ADIABATIC ROTATION OF EFFECTIVE SPIN .1. NEW INSIGHT INTO SPIN-ROTATIONAL INTERACTION

The effect of electron spin-rotational (SR) interaction in species wit h an effective-spin S-eff=1/2 is treated for molecular rotations repre senting an adiabatic perturbation to the electronic system. Based on t he adiabatic rotation of effective spin (ARES) concept as well as an t he transformation of the Hamiltonian to the rotating molecular frame a useful expression is obtained for the effective spin-Hamiltonian of S R interaction whereby the. spin-rotational tenser is represented in te rms of,a tenser gamma(($) over cap) (relating L + S to S-eff) instead of the gamma tenser (related to L + g(e)S). This relation is valid for rotating systems with arbitrary strength of spin-orbit coupling (SOC) . An example is provided to demonstrate the dependence of the gamma te nsor on the SOC parameter. It is shown that only in the case of weak S OC, the traditional representation of the SR Hamiltonian, is equivalen t to the more general ARES Hamiltonian. Furthermore, a direct relation between the ARES and the geometrical (topological phase) approach to the problem is pointed out. The ARES spin-Hamiltonian can be represent ed in terms of a U(2) non-Abelian gauge potential.