Knowledge Bank

The \textit{in situ} photochemistry of dopant molecules isolated in solid parahydrogen (pH$2$) typically differs from analogous studies in rare gas crystals for two main reasons: (1) solid pH$2$ has a negligible cage effect so that photodissociation of a precursor molecule can lead efficiently to well-separated fragments, and (2) radical fragments can potentially react with the pH$2$ matrix. Our group is currently studying the 193 nm photochemistry of a number of precursor molecules isolated in solid pH$2$ via high-resolution FTIR spectroscopy. In this talk I will present results for the 193 nm photolysis of formic acid (FA) in solid pH$2$. In rare gas matrices, the analogous FA photochemistry proceeds via the CO+H$2$O and CO$2$+H$2$ photodissociation channels.$s$\ddot{a}$nen,\; \backslash textit\{J.\; Mol.\; Struct.\; \}\backslash textbf\{436-437\},\; 349\; (1997).\}\; In\; solid\; pH$_2$, in addition to these channels we observe the production of HCO and HOCO. Further, after the UV laser is turned off, the HOCO concentration continues to increase with a slow first-order rate constant for a period of 10 hours. At this point, we still do not have a full explanation of the chemical mechanism leading to the post-photolysis increase in the HOCO concentration.