Susan M. Dounce

Crystallization at the glass transition in supercooled thin films of methanol.

The stability of an amorphous material depends on how fast and by what mechanism crystallization occurs. Based on crystallization rate measurements through optical reflectivity changes in supercooled methanol thin films, it is observed for the first time that there is a definitive and detectable change of the crystallization mechanism at the glass transition temperature T(g). For methanol glasses below T(g)=103.4 K, crystallization occurs as an interface controlled, one-dimension process at frozen-in embryo sites, while in the deep supercooled liquid phase above T(g) crystallization is diffusion controlled in two dimensions with a constant nucleation rate and an activation energy of 107.8(+/-4.7) kJ/mol.

The wetting-dewetting transition of monolayer water on a hydrophobic metal surface observed by surface-state resonant second-harmonic generation

The isothermal adsorption and desorption of monolayer water on a Ag(110) surface in the temperature range of 130-137 K were characterized by monitoring second-harmonic (SH) generation from the silver surface. The SH intensity resonantly enhanced by the silver surface-state transition is highly sensitive to the amount of silver surface area covered by water and allows the observation of an abrupt change in the adsorption/desorption behavior at 133.5 K. At temperatures below 133.5 K water wets the Ag surface in a two-dimensional structure with a measured desorption energy of 25.0 (+/-3.3) kJ/mol. At temperatures greater than 133.5 K water desorbs from three-dimensional clusters with a measured desorption energy of 48.3 (+/-2.2) kJ/mol, in agreement with temperature-programmed desorption measurements. This wetting-dewetting transition of water adsorbed on the silver surface at 133.5 K is supported by classical nucleation theory calculations.

Nonlinear optical probe of structures and phase transitions in ultrathin molecular films

The study of nanometer-thick molecular thin films deposited on a solid surface, due to recent technology applications, has become an important subject. Effective tools for unraveling the intrinsic structure within the molecular films and their growth mechanism, however, are still in the searching. Consequently, little is known about the structure and the most important factors controlling deposition of thin molecular films. This paper summarizes the demonstration showing that the nonlinear optical phenomenon- second harmonic generation- because of its symmetry properties can be used effectively to characterize the structure within the molecular films. Experiments show that disorder-order phase transitions, glass transitions, crystallization kinetics and nucleation processes, and interfacial molecular structure within the thin molecular films can be characterized. The nonlinear optical studies have revealed the mechanisms and established the most important criteria for the deposition and growth of ultrathin molecular films.