TPD NH3 HF H2O Au(111) - The influence o f a...

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The influence of adsorbate-absorbate hydrogen bonding in molecular chemisorption: NH 3 , HF, and H 2 0 on Au(111) Bruce D. Kay, Keith R. Lykke, J. Randall Creighton, and Stephen J. Ward Sandia National Laboratories, Albuquerque, New Mexico 87175 (Received 2 May 1989; accepted 30 June 1989) Hydrogen bonding plays a pervasive role in nature, be- ing important in such diverse areas as the nonideal gas be- havior and anomalous boiling points of associated vapors and liquids, the crystal structure of molecular solids, the homogeneous nucleation of atmospheric aerosols, and the binding of nucleic acid base pairs in biological systems. 1,2 Presently, surprisingly little is known about the role hydro- gen bonding plays in the molecular chemisorption of mole- cules on surfaces. In this note we report results of recent molecular beam and temperature programmed desorption (TPD) experiments which examine the adsorption-desorp- tion kinetics of three classically hydrogen bonded molecules, NH 3 , HF, and H 2 0, from a relatively chemically inert metal- lic substrate, Au ( 111 ) . Figure 1 displays representative TPD spectra for var- ious coverages ofNH 3 , HF, and H 2 0 adsorbed on Au( 111) at 85 K which are subsequently desorbed during a linear thermal ramp of2.0 K/s. The Au( 111) sample resides in an UHV surface analytical chamber (base pressure < 2 X 10- 10 Torr) and is dosed by a quadruply differentially pumped 300 K effusive molecular beam source having a flux of - 1013 molecules cm - 2 S - 1 and a cross-sectional area of -0.1 cm 2 [roughly 5% of the -1 cm diameter Au(111) sample]. This dosing source enables precise and reproduc- ible exposures to be attained without appreciable adsorption on surfaces other than the crystal target. The beam dosing technique also allows the absolute molecular sticking coeffi- cient to be directly determined by the beam reflection tech- nique of King and Wells. 3 At 85 K the molecular sticking coefficients for NH 3 , HF, and H 2 0 are always greater than 0.95 and are independent of coverage (including multi- layers) and incident angle (9;<60°) indicting the adsorp- tion kinetics are markedly non-Langmuirian. For each molecule the adsorption is molecular in na- ture, consistent with the chemically inert nature of Au ( 111 ), and in each case desorption is complete by 170 K indicating that the adsorbates are only weakly bound to the metallic substrate. Interestingly, the resultant TPD spectra are qualitatively different for the various species. The NH3 spectra exhibit a submonolayer peak whose maximum shifts to lower temperature (145 .
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TPD NH3 HF H2O Au(111) - The influence o f a...

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