Ultrafast response of water near hydrophobic and hydrophilic surfaces

Ultrafast response of water near hydrophobic and hydrophilic surfaces is investigated by femtosecond pump-probe ellipsometry. Pump and probe pulses are from a dual Ti:sapphire laser with stable difference repetition rate. Every difference repetition rate, time delay is swept the whole pulse-to-pulse interval without an optical delay stage. Pump pulses induce heating and acoustic vibration to a Pd surface. The Pd surface is modified by thiol chemistry. Thiols with --OH and -CH3 end groups generate uniform hydrophilic and hydrophobic surfaces, and these mixtures modulate hydrophobicity two-dimensionally. Probe pulses with circular polarization impinge at the Brewster's angle and are analyzed by a polarizer. The transient ellipticity shows a refractive index change of water by thermal conductance and novel insight into the peculiar qualities of interfacial water.