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| - Picosecond Dynamics of Adsorbed Dyes: A Time-Resolved Surface Second-HarmonicGeneration Study of Rhodamine 110 on Silica
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| - The picosecond dynamics of adsorbed rhodamine 110 have beenobserved by time-resolved surface secondharmonic generation (SHG). This technique reveals details of thecoverage-dependent photophysics andphotochemistry of adsorbed dyes which are not available from, forexample, fluorescence decay experiments.In particular the ground-state recovery was found to be biphasic,with recovery times of a few picosecondsand tens of microseconds. This behavior is attributed to anultrafast internal conversion of the initially excitedstate followed by either fast vibrational relaxation to the initialground state or a disruption of the dye layerin the hot ground state leading to a metastable form of the groundstate with a reduced hyperpolarizability.Other models are discussed, but none readily reproduce thetime-resolved SHG data. The large differencebetween solution phase and adsorbate photophysics is attributed to theformation on the surface of dimersand higher aggregates. The existence of such aggregates isrevealed in the absorption spectra. Stationary-state SHG measurements show that the most probable adsorption geometryis with the molecular plane of thedye perpendicular to the surface; this is consistent with the formationof cofacial aggregates. With polarization-resolved SHG it was shown that the adsorbate orientation is not coveragedependent, at least between 0.2 and1 monolayer; however, a meaningful angle of adsorption could not bedetermined.
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