Spin relaxation of muonium-substituted ethyl radicals (MuCH(2)CH(2)) in the gas phase

The spin relaxation of the muonium-substituted ethyl radical (MuCH(2)CH(2)) and its deuterated analog (MuCD(2)CD(2)) has been studied in the gas phase in both transverse and longitudinal magnetic fields spanning the range similar to 0.5-35 kG, over a pressure range from similar to 1-16 atm at ambient temperature. The Mu(13)CH(2)(13)CH(2) radical has also been investigated, at 2.7 atm. For comparison, some data is also reported for the MuCH(2)C(CH3)(2) (Mu-t-butyl) radical at a pressure of 2.6 atm. This experiment establishes the importance of the mu SR technique in studying spin relaxation phenomena of polyatomic radicals in the gas phase, where equivalent ESR data is sparse or nonexistent. Both T-1 (longitudinal) and T-2 (transverse) mu SR relaxation rates are reported and interpreted with a phenomenological model. Relaxation results from fluctuating terms in the spin Hamiltonian, inducing transitions between the eigenstates assumed from an isotropic hyperfine interaction. Low-field relaxation is primarily due to the electron, via both the nuclear hyperfine (S . A . I) and the spin rotation interactions (S . J), communicated to the muon via the isotropic muon-electron hyperfine interaction. At the highest fields, direct spin flips of the muon become important, due to fluctuations in the anisotropic part of the muon-electron hyperfine interaction. In the intermediate held region a muon-electron ''flip-flop'' relaxation mechanism dominates, due partly to the anisotropic hyperfine interaction and partly to modulation of the isotropic muon-electron hyperfine coupling. In the case of the T-2 rates, electron relaxation mechanisms dominate over a much wider field range than for the T-1 rates, and inhomogeneous line broadening also contributes. The fluctuations that induce both the T-1 and T-2 relaxation rates are described by a single correlation time, tau(c), inversely proportional to the pressure. An effective spin-reorientation cross section is deduced from this pressure dependence, sigma(J) similar to 100+/-20 Angstrom(2), for all topically substituted ethyl radicals. This is similar to the geometrical cross section, but about a factor of 4 larger than values of sigma(J) found for similar-sized diamagnetic molecules by gas phase NMR, primarily reflecting the longer range of the electron-induced intermolecular potential. (C) 1996 American Institute of Physics.