Picosecond dynamics of surface electron transfer processes: Surface restricted transient grating studies of the n-TiO2/H2O interface

Abstract

The surface restricted transient grating is demonstrated as a sensitive probe of ultrafast surface reaction dynamics. Studies of doped single crystal n-TiO2 (001) surfaces in air demonstrate linear trapping processes, assigned to crystal defects within the surface deformation layer, that limit carrier lifetimes to 5 ns. Direct in situ grating studies at photochemically active n-TiO2/H2O interfaces demonstrate that the dominant mechanism of interfacial electron transfer in this system involves thermalized hole carriers at the atomic surface. The dynamics are consistent with adsorbed OH− as the initial hole acceptor. In addition, optical generation of coherent surface acoustic modes is demonstrated. A detailed theory is presented for the grating excitation of the surface acoustics. Acoustic propagation in the H2O half-space of the TiO2/H2O liquid interface gives evidence for a phase change of the water layer at the polar TiO2 (001) surface to a solid phase.