Molecular excited-state relaxation dynamics at the colloidal microparticle interface monitored with pump-probe second harmonic generation

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Time-resolved second harmonic generation is used to monitor the excited-state relaxation dynamics of molecules adsorbed to the surface of colloidal microparticles suspended in solution. The cationic organic dye, malachite green (MG), is adsorbed to the negatively charged surface of polystyrene sulfate microparticles in water. MG is photoexcited to the S 1 excited state by a 615 nm pump pulse. The time-dependent change of the S0 ground-state depletion is probed by second harmonic generation of an 800 nm pulse as a function of pump-probe delay to obtain a lifetime of 5.7 ± 0.4 ps. This excited-state lifetime is approximately three times longer than the corresponding lifetime at the air/water interface, showing the significant effect of the negatively charged surface on local friction, which is important in the energy relaxation of photoexcited MG. © 2012 American Chemical Society.

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Journal of Physical Chemistry B

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