Slow relaxations in the supercooled state of a model diatomic molecular liquid

We report results of molecular dynamics simulation for the supercooled state of a model diatomic molecular liquid possessing a rotational degree of freedom. We develop a new technique to extract the contribution of the internal motion in the intermediate scattering function and in the imaginary part of the generalized susceptibility, and show that the rotational relaxation becomes well separated from the center-of-mass relaxation as temperature is reduced. We identify three processes in the susceptibility. The relaxation time of the α relaxation produced mainly by the slow relaxation of the center-of-mass motion can be fitted well with the Vogel-Fulcher low. A secondary peak is observed at the high frequency side of the α-peak, and is shown to be produced by the internal degree of freedom. The features of the secondary relaxation process are consistent with the Johari-Goldstein process. The relaxation process of an elongate model and a rigid model molecular liquid are also studied and we show that the Johari-Goldstein process cannot be identified in the elongate model system.