Averaged local field intensities in composite films

A270 calculated. It is shown that these results can be qualitatively understood by folding three times the dispersion relation of the (001) face which is the crystallographic plane of the terrace. In the direction parallel to the step, the dispersion relations are obtained by the superposition of the sagittal and transverse dispersion relation of the (001) face. The (117) configuration introduces a coupling between these two polarizations and some resonances appear. A general rule which enables one to predict qualitatively the shape of the dispersion relation for any vicinal surface of type ( l l m ) is expressed,

124 AVERAGED Zvi KOTLER

Surface Science 130 (1983) 124 154 North-Holland Publishing Company LOCAL

FIELD

and Abraham

INTENSITIES

IN COMPOSITE

FILMS

NITZAN

Department of Chemistry. Tel A t~iv Unit~ersi(v, Tel A t:it, 69978. Israel

Received 28 December 1982 The photophysical properties of molecules adsorbed in composite films (e.g. surface island films) depend on the local electromagnetic field within the film. The ratio between the average field intensity (IEI 2) in the film and the intensity IEII 2 associated with the incident field is a measure of the electromagnetic contribution to the surface influence on molecular photophysical phenomena. This ratio depends on the film composition and morphology, on the dielectric properties of the pure components making the film and on the frequency, direction and polarization of the incident radiation. Calculations of this ratio as a function of these parameters for several models of composite films are presented. Image interactions and retardation effects as well as radiative damping and finite size contributions to the dielectric response of the films are taken into account. In addition, an estimate of the field inhomogeneity within the film is obtained by calculating also the ratio (IEl2)shell/IEiI 2 associated with the field in thin shells surrounding the dielectric particles which constitute the film.

Surface Science 130 (1983) 155-162 North-Holland Publishing Company EXCITON-INDUCED

PHOTOEMISSION

155 FROM

EXTRINSIC

SURFACE

STATES: N a l Lajos ERNST

*

Kaliforsehungs - lnstitut der Kali und Salz A G, Hannover, Fed. Rep. of Germat~v

Received 6 January 1983; accepted for publication 16 February 1983 Spectral photocurrents emitted by Nat(001) crystal faces subsequent to air and ultrahigh vacuum cleavage have been measured in the near-ultraviolet excitation region at room temperature. On the air-cleaved face, on which a hydrate layer forms, a broad surface state band exists within the energy gap, with an upper edge at about 3.5 eV below the vacuum level. In general the major part of the states are empty, but may be filled by illuminating the sample at a wavelength corresponding to the lowest exciton peak: excitons can decay by dissociation at the surface, populating empty surface states. The excitons may also release photoelectrons, presumably from imperfections of the crystal samples used in the experiments. These photoelectrons lead to an enhancement of the photocurrent at hp > 5.1 eV.