is to measure the conversion of ortho-H$_{2}$ to para-H$_{2}$ within the solid that is catalyzed by the presence of H atoms. The H atom is a good ortho-para catalyst because it is paramagnetic and mobile within the solid even at extremely low temperatures. We have recently conducted a number of studies where we purposely synthesize solid para-H$_{2}$ samples with approximately 3\% ortho-H$_{2}$ concentrations (slightly elevated). In the absence of H atoms, the ortho-H$_{2}$ concentration in the solid is stable on the order of days due to slow self-conversion. We can quantitatively detect the ortho-H$_{2}$ fraction using the overlapping Q$_{1}$(0)+S$_{0}$(1) and Q$_{1}$(1)+S$_{0}$(1) double transitions of solid molecular hydrogen. By rapidly generating H atoms via in situ photolysis of various H atom precursor molecules (NO and N$_{2}$O), we can initiate ortho-para conversion and follow the ortho-H$_{2}$ fraction in real time. This H atom catalyzed ortho-para conversion data therefore has the time dependent H atom concentration encoded in the signal; the challenge is to extract it. We observe qualitative differences in the shape of the ortho-H$_{2}$ fraction decay curve depending on the specific precursor used, the specific photolysis conditions, and the temperature of the sample. We will present the latest results and analysis at the meeting