Role of adsorbed water in diffusion of rhodamine 6G on glass surface

Diffusion of rhodamine 6G (R6G) adsorbed on glass surface was measured. A dilute solution of R6G was spin coated on a glass plate. A small area of the film was photo-bleached by a focused laser beam, and subsequent disappearance of the dark spot in the fluorescent image due to diffusion of the surrounding molecules was measured as a function of time. It was found that the diffusion of adsorbed R6G over the glass surface was enhanced strongly at high humidity. A simple model, which assumes an adsorption–desorption equilibrium of the dye molecules within a thin adsorbed water layer, is proposed.