Inelastic scattering of atoms and molecules from liquid crystal surfaces

Collisions of an atomic beam of He atoms with the surface of an aligned nem atic liquid crystal film (4'-pentyl-4-cyano biphenyl, 5CB) initiate a trans ient change in the alignment of the surface region of the film. The effect is greater when the He atoms approach perpendicular to the alignment direct or of the sample than when the approach is parallel to the director. The re laxation of the system back to its initial alignment has a temperature depe ndence which suggests that rotational diffusion is the relaxation mechanism . Laser induced fluorescence spectra of I-2 molecules scattered from the is otropic phase and from the nematic phase of 5CB indicate that molecules sca ttered from aligned 5CB are more internally excited than those scattered fr om an isotropic 5CB sample. In total, these results indicate that the dynam ics of collisions at the gas-liquid crystal interface are strongly dependen t upon the molecular order in the liquid crystal. (C) 1999 American Institu te of Physics. [S0021-9606(99)70216-3].