Secondary ion yields produced by keV atomic and polyatomic ion impacts on a self-assembled monolayer surface

A suite of keV polyatomic or 'cluster' projectiles was used to bombard unoxidized and oxidized self-assembled monolayer surfaces. Negative secondary ion yields, collected at the limit of single ion impacts, were measured and compared for both molecular and fragment ions. In contrast to targets that are orders of magnitude thicker than the penetration range of the primary ions, secondary ion yields from polyatomic projectile impacts on self-assembled monolayers show little to no enhancement when compared with monatomic projectiles at the same velocity. This unusual trend is most likely due to the structural arrangement and bonding characteristics of the monolayer molecules with the Au(111). Copyright 1999 John Wiley & Sons, Ltd.     

Sum-frequency observation of solvent structure at model chromatographic interfaces: acetonitrile-water and methanol-water systems

The adsorption of methanol-D2O and acetonitrile-D2O solutions at model chromatographic interfaces (octadecylsiloxane and quartz) was studied using sum-frequency spectroscopy. Methanol did not adsorb at either interface in detectable quantities, while acetonitrile adsorbs at the octadecylsiloxane- and quartz-solution interfaces in a concentration-dependent manner and is well ordered at the interface. Adsorption of acetonitrile was decreased by the addition of KCl at 10 and 100 mM. Acetonitrile adsorption was also observed during simulated gradient elution, demonstrating that adsorption of acetonitrile occurs on a time scale relevant to actual chromatographic separations. Examination of the OH stretch spectra of acetonitrile-H2O and methanol-H2O solutions at the interface revealed concentration-dependent changes in the acetonitrile-H2O spectra that are consistent with hydrogen bonding between interfacial water and acetonitrile, indicating that interfacial water is involved in mediating acetonitrile adsorption. The OH stretch spectra of methanol-H2O solutions showed no such changes.