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)C H(2)) and its deuterated analog (MuCD(2)CD(2)) has been studied in the gas phase in both transverse and longitudinal magnetic fields spannin g the range similar to 0.5-35 kG, over a pressure range from similar t o 1-16 atm at ambient temperature. The Mu(13)CH(2)(13)CH(2) radical ha s also been investigated, at 2.7 atm. For comparison, some data is als o 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 t echnique 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 ar e reported and interpreted with a phenomenological model. Relaxation r esults from fluctuating terms in the spin Hamiltonian, inducing transi tions between the eigenstates assumed from an isotropic hyperfine inte raction. Low-field relaxation is primarily due to the electron, via bo th the nuclear hyperfine (S . A . I) and the spin rotation interaction s (S . J), communicated to the muon via the isotropic muon-electron hy perfine interaction. At the highest fields, direct spin flips of the m uon become important, due to fluctuations in the anisotropic part of t he muon-electron hyperfine interaction. In the intermediate held regio n a muon-electron ''flip-flop'' relaxation mechanism dominates, due pa rtly 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 re laxation rates are described by a single correlation time, tau(c), inv ersely proportional to the pressure. An effective spin-reorientation c ross section is deduced from this pressure dependence, sigma(J) simila r to 100+/-20 Angstrom(2), for all topically substituted ethyl radical s. This is similar to the geometrical cross section, but about a facto r of 4 larger than values of sigma(J) found for similar-sized diamagne tic molecules by gas phase NMR, primarily reflecting the longer range of the electron-induced intermolecular potential. (C) 1996 American In stitute of Physics.