ALBERT

Notes: The spin relaxation of the muonium-substituted ethyl radical (MuCH2C(overdot)H2) and its deuterated analog (MuCD2C(overdot)D2) has been studied in the gas phase in both transverse and longitudinal magnetic fields spanning the range ∼0.5–35 kG, over a pressure range from ∼1–16 atm at ambient temperature. The Mu13CH213C(overdot)H2 radical has also been investigated, at 2.7 atm. For comparison, some data is also reported for the MuCH2C(overdot)(CH3)2 (Mu-t-butyl) radical at a pressure of 2.6 atm. This experiment establishes the importance of the μSR technique in studying spin relaxation phenomena of polyatomic radicals in the gas phase, where equivalent ESR data is sparse or nonexistent. Both T1 (longitudinal) and T2 (transverse) μSR 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 field 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 T2 rates, electron relaxation mechanisms dominate over a much wider field range than for the T1 rates, and inhomogeneous line broadening also contributes. The fluctuations that induce both the T1 and T2 relaxation rates are described by a single correlation time, τc, inversely proportional to the pressure. An effective spin-reorientation cross section is deduced from this pressure dependence, σJ∼100±20 A(ring)2, for all isotopically substituted ethyl radicals. This is similar to the geometrical cross section, but about a factor of 4 larger than values of σJ found for similar-sized diamagnetic molecules by gas phase NMR, primarily reflecting the longer range of the electron-induced intermolecular potential. © 1996 American Institute of Physics.