Coherent Cross-Polarization Theory for a Spin- Coupled to a General Object.
In: Journal of Magnetic Resonance, Jg. 143 (2000) ; Nr. 2, S. 243-254
Zeitschriftenaufsatz / Fach: Chemie
Zero-order av.-Hamiltonian theory is used to extend the product-operator description of coherent spin-spin cross-polarization to the case of a spin- coupled to a general object, like a mol. rotor or a quantum oscillator. The object, which is not necessarily in a Boltzmann equil. state, is assumed to have no interaction with the lattice and no internal relaxation capacity. The Bloch-Wangsness-Redfield (BWR) theory for incoherent processes like spin-lattice relaxation does not apply for such an isolated spin-object pair. Nevertheless spectral d. at the Larmor frequency, of key importance in BWR theory, also plays a central role in object-induced spin polarization. Spectral d. in our theory is represented by quantum operators J- and J+. If J- and J+ do not commute, the spin-object coupling may cause spin polarization in an initially satd. spin system. This represents a coherent mechanism for spin cooling, which in specific cases may lead to enhanced spin polarization above the thermal equil. value. A master equation is derived for general spin-object cross-polarization, and applied to the case of a spin pair inside a uniaxial rotor, and a spin coupled to a microelectronic LC circuit. (c) 2000 Academic Press.