Disorder/reorientation dynamics of 4-methoxybenzylidene-4-n-butylaniline observed by heterodyne transient grating method

Molecular dynamics of 4-methoxybenzylidene-4-n-butylaniline (MBBA) in a liquid crystal cell after the perturbation of a UV pulse was observed using the polarization dependent heterodyne transient grating (HD-TG) method. The response of a refractive index change showed different signs depending on the probe polarization, and each response was composed of 4 exponential components. These components were assigned to disordering, 2 types of ordering, and molecular diffusion, judged from the sign of the refractive index change and the dependence on the grating spacing and the pump intensity. The time constants for the disordering and the ordering processes drastically increased for the pump intensity above 0.10 mJ/pulse. To explain the threshold, we proposed that disordering/ordering of domains were observed for the higher pump intensities, while an individual molecule is subject to disordering/ordering for the lower pump intensity.