Interaction of coadsorbed CH3Cl and D2O layers on Pd(111) studied by sum frequency generation.

Adsorption of methyl chloride and coadsorption of CH₃Cl and D₂O on Pd(111) surfaces at T=100 K have been studied under ultrahigh-vacuum conditions using femtosecond sum frequency generation (SFG) spectroscopy in the spectral regions of CH and OD bands. On the bare Pd(111) substrate, the CH₃Cl coverage dependence of the resonant SFG signal is consistent with a progressive molecular rearrangement starting at half saturation followed by the growth of two ordered monolayers in which the molecular axes are perpendicular to the surface. When CH₃Cl is adsorbed on top of predeposited D₂O on Pd(111), the SFG signals as a function of the CH₃Cl exposure indicate that methyl chloride is adsorbed onto D₂O through hydrogen bonding. On the contrary when the adsorption order is reversed the strong decrease of the CH₃ signal as a function of the D₂O exposure is explained by assuming that water molecules penetrate inside the CH₃Cl layers, leading to the formation of disordered CH₃Cl clusters. In all cases a nonresonant contribution due to molecular adsorption is observed and it shows a dependence upon surface structure and coverage significantly different from that of the resonant vibrational bands. [ABSTRACT FROM AUTHOR]