Enhancement of the ESDIAD method for imaging bond directionality in chemisorbed species

Al77 disordering transition. For purposes of comparison. simulations of the commensurate phase at these temperatures are reported, together with a 15 K simulation of a phase that has been uniaxially compressed in the direction perpendicular to that of the UXI phase (UYI phase). The simulations indicate that the UXI phase is stable hut the UYI phase tends to transform mto domains of UXI; it is also concluded that the compression necessary to form the UXI phase from the commensurate does not produce a significant change in the out-of-plane ordering. at least at the lower temperature, but does bring about changes in the in-plane ordermg of these molecules.

Surface Science 169 (1986) 91-103 North-Holland. Amsterdam

91

ENHANCEMENT OF THE ESDIAD * METHOD FOR IMAGING DIRECTIONALITY IN CHEMISORBED SPECIES M.J. DRESSER **, M.D. ALVEY and J.T. YATES. Jr. SurJuce Science Center, Department Pittsburgh.

Received

Pennsyluunm

15260.

7 June 1985: accepted

of Chemrstrv,

L’nrwrsitv

BOND

of Plttshur~h,

USA

for publication

26 August

1985

The ESDIAD method for imaging adsorbate bond directions by photographic observation of positive ion angular distributions during electron stimulated desorption suffers from inherent low contrast due to background effects. The use of a digital acquisition system designed to overcome this difficulty in ESDIAD measurement is presented. Measurements on a Ni( 1 IO) single crystal substrate show the presence of a significant background signal due to soft X-ray generation by electron impact. By subtraction of the background signal. a significant enhancement of positive ion signal-to-noise ratio is achieved in ESDIAD, converting the ESDIAD method into a high contrast, high resolution surface measurement technique. Quantitative studies of the soft X-ray background have shown it to be linearly dependent on electron current density and electron energy, with no change in angular shape. These properties permit an accurate background subtraction procedure to be employed to significantly enhance the capability of the ESDIAD method.

Surface Science 169 (1986) 104-122 North-Holland. Amsterdam

104

ADSORPTION ON THE SURFACE OF IONIC EFFECT OF APPLIED ELECTRIC FIELD E.N. KOROL Pmmhecisk.v

Received

AND

THE

and O.Yu. POSUDIEVSKY

Institute

Prospect Nauki

CRYSTALS

of Physicul

31, KIeu-28,

1 February

Chemistry,

Academ_y of Sctences of the tikrurntan

SSR,

USSR

1985; accepted

for publication

21 October

1985

Adsorption of molecules on an ideal surface of ionic crystals and the effect of an external uniform electric field on this process have been investigated using a model Hamiltonian. which takes account of the Coulomb interaction between electrons and atomic cores of the system under consideration, all valence orbitals of the admolecule. as well as their nonorthogonality to the crystal wavefunctions, the substrate band structure. The binding energy as a function of the adsorbate-to-substrate distance is calculated for H,O and CO on NaCl and MgO (001) surfaces. The main attention is focused on the contributions of admolecule-substrate interaction of various types to the binding energy.