Two-dimensional spectroscopy for a two-dimensional rotator coupled to a Gaussian–Markovian noise bath

The dynamics of a system in the condensed phase are more clearly characterized by multitime correlation functions of physical observables than by two-time ones. We investigate a two-dimensional motion of a rigid rotator coupled to a Gaussian–Markovian harmonic oscillator bath to probe this issue. The analytical expression of a four-time correlation function of a dipole that is the observable of two-dimensional microwave or far-infrared spectroscopy is obtained from a generating functional approach. The spectra in the absence of damping are discrete and reveal transitions between eigenstates of the angular momentum quantized due to the cyclic boundary condition. For a weakly damped case, the two-dimensional spectrum predicts three echolike peaks corresponding to transition processes between the rotational energy levels, which cannot be observed in one-dimensional (linear-absorption) spectroscopy related to the two-time correlation function of the dipole [J. Phys. Soc. Jpn. 71, 2414 (2002)]. The two-dimensi...