Donald Fleming

My research (and teaching) interests lie in the general field of nuclear and particular muon chemistry in two quite distinct areas: isotopic mass effects in chemical kinetics and reaction dynamics of both the lightest isotope of the H-atom, muonium (Mu=µ+e-), with a mass of 0.113 amu, and, more recently, of its heaviest isotope, muonic He (4Heµ), formed by negative muon (µ-) capture on He, with a mass of 4.11 amu [Science, 331, 448 (20110]; and the study of the hyperfine interactions and molecuar motion of muoniated free radicals formed by Mu addition reactions to unsaturated bond systems, with a particular interest in catalytic enviornments, zeolites and nanomaterials .

In both instances the  "µSR" (Muon Spin Rotation or Relaxation or Resonance) technique employed relies on parity violation in the π-μ-e decay sequence in which the muon is produced 100% spin-polarized and electrons (e+ or e-) from muon decay are emitted preferentially along (or opposite to) the muon spin. This anisotropy effect provides the basis for detecting the interaction of the muon spin with its environment. The "µSR" signal is analyzed to yield amplitudes, frequencies and relaxations characteristic of these interactions in either a tranverse (TF) or longitudinal (LF) magnetic field, as indicated in the diagram below, and similar in scope to related studies in magnetic resonance.

In the field of chemical reaction dynamics, the unprecedented isotopic mass ratio of 36.4 between 4Heμ and Mu provides for truly unique tests of quantum mass effects in reaction rate theory [Science, 2011; J. Chem. Phys., 2011]. However the study of Mu reactivity, as the lightest isotope of the H-atom, is important in its own right. In a novel study of Mu addition reactions, Mu + CO, the MuCO complex formed has only one bound vibrational state, due to the remarkable shift in zero point energy (ZPE) effected by the light muon mass;  as a result comparison between experiment and theory provided an important new test of the Isolated Resonance Model of unimolecular decay [J. Chem. Phys., 125,  014307 (2006)]. More recently, a new direction in reaction rate studies of mounium, is to use pulsed lasers to state-select reactants in the gas phase in concert with a pulsed μ+ beam at the Rutherford Laboratoy in the UK. Reaction rates from vibrational excited states probe a different region of the potential energy surface (PES). The first study is a meaurement of the kinetics of the Mu + H2*(v=1) reaction rate at 300 K, in comparison with the results of rigorous quantum theory has recently been published [J. Phys. Chem. Letts, 2012].

In the realm of muoniated radicals, the muon polarization can be efficiently transferred to the radical by Mu addition reactions, allowing measurements of both the muon and nuclear hyperfine coupling constants (Hfcc) in a variety of different enviornments. Recently the MuC6H6 radical has been studied in the Y-zeolites in which pronounced radical motion was indicated from the widths of Level Crossing Resonances in USY and HY [J. Phys. Chem. C, 115, 11177 (2011)], important industrial catalysts about which nothing is  known from ESR studies. This work complements an earlier study of the MuC2H4 radical in these same zeolites [J. Phys. Chem. C, 111, 9779 (2007)]. The motivation for these studies is to use the μSR technique as a "template" to learn about binding sites and the nature of free radical intemeadiates that might well be of importance for their H-adduct cousins in particularly hydrocarbon cracking in zeolites. New directions for the study of Mu-radicals are envisaged in mettalic nanomaterials (Au,Pt,Pd), also important catalytic environments. It has also been important to establish agreement between experimental determinations of Hfcc for muoniated radicals and ab-initio calculations of these values; this has recently been established for neat muoniated sec-butyl radicals in condensed phases [J. Phys. Chem. A, 115, 2778 (2011)], where generally excellent agreement between theory and experiment was found for both the magnitude and the temperature dependence of muon and proton Hfcc [J. Phys. Chem. A, 115, 2765 (2011)]. In a novel direction as well the existence of the Heavy-Light-Heavy Br-Mu-Br radical has recently been inferred, which has important implications for the possible existence of vibrational bonding, which has not heretofore been experimentally established [PCCP, 2012].

Experimental setup and detection schemes for important muon spin properties.