Toward the first study of chemical reaction dynamics of Mu with vibrational-state-selected reactants in the gas phase: The Mu þ H  2 ðv ¼ 1Þ reaction by stimulated Raman pumping Pavel Bakule a,Ã , Oleksandr Sukhorukov b , Yasuyuki Matsuda c , Francis Pratt d , Peter Gumplinger e , Takamasa Momose f , Eiko Torikai g , Donald Fleming b a RIKEN-RAL Muon Facility at ISIS, Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan b TRIUMF and Department of Chemistry, University of British Columbia, Canada c Graduate School of Arts and Sciences, University of Tokyo, Komaba 3-8-1, Meguro, Tokyo 153-8902, Japan d ISIS, ISIS Muon Facil, Didcot OX11 0QX, Oxon, UK e TRIUMF Laboratory, 4004 Wesbrook Mall, Vancouver, BC, Canada f Departments of Chemistry and Physics, University of British Columbia, Canada g Faculty of Engineering, Yamanashi University, Kofu, Yamanashi 4008511, Japan article info Keywords: Muonium Stimulated Raman pumping Hydrogen Laser setups Reaction dynamics abstract Stimulated Raman pumping (SRP) is used to produce H 2 in its first vibrational state, in order to measure, for the first time, the Mu þ H  2 ðv ¼ 1Þ! MuH þ 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 H 3 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 cm 3 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 H 2 molecules and their overlap with the stopping profile of the muon beam in the reaction cell (total volume 100  40  4 mm 3 ). The first result of this experiment gives a measured relaxation rate due to laser excitation of l  ¼ 0:085  0:051 ms 1 , consistent with theory but limited by both low statistics and particularly a high background relaxation rate. & 2009 Elsevier B.V. All rights reserved. 1. Introduction and scientific motivation The utility of the Mu atom as a remarkably light H-atom isotope and its role in elucidating quantum mass effects in gas-phase chemical reactivity is well established. However, to date, all reaction rate studies of this nature have been for reactants in their ground states (GS). The present paper charts new directions for Mu reaction rate studies from specific state-selected reactants in the gas phase, as part of the development of a dedicated new laser-mSR setup at the RIKEN/RAL laboratory. We report on a first attempt to measure the rate of the Mu þ H  2 ðv ¼ 1Þ! MuH þ H reaction, using stimulated Raman pumping (SRP) to prepare H 2 in its first vibrational state ðv ¼ 1Þ, hereafter denoted H  2 . The Mu þ H  2 reaction is chosen for this initial study for two reasons. First, the theory of the H 3 system has evolved to such an extent that truly unprecedented tests by experiment have been realized, for both the GS H þ D 2 and D þ H 2 reactions [1], and for the GS Mu þ H 2 reaction [2,3]. The 3D quantum reaction rates from these calculations, in the form of Arrhenius plots, are shown in Fig. 1 . The ultimate goal of our experiments is to provide, for the first time, a similar test for the Mu þ H  2 reaction. Second, though untested, we can still expect the calculated rates for Mu þ H  2 (Fig. 1) to be quite accurate, thereby allowing us to ‘‘benchmark’’ the experimental results, providing a valuable assessment of both the methodology and data analysis procedures. For a collinear transition state (TS), the interaction potential is a function of only two coordinates describing the reactant ðMu þ H2HÞ and product ðMu2H þ HÞ channels. A schematic plot of the ‘‘reaction path’’ between these initial and final states is shown in Fig. 2, along with the transitions that are important in pumping H 2 by SRP. Since H 2 has no dipole moment, direct excitation of ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2008.11.230 Ã Corresponding author. E-mail address: pavel.bakule@stfc.ac.uk (P. Bakule). Physica B 404 (2009) 1013–1016