Nonlinear and linear optical studies of the structure and dynamics of supercooled molecular thin films and metal surfaces

Linear and nonlinear optical techniques are used to study the dynamical and structural properties of a bare metal surface, single monolayer of molecules, and bulk molecular thin films. The resonance enhancement of Second Harmonic Generation (SHG) from a bare Ag(110) surface due to electronic transitions between surface states is characterized. Changes in resonant SH intensity due to temperature and adsorbate effects on surface state energies are well described by a quantum mechanical treatment of the second order susceptibility of the Ag surface. A fit of the experimental SH intensity to the model allows information on the relaxation dynamics of the surface state electrons to be extracted. By exploiting surface state resonant SHG as a highly sensitive probe of surface coverage we are able to monitor in real time the adsorption and desorption of a single monolayer of water on the Ag(110) surface and reveal the existence of a wetting/dewetting transition at 133.5 K.; The structure and dynamics of bulk amorphous solid and supercooled liquid molecular thin films are probed using linear and nonlinear optical techniques. SHG in combination with Rayleigh scattering exposes the sudden formation of small heterogeneous domains in water and methanol films at particular deposition temperatures. Linear reflectivity is used to identify numerous stable and metastable phases of methanol. The thermal pathway, with respect to the glass transition temperature, Tg, is important in determining film structure since the slow dynamics near Tg produce a previous-state memory effect. Furthermore, a crystallization transition to a metastable state is observed below the true crystallization. This additional transition occurs only in films deposited at temperatures below their onset of heterogeneity. The kinetics of methanol crystallization near the glass transition temperature are investigated and an abrupt change in the nucleation mechanism at T g is observed. Finally, reflectivity reveals that water films grown at temperatures above the onset of heterogeneity access a new and distinct state of amorphous water. This heterogeneous amorphous phase appears to be more stable than cubic ice as evidenced by its higher desorption activation energy.