Nuclear-Spin-Symmetry Conversion of Tunneling Methyl Groups

Conversion from E to A proton spin symmetry of methyl groups undergoing tunneling rotation at 4.2 K is observed by means of nuclear magnetization induced by resonant processes which, besides symmetry conversion, involve simultaneous flips of nuclear and impurity electronic spins. Nonequilibrium populations of spin symmetries, frozen in at 4.2 K, relax when electronic Larmor and tunneling frequencies are close. A spread of tunneling frequencies inhomogeneously broadens the resonance which exhibits "hole burning" and similar distortion effects.