Abstract
Ultrafast surface second harmonic generation (SHG) studies are reported for several oxazine dyes at the air/water interface. As we demonstrated previously [J. Phys. Chem. B 2001, 105, 3062] in our steady-state SHG studies of aqueous oxazine dye solutions, the SHG signals are almost entirely due to dimers. The transient SHG recovery is biexponential with time constants of 4−8 ps and 22−44 ps depending on the dye and the solution composition. The SHG recovery is one-photon resonant and is attributed to two parallel ground state recovery mechanisms. Transient absorption signals were also measured for bulk dye solutions of water and methanol, and the results indicate a fast (2 ps) solvation dynamics component as well as a longer component due to the excited state lifetime (>400 ps) of the dye monomer. Therefore, both decay times observed for the dyes at the surface are different than what is seen for bulk solution, and they resemble what has been previously reported for dye aggregates.
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