Picosecond dynamics of adsorbed dyes: A time-resolved surface second-harmonic generation study of rhodamine 110 on silica

The picosecond dynamics of adsorbed rhodamine 110 have been observed by time-resolved surface second harmonic generation (SHG). This technique reveals details of the coverage-dependent photophysics and photochemistry of adsorbed dyes which are not available from, for example, fluorescence decay experiments. In particular the ground-state recovery was found to be biphasic, with recovery times of a few picoseconds and tens of microseconds. This behavior is attributed to an ultrafast internal conversion of the initially excited state followed by either fast vibrational relaxation to the initial ground state or a disruption of the dye layer in the hot ground state leading to a metastable form of the ground state with a reduced hyperpolarizability. Other models are discussed, but none readily reproduce the time-resolved SHG data. The large difference between solution phase and adsorbate photophysics is attributed to the formation on the surface of dimers and higher aggregates. The existence of such aggregates is revealed in the absorption spectra. Stationary-state SHG measurements show that the most probable adsorption geometry is with the molecular plane of the dye perpendicular to the surface; this is consistent with the formation of cofacial aggregates. With polarization-resolved SHG it was shown that the adsorbate orientation is not coverage dependent, at least between 0.2 and 1 monolayer; however, a meaningful angle of adsorption could not be determined.