OBSERVATION OF NUCLEAR SPIN SELECTION RULES IN SUPERSONICALLY EXPANDING PLASMAS CONTAINING H$_3^+$

M. Cordonnier, D. Uy, R. M. Dickson, K. E. Kerr, Y. Zhang, and T. Oka J. Chem. Phys.Author Institution: Department of Chemistry, University of Illinois at Urbana-Champaign; Urbana, IL 61801The reaction of {\em para}-H$_2^+$ with {\em para}-H$_2$ can form only the {\em para} spin modification of H$_3^+$, according to nuclear spin selection rules. A proton hop from {\em para}-H$_3^+$ to {\em para}-H$_2$ maintains the {\em para}-H$_3^+$, but a hydrogen exchange reaction can produce {\em ortho}-H$_3^+$. In the hopes of developing a source of H$_3^+$ that is almost entirely in a single quantum state ($J=K=1$), we have investigated a supersonically expanding plasma using a pulsed discharge nozzle with pure {\em para}-H$_2$ as a precursor gas. The plasma has been interrogated via continous-wave cavity ringdown spectroscopy using a homemade difference frequency laser that combines a tunable Ti:Sapphire laser and a fixed frequency Nd:YAG laser. With this system, we have measured the relative intensities of the $R(1,0)$, $R(1,1)^u$, and $R(2,2)^l$ transitions of the $\nu_2$ band of H$_3^+$ near 3.67 $\mu$m. \mbox{} We will discuss the construction of the difference frequency laser, our adaptation of continous-wave cavity ringdown spectroscopy to a pulsed source, and the results of our spectroscopic study. We have found that it is possible to produce highly enriched {\em para}-H$_3^+$, especially when an inert gas is used as a buffer to reduce the rate of reactions between H$_3^+$ and H$_2$. We have also found that the ratio between the rate coefficients of the proton hop and hydrogen exchange reactions $\alpha = k_H / k_E \sim 1$ at the low temperatures of our expansion, in contrast to the value of $\alpha \sim 2.4$ found by the Oka group} \underline{\textbf{113}}, 3181 2000.} at the higher temperatures of a water-cooled hollow cathode discharge