[Triumf-seminars] TRIUMF Colloquium today at 14:00

[triumf-seminars at lists.triumf.ca]

Date/Time: Thu 2011-05-12 at 14:00

Location:  Auditorium          

Speaker:   Don Fleming (UBC)

Title:     From the simple to the important: muon studies of isotopic H-atom chemical reaction rates as tests of rate theory over a remarkable range of 36 in isotopic mass

Abstract: Our understanding of the underlying basis of chemical reaction rates and dynamics had its genesis with the first Potential Energy Surface (PES) for the H + H2 reaction published in the early 1930s by London, Eyring and Polanyi. This simplest and most fundamental of chemical reactions can be thought of as the "quark of the molecular world". It has, accordingly, been studied intensely ever since.

Deuterium (D) was discovered by Harold Urey around the same time, ushering in studies of "kinetic isotope effects" (KIEs), in which the heavier D-atom mass affected the dynamics and hence the reaction rate but not the underlying PES, which is an implicit statement of the Born-Oppenheimer approximation. However the masses of H and D atoms differ by only a factor of two in mass, limiting the range of KIEs that can be probed by this substitution. (Though tritium, with a mass of 3 amu would be a useful complement in this regard, it is dangerously radioactive and has seen only limited use in experimental reaction rate studies.)

It fell then to muon science to extend the isotopic H-atom mass range, first to its lowest end in a series of experiments carried out at TRIUMF with the muonium atom (Mu), with a mass of only 0.113 amu, and most recently to its heaviest end in studies of the reaction rates of the muonic He atom (4Heu), with a mass of 4.11 amu. This has in turn facilitated unique tests of the fundamental H3 reaction system, in which fully accurate quantum rate theory results, that include as well mass corrections to the BO surface (BODC), agree quantitatively with experiment over an unprecedented range of a factor of 36 in H-atom isotopic mass.

In this talk theory and experiment for the Mu + H2 (J. Chem. Phys., 1987) and 4Heu +H2 (Science, 2011) reaction rates will be compared, along with related studies of D +H2. Similar comparisons with recently completed results for the Mu +H2*(v=1) reaction will also be presented, in which, for the first time, Mu reactivity with state-selected reactants has been measured, using Stimulated Raman Pumping to populate the 1st vibrational state of H2 and the pulsed muon beam at the RIKEN/RAL Facility at ISIS.

Stimulants available 15 minutes before the talk.

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