Picosecond excited state dynamics and excitation transport in solution and on surfaces

Time correlated single photon counting was employed in studies of excitation transport (ET) depolarization in 2- and 3-dimensional disordered systems. Reduced concentrations ranged up to ~3 in ethylene glycol solutions of DODCI (3-dimensional system) and ~5 in Langmuir-Blodgett monolayers containing octadecyl rhodamine B (2-dimensional system). Discrepancies between experiment and current ET theories (3-body Gochanour-Andersen-Fayer and/or 2-particle Huber) are small when artifacts due to reabsorption, dimerization (and concomitant trapping), solvent reorganization, and molecular reorientation are minimized or correctly modeled. Discrepancies persist in glycerol solutions of DODCI; they are attributed to orientational correlation between chromophores. The orientational correlation apparently arises from 3-dimensional liquid glycerol structure and extends to ~R[subscript]0;An optical shot noise limited detection scheme was developed which enabled pump-probe transient absorption studies of submonolayer rhodamine 640 adsorbed on fused silica and ZnO. The ground state recovery dynamics on silica are coverage dependent due to excitation trapping by dye aggregates. In contrast, the recovery on ZnO is significantly faster and essentially coverage independent. This provides strong evidence for efficient dye → semiconductor nonradiative excitation decay. ftn[superscript]1DOE Report IS-T-1291. This work was performed under contract No. W-7405-Eng-82 with the U.S. Department of Energy