Quantum theory of a two-dimensional rotator in a dissipative environment: Application to far-infrared spectroscopy

Quantum coherence and its destruction by coupling to a dissipative environm ent play important roles in time-resolved optical response. We study a two- time correlation function of a two-dimensional rotator coupled to a harmoni c-oscillator bath. Generating functionals of reduced density matrix element s for the rotator degrees of freedom are calculated by diagonalizing the to tal Hamiltonian with the use of unitary transformations and then performing path integrals. A closed-form expression of linear absorption spectrum for a dipole rotator, i.e., a Fourier transformation of the dipole two-time co rrelation function, is derived from the generating functionals characterize d by the bath spectral density. Based on the theory, the spectra for a meth yl rotation in a toluene are depicted for various damping constants and tem peratures. Because of the cyclic boundary condition that is constrained to fit the rotator degree of freedom, the energy states of the rotator in the absence of damping are discrete: the spectra consist of rotational branches , which correspond to change of the angular momentum. Owing to damping, the spectra exhibit a continuous band which is broadened as temperatures incre ase.