MU-SR STUDIES OF FREE-RADICALS IN THE GAS-PHASE

Muonium (Mu=mu(+)+e(-)) is the bound state of a positive muon and an electron. Since the positive muon has a mass about 1/9 of the proton, Mu can be regarded as an ultra light isotope of hydrogen with unusually large mass ratios (Mu:H:D:T=1/9:1:2:3). The muon spin rotation technique (mu SR) relies on the facts that (1) the muon produced in pion decay, pi(+) –> mu(+) nu(mu), is 100% spin polarized and (2) the positron from muon decay is emitted preferentially along the instantaneous muon spin direction at the time of the muon decay. In transverse field mu SR (TF-mu SR), the precession of the muon spin in muonium substituted radicals is directly observed by detecting decay positrons time differentially. From observed radical frequencies, the hyperfine coupling constants (A(mu)) of C(2)H(4)Mu, C(2)D(4)Mu, (13)C(2)H(4)Mu, C(2)F(4)Mu, and C(2)H(3)FMu are determined. In the longitudinal field avoided level crossing (LF-ALC) technique, one observes the resonant loss of the muon spin polarization caused by the crossing of hyperfine levels at particular magnetic fields. The LF-ALC method together with the information on A(mu) obtained from TF-mu SR allows one to determine the magnitude and sign of the nuclear hyperfine constants at alpha- and beta-positions. Results are compared with hydrogen substituted ethyl-radicals and isotope effects are discussed.