Coupled harmonic oscillator models of carbon monoxide adsorbed on stepped, platinum surfaces

A4 90

Surface

INVERSE PHOTOEMISSION WITH K ON Pt(ll1) V. DOSE *, J. ROGOZIK

FROM CO CO-ADSORBED

*

Physrkalisches Institut der Universitiit Fed. Rep. of Germany

A.M. BRADSHAW

Science 179 (1987) 90-100 North-Holland, Amsterdam

Wiirzburg, Am Hubland,

and K.C. PRINCE

D-8700 Wiirzburg,

**

Fritz-Huber-Institut der Max-Plunck-Gesellschaft, Fed. Rep. of Germany

Furadayweg 4- 6, D-l 000 Berlin 33,

Received

26 August

16 June 1986; accepted

for publication

1986

For a pure CO layer on Pt(lll) inverse photoemission measurements reveal a Zn-derived band at 4.3 eV above E,. On co-adsorption with potassium this feature is shifted down to 2-3 eV above E, depending on the relative coverage of the two adsorbates. This lowering in energy of the observed 2r-derived level can be correlated with the observation of species exhibiting considerably reduced C-O stretching frequencies in vibrational spectroscopy of the same system. Both effects are ascribed to a weakening of the C-O bond due to a short-range interaction with potassium atoms. The reduction in the local electrostatic potential in the immediate vicinity of a K atom lowers the energy of the 2a level which then forms a stronger bond to the metal.

101

Surface Science 179 (1987) 101-118 North-Holland, Amsterdam

COUPLED HARMONIC OSCILLATOR MODELS OF CARBON MONOXIDE ADSORBED ON STEPPED, PLATINUM SURFACES Frederick M. LEIBSLE, Department Milwaukee, Received

Richard S. SORBELLO

of Physics and Laborutoty for Surfuce Studies, WI 53201, USA

25 April 1986; accepted

for publication

and Robert G. GREENLER

Umversity of Wtsconsin - Milwuukee.

15 July 1986

Harmonic oscillator models are used to explain recent experimental data on infrared absorption by CO molecules adsorbed on two stepped platinum surfaces. These data reveal only a lower frequency band at low coverage and only a higher frequency band at high coverage. Both bands exist over a range of intermediate coverages. The data are explained by a coupled-dipole model which includes the effects of electronic polarizability, the tilted orientation of CO molecules at step sites, and the electric field enhancement at step sites. The lower-frequency band is associated with CO molecules adsorbed on step sites and the higher-frequency band is associated with two-dimensional islands consisting of both step and terrace CO. The model explains the observed variation of frequency and intensity with coverage for CO adsorption on Pt(533) and Pt(432) surfaces. The model calculations indicate that the wavenumber for a single, linearly bonded CO molecule is about 9 cm- 1 higher on a terrace site than on a step site.