Subpicosecond spectroscopy of thioglycerol-capped CdS colloidal solutions: influences of the solvent composition and the pump pulse intensity

Abstract

The dependencies of the transient absorption of thioglycerol-capped CdS (TG-CdS) particles on the solvent and the pumping pulse intensity were investigated in water, dimethylformamide (DMF) and their mixtures by excitation with a subpicosecond laser pulse of λ max=396 nm to elucidate the more detailed contribution of hydrated electrons to the transient absorption of the surface modified CdS particles in longer wavelength region. It was observed that the transient absorption of TG-CdS in water–DMF mixed solvents, obtained immediately after the excitation, reflected the difference in the amount of the hydrated electron produced that depending on both the water content of the mixed solvents and the pump pulse intensity. As the relationship between the pump pulse intensity and the intensity (at 0.5 ps after laser excitation) of the transient absorption of TG-CdS at 720 nm which nearly corresponds to the absorption maximum wavelength of the hydrated electron is non-linear, it seems that the absorption of two or more photons per a particle is essential for the production of the hydrated electron. The production of hydrated electron in TG-CdS colloidal solutions seems to occur by electron ejection via an Auger-like process which needs two electron–hole pairs per a particle as in the case of non-capped CdS particles. However, even at the pump pulse energy of 160 μJ per pulse, where TG-CdS particles absorb about four photons per a particle, the production of the hydrated electrons was not observed in the water–DMF solvents with the water content of less than 40%. This result suggests that the mixed solvent containing water of more than about 50% is necessary for the production and stabilization of the hydrated electron following electron ejection from TG-CdS particles in water–DMF mixed solvents.