Dynamics of metal electron excitation in molecular dipole-surface collisions

A87 Surface Science 151 (1985) 521-530 North-Holland, Amsterdam

ADSORPTION

521

OF FLUORINE ON Pt(100)

E. BECHTOLD and H. LEONHARD InsMut fir Physikalische Received

1 October

Chemie, Uniuersiiiit Innsbruck, A -6020 Innsbruck, Austria

1984; accepted

for publication

12 November

1984

The adsorption of elemental fluorine on a Pt(lOO)-(5 X 20) surface has been studied by thermal desorption mass spectrometry, LEED and work function measurements. Fluorine atoms are adsorbed with high efficiency via a precursor state. At monolayer coverage on heating a (1 x 1)F structure is formed which is interpreted to consist of fluorine ‘atoms adsorbed on a (1 x 1) substrate. In this stage the work function is increased by A+ = 0.5 eV. Thermal desorption yields fluorine atoms which are desorbed in a narrow peak with the peak maximum shifting to higher temperatures (925-950 K) as the initial coverage increases. The typical features of the desorption traces seem to be effected by the transformation of the (1 X 1) into the reconstructed (5 X 20) substrate structure occuring during desorption. The low coverage desorption energy is estimated to be 54 kcal mol-t. At high coverage a broad unresolved additional fluorine atom peak emerges at the leading edge of the main peak. Fluorine exposures beyond saturation of the chemisorption layer produce a surface fluoride which is desorbed as PtF, in a very sharp peak at 740 K.

Surface Science 151 (1985) 531-542 North-Holland, Amsterdam

531

DYNAMICS OF METAL ELECTRON DIPOLE-SURFACE COLLISIONS Z. KIRSON

EXCITATION

IN MOLECULAR

Department of Physical Chemistty and the Fritz Haber Research Center /or Molecular Dynamics, Hebrew University, Jerusalem 91904, Israel and Department of Chemistry Tel Aviv University, Tel Aviv 69978, Israel

R.B. GERBER Depariment of Physical Chemistry and the Fritz Haber Research, Center for MoIecular Dynamics, Hebrew University, Jerusalem 91904, Israel

A. NITZAN Department

of Chemistry,

Tel Aviv University, Tel Aviv 69978, Israel

and M.A. RATNER Department Received

of Chemistry, Northwestern

19 June 1984; accepted

University, Evanston, Illinois 60201, USA

for publication

8 November

1984

Electron-hole pair excitations in low energy collisions of dipolar molecules with metal surface are studied in the framework of one-dimensional independent electron model. The motion of the incoming (rigid) molecule is treated classically and is coupled to the electron dynamics, which is treated quantum mechanically through the timedependent self-consistent field (TDSCF) approximation. Model calculations were carried out for NO and HCl molecules colliding with surface of

A88 Li and Al. The average fraction of collision energy converted to electron-hole pair excitation (A E)/E and the probability for trapping due to this process were evaluated for collision energies in the range 0.01-10 eV. The effects of the pure dipolar electron-molecule interaction is compared to that of the short range interaction. It is concluded that the (screened) long range dipolar part of the electron-molecule interaction can play an important role in the collisional energy transfer between dipolar molecules and metal surfaces.

543

Surface Science 151 (1985) 543-552 North-Holland, Amsterdam

THEORETICAL MICROSCOPE

DISCUSSION

OF THE SCANNING

TUNNELING

J. BONO and R.H. GOOD, Jr. Deparrmenr USA Received

of Physics,

10 October

The Pennsyhwzia

1984; accepted

State

Unioersity,

for publication

Untuerstiy

2 November

Park,

Pennsylvania

16802,

1984

A free-electron model of the device is developed. The electrodes are taken to be plane but with a hemispherical projection on the cathode. The potential energy of the tunneling electron is found numerically by summing, in a special way, over many of the images in the electrodes. The penetration probability is assumed to be the WKB result along the central line, effective over the cross section of the projection. The results are in agreement with the first experimental reports.

Surface Science 151 (1985) 553-569 North-Holland, Amsterdam

553

MANY-BODY INTERACTIONS AND EFFECTIVE POTENTIALS IN TWO- AND THREE-DIMENSIONAL SOLIDS AND ADSORBATES David NICHOLSON Deparrment Received

of Chemistry,

14 August

Imperial College, London S W7 2A Y, UK

1984; accepted

for publication

29 October

1984

Calculations have been made of the lattice energy in two-dimensional CP and three-dimensional fee arrays and for self-adsorbed monolayers, taking Ar as an example. The potentials include “true” 2-body interaction, 3-body terms up to triple quadrupole interactions and the fourth order 3-body triple dipole-term all of which originate from perturbation theory and which are supplemented here by repulsive terms and exchange interaction damping. A comparison has been made with pairwise calculations using the effective Lennard-Jones 12-6 potential. In addition the Lifshitz potential and the McLachIan expression for substrate mediated interaction have been investigated from the point of view of a discrete atomic model. The first two terms of a series expanded form for the Lifshitz potential correspond to the undamped dipole terms obtained from perturbation theory, provided that integration over a continuum is replaced by summation. There are deviations between the two approaches for higher order many-body terms; in particular the important fourth order 3-body term does not appear in the series expansion. It also emerges that the first few terms from an expanded McLachIan energy are in rather close agreement with results from perturbation theory. These calculations are useful in assessing both the relative contributions