Time-resolved second harmonic generation near-field scanning optical microscopy.

Novel scanning probe microscopy (SPM) spectroscopic techniques are currently receiving much attention. The increasing number of spectroscopic techniques, when combined with the high spatial resolution that is attainable with SPM, makes it possible to probe a large range of material characteristics on the nanoscopic scale. The ability to probe samples on the nanoscale with femtosecond time resolution promises to increase our knowledge of nanoscale electronic devices and their dynamics. Several time-resolved spectroscopic probes have recently been demonstrated in combination with SPM techniques. Ambrose et al. as well as Xie and Dunn and Trautman and Macklin have performed fluorescence lifetime measurements in combination with near-field scanning optical microscopy (NSOM), and found that the excited-state lifetimes of emissive dye molecules were sensitive to the proximity of a metal-coated NSOM probe. This change in lifetime was explained in terms of energy transfer between the dye and the metal coating of the probe. Transient linear reflectivity, 6] transient transmissivity, and transient absorption NSOM techniques have also been demonstrated. Shigekawa and co-workers have also demonstrated transient conductivity scanning tunneling microscopy (STM) on a GaAs sample using resonant, femtosecond-pulsewidth excitation. Time-resolved second harmonic generation (TRSHG) has been utilized in far-field measurements of changes in molecular orientation at interfaces, in dynamic studies of carrier lifetimes in semiconductors, and as a probe of coherent phonon relaxation. The magnitude of the second-order induced polarization, P produced in a sample (the square of which is equal to the magnitude of the detectable SHG signal) is proportional to the elements of a nonlinear susceptibility tensor reducing agent. Because of the interference of TPA derivatives, our potential calibration with Fc/Fc couple may not be very accurate either, which leaves little room for accommodating the contradictory results. In summary, a stable and electrochemically reversible product can be voltammetrically detected after TPA is anodically oxidized in acetonitrile. Although it is not yet certain whether the observed redox wave is linked to the long-awaited reducing radical responsible for the generation of ECL excited state, the finding is an important clue to understanding the oxidation of TPA and ECL generation. With many follow-up reactions and short-lived intermediates, the anodic oxidation of aliphatic amines is very complicated. It is understandable that a convincing identification of the wave in question will need more experiments, which are under way in our laboratories and further results will be reported in due course.