Ultrafast processes at liquid interfaces investigated with time-resolved sum frequency generation

The molecules at the interface between two phases are subject to inherently anisotropic forces that result in its unique properties, structure, and dynamics [1-7]. The asymmetric environment is manifested in the equilibrium characteristics of the interface such as molecular orientation [8-10], chemical composition [11-13], and polarity [14-20]. When subject to outside stimuli, such as photoexcitation or an external electric field [1-7,21-23], the response of the interfacial molecules often differs from the dynamics observed in bulk media; for instance, transport kinetics [21-25], the dynamics of molecular rotation [26-33], solvation [29,34-40], energy relaxation [41,42], electron transfer [43], and chemical reactions [23,44,45] can be different at interfaces as compared to the bulk material. Since numerous practical devices rely on junctions between two materials for charge separation and biological systems make use of interfaces for specificity in transport and chemical reactions, it is of fundamental and pragmatic importance that equilibrium and dynamical aspects of interfaces be explored [1-3,46,47].