Inelastic scattering of atoms and molecules from liquid crystal surfaces

Abstract

Collisions of an atomic beam of He atoms with the surface of an aligned nematic liquid crystal film (4′-pentyl-4-cyano biphenyl, 5CB) initiate a transient change in the alignment of the surface region of the film. The effect is greater when the He atoms approach perpendicular to the alignment director of the sample than when the approach is parallel to the director. The relaxation of the system back to its initial alignment has a temperature dependence which suggests that rotational diffusion is the relaxation mechanism. Laser induced fluorescence spectra of I2 molecules scattered from the isotropic phase and from the nematic phase of 5CB indicate that molecules scattered from aligned 5CB are more internally excited than those scattered from an isotropic 5CB sample. In total, these results indicate that the dynamics of collisions at the gas–liquid crystal interface are strongly dependent upon the molecular order in the liquid crystal.