Structures, phase transitions, and nonlinear optical properties of ultra-thin molecular films

Ultra-thin molecular films with a thickness below a few hundred nanometers are currently of special interest in surface and material science/technology. Although many studies have concentrated on adsorbate-substrate and inter-molecular interactions, it still remain to be explored how adsorbate-substrate and inter-molecular interactions affect the growth and structure of ultra-thin molecular film. In this thesis, second harmonic generation (SHG), a nonlinear optical phenomenon, has been utilized, in combination with traditional surface techniques such as TPD and EELS, to investigate structure, growth mechanisms, and phase transitions in ultra-thin molecular films. The ultra-thin molecular films of our interest consist of small/moderate polyatomic molecules, such as benzene, pyridine, aniline, and water. Crystallization kinetics and premelting in pyridine films (within 100 nm thickness) have been characterized. Non-equilibrium nucleation, followed by pre-melting, was first observed for thin films of aniline. Various types of layer-by-layer structures for benzene, pyridine, and aniline (within 5 nm thickness) were observed, and the influence of both molecular geometry and interactions on nanostructures is discussed. Second harmonic Rayleigh scattering was introduced to monitor the glass transition of supercooled water produced in the form of a molecular thin film. This is the first reported optical detection of the glass transition for supercooled water. In addition, SHG was successfully used for the first time, to investigate nucleation and wetting/dewetting processes for a weakly binding system such as water molecules on silver surfaces. Finally, the temperature dependence and effect of adsorption on SHG resonantly enhanced by a surface-state transition of the Ag(110) surface was investigated