Soft-landed ion diffusion studies on vapor-deposited hydrocarbon films

Cesium and hydronium ions were deposited with a "soft-landing" ion beam (1 eV) on n-hexane and 3-methyl-pentane vapor-deposited thin films on a Pt (111) surface at 27 K. Dielectric properties and ion migration were studied during the ion deposition and during a temperature ramp up to the desorption temperature of the molecular films. The ions were found to migrate through amorphous versions of these films as expected by simple viscosity models near 90 K with ion mobilities of about 10(-18) m(2) V-1 s(-1). No, or very limited, diffusion was observed through crystalline films. The n-hexane films crystallize during the ion motion. Analysis of this permits the estimation that average diffusional motion for a neutral hexane during crystallization is about 1 molecular diameter. (C) 1999 American Institute of Physics. [S0021-9606(99)72127-6].