Toward the first study of chemical reaction dynamics of Mu with vibrational-state-selected reactants in the gas phase: The Mu+H2*(v=1) reaction by stimulated Raman pumping
Stimulated Raman pumping (SRP) is used to produce H2 in its first vibrational state, in order to measure, for the first time, the Mu+H2*(v=1)→MuH+H reaction rate at room temperature, as a prototypical example of new directions in gas-phase muonium chemistry, utilizing the pulsed muon beam and a new dedicated laser system at the RIKEN/RAL Laboratory. Reported here is a preliminary result but the final results are expected to provide definitive new tests of reaction rate theory on the highly accurate H3 potential energy surface. The major difficulty in this experiment, compared to the standard SRP process, is to ensure a homogeneous excitation over a volume of several cm3 and of sufficient intensity to ensure a measurable Mu relaxation rate. The techniques used to accomplish this are described. The experiment utilizes the 2nd harmonic output of a Nd:YAG laser (532 nm) with pulse energies up to 500 mJ at a repetition rate of 25 Hz. Different optical setups have been constructed and tested in order to optimize the number of laser-pumped H2 molecules and their overlap with the stopping profile of the muon beam in the reaction cell (total volume ∼100×40×4 mm3). The first result of this experiment gives a measured relaxation rate due to laser excitation of λ*=0.085±0.051 μs-1, consistent with theory but limited by both low statistics and particularly a high background relaxation rate.