Optical Pump-Probe Spectroscopy of Dyes on Surfaces: Ground-State Recovery of Rhodamine 640 on ZnO and Fused Quartz

In recent years, time-resolved fluorescence spectroscopy has been investigated for dyes adsorbed onto fused quartz [1] and semiconductors [2,3]. On quartz, the monomer fluorescence dynamics are nonexponential and tend to dominated by excitation trapping by dye aggregates; the phenomenological lifetime measured in the limit of low coverage is often comparable to the fluorescence lifetime observed in solution [1]. On ultraviolet-bandgap semiconductors like TiO2, much more rapid fluorescence decay is typically found [2,3], even at low coverage. The channel responsible for this accelerated decay on semiconductors is widely believed to be electron injection into the semiconductor space-charge region. However, the photocurrent efficiencies of liquid-junction solar cells with dye-coated single-crystal semiconductor photoelectrodes are generally small, and the possibility exists that dye electronic excitation may instead decay rapidly and nonradiatively into semiconductor modes [3]. To differentiate between these decay mechanisms, we have done optical pump-probe measurements of ground-state recovery dynamics of rhodamine 640 adsorbed on fused quartz and on ZnO at submonolayer coverages.</jats:p