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.