First accurate experimental study of Mu reactivity from a state-selected reactant in the gas phase: the Mu + H 2 1 reaction rate at 300 K

This paper reports on the experimental background and methodology leading to recent results on the first accurate measurement of the reaction rate of the muonium (Mu) atom from a state-selected reactant in the gas phase: the Mu + H 2 ##IMG## [http://ej.iop.org/images/0953-4075/48/4/045204/jpb505072ieqn3.gif] {$\{1\}\to $} MuH + H reaction at 300 K, and its comparison with rigorous quantum rate theory, Bakule et al (2012 J. Phys. Chem. Lett. 3 [http://dx.doi.org/10.1021/jz3011496] 2755 ). Stimulated Raman pumping, induced by 532 nm light from the 2nd harmonic of a Nd:YAG laser, was used to produce H 2 in its first vibrational ( v = 1) state, H ##IMG## [http://ej.iop.org/images/0953-4075/48/4/045204/jpb505072ieqn4.gif] {$_{2}\{1\}$} , in a single Raman/reaction cell. A pulsed muon beam (from ‘ISIS’, at 50 Hz) matched the 25 Hz repetition rate of the laser, allowing data taking in equal ‘Laser-On/Laser-Off’ modes of operation. The signal to noise was improved by over an order of magnitude in comparison with an earlier proof-of-principle experiment. The success of the present experiment also relied on optimizing the overlap of the laser profile with the extended stopping distribution of the muon beam at 50 bar H 2 pressure, in which Monte Carlo simulations played a central role. The rate constant, found from the analysis of three separate measurements, which includes a correction for the loss of ##IMG## [http://ej.iop.org/images/0953-4075/48/4/045204/jpb505072ieqn5.gif] {${{\rm H}_{2}\{1\}$} concentration due to collisional relaxation with unpumped H 2 during the time of each measurement, is ##IMG## [http://ej.iop.org/images/0953-4075/48/4/045204/jpb505072ieqn6.gif] {${k}_{\rm Mu}}\{1\}$} = 9.9[(−1.4)(+1.7)] × 10 −13 cm 3 s −1 at 300 K. This is in good to excellent agreement with rigorous quantum rate calculations on the complete configuration interaction/Born–Huang surface, as reported earlier by Bakule et al , and which are also briefly commented on herein.