BRIAN A. TOM, MICHAEL B. WICZER, ANDREW A. MILLS, KYLE N. CRABTREE AND BENJAMIN J. MCCALL, Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801.

The reaction of para -H2+ with para -H2 can form only the para spin modification of H3+, according to nuclear spin selection rules. A proton hop from para -H3+ to para -H2 maintains the para -H3+, but a hydrogen exchange reaction can produce ortho -H3+. In the hopes of developing a source of H3+ 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 para -H2 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 nu2 band of H3+ near 3.67 µm.

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 para -H3+, especially when an inert gas is used as a buffer to reduce the rate of reactions between H3+ and H2. We have also found that the ratio between the rate coefficients of the proton hop and hydrogen exchange reactions alpha = kH / kE sim 1 at the low temperatures of our expansion, in contrast to the value of alpha sim 2.4 found by the Oka group at the higher temperatures of a water-cooled hollow cathode discharge.