TIME-RESOLVED OPTICAL 2ND-HARMONIC GENERATION MEASUREMENTS OF PICOSECOND BAND FLATTENING PROCESSES AT SINGLE-CRYSTAL TIO2 ELECTRODES

Time-resolved optical second harmonic generation (SHG) is used to prob e the variations in the surface electrostatic fields that occur under pulsed UV (supra-bandgap) illumination at a n-TiO2(001) electrode. The interfacial SHG is dominated-by the electric field-induced second har monic response from the first 20 nm of the,semiconductor surface; this large SHG signal decreases sharply under UV illumination due to the c reation of a steady-state photogenerated hole population at the interf ace. An additional transient drop in SHG intensity is observed when th e pump and probe beams are overlapped temporally on the TiO2 surface. The time scale for this additional band flattening is determined by th e finite time required for photogenerated holes to migrate to the inte rface under the influence of the surface electrostatic fields. An aver age transit time of 25 ps that is independent of applied potential or solution composition is observed; this corresponds to a hole drift vel ocity of 4.0 x 10(4) cm s(-1) at the TiO2 surface. The SHG intensity i s found to return to its steady-state level in 3-4 ns due to-the remov al of the excess holes at the surface by, electrochemical charge trans fer and surface recombination processes.