Molecular interactions are partitioned in SCF and correlation energy parts. It is shown that for atomic systems, one can join high quality ’’a priori’’ SCF calculations with a semiempirical estimate of the correlation energy, made using the standard long range multipolar expansion and corrected assuming the 3Σ+u state of H2 as a model (scaling the ’’size’’ of the atomic charge distribution as the ionization potential to the negative 2/3 power) to obtain very good agreement with the available experimental information.

High-resolution scanning-tunneling-microscope (STM) topographic images of vacuum-deposited Ag films are reported. Films were formed and imaged at 100 and 300 K. Images of films deposited at 300 K, annealed to 560 K, and then returned to 300 K are also presented. The topographic surfaces of the low-temperature films are found to be self-affine fractals with a local Hausdorft-Besicovitch dimension D =2.5. The low-temperature films exhibit intense surface-enhanced Raman spectra (SERS). Films deposited at 300 K do not possess significant fractal character and are not SERS active. We show that the apparent local fractal dimension obtained by analyzing STM topographic images depends critically on the algorithm used. Three such methods (cube counting, triangulation, and power spectrum analysis) are assessed. A method is proposed for obtaining reliable fractal dimensions by analyzing the experimental STM topographic images using several algorithms and comparing the results to a calibration curve generated by applying the same algorithms to simulated fractal surfaces of known HausdorA'-Besicovitch dimension. INTRODUCTIVESince the discovery of surface-enhanced Raman scattering (SERS) from pyridine adsorbed onto electrochemically roughened Ag (Ref. l) there has been continued interest in the optical properties of these and other structurally inhomogeneous metal surfaces. Specifically, metal films produced by metal vapor condensation onto very cold surfaces have been the subject of some discussion. While it is universally conceded that cold-deposited films are disordered due to the restricted mobility of the atoms landing on the low-temperature surfaces, there is some controversy regarding the specific morphology of these surfaces. Albano et al. suggested that these surfaces consist of many rather Oat grains separated by narrow intergrain regions. It was proposed that these intergrain regions are the areas of large electromagnetic enhancement and that only molecules localized in those intergrain regions benefit from the enhancement.The evidence for such a model has been discussed previously and need not be repeated here. There is also an early scanning-tunneling-microscope (STM) study related to cold-deposited films that purports to support this structural model of cold-deposited surfaces.Unfortunately, those images were recorded from cold-deposited films that had been annealed to room temperature. Consequently, it was not possible to surmise their original structure with confidence.The alternative view of these rough surfaces is given by Bales et al. , albeit indirectly.Sputtered films of refractory metals, deposited at room temperature, should display some of the characteristics of cold-deposited films provided the melting temperature of the metal is high enough. Such films have been investigated quite thoroughly. Briefly, they are randomly rough self-affine objects of two limiting types. If the metal vapor source is well collimated so that the atoms approach the substrate, more or less normal to the receiving surface, the...

Near-field optical microscopy with simultaneous topographic imaging shows that for a given excitation wavelength the electromagnetic optical fields excited in a self-affine film are highly localized at ''hot spots'' whose dimensions are often smaller than the wavelength. The pattern of hot spots varies greatly with excitation wavelength and does not correlate simply with the film topography. This observation is consistent with a recent theory of the optical response of self-affine fractal objects and also indicates that the locus of optical effects such as surface-enhanced Raman does not reside in special surface sites such as interstices. Additionally, we show that the conclusions of the theory, which was based on a quasilocal approximation, are qualitatively valid for self-affine films considerably larger than the exciting optical wavelength. ͓S0163-1829͑98͒01223-5͔ INTRODUCTIONSurface-enhanced Raman spectroscopy ͑SERS͒ and other surface-enhanced optical effects have been known for some 20 years. 1 It is recognized that the sometimes enormous increase in signal intensity comes about to a large extent from the increase in the magnitude of both the incident and the scattered electromagnetic fields resulting from the excitation of surface plasmon polaritons in the active system. These resonances lead to particularly large effects when one deals with small metal particles of an appropriate metal whose geometry is auspicious for field enhancement, i.e., particles with high local curvatures, or with assemblies of small metal particles where the interaction between the individual dipolar surface plasmons of the particles couple to produce normal modes. Recently a great deal of attention has been given theoretically to such assemblies, especially to assemblies with fractal character. 2 This is because many of the most efficient SERS-active systems are found to be fractal clusters of adsorbate-covered colloidal particles, and because the optical response of fractal aggregates have been shown to manifest a number of surprising physical attributes. In brief, for a periodic or compact assembly of identical coupled dipolar oscillators the spatial extent of the normal modes will span the entire assembly. That is, in general, in any normal mode, every oscillator is significantly excited. This will be true of both two-dimensional and three-dimensional clusters of oscillators. In contrast, it has been suggested that fractal clusters of oscillators will possess many normal modes that are significantly localized with respect to the dimension of the entire cluster 2 and that of the exciting wavelength. Moreover, it has been suggested that the pattern of localized excitation will vary substantially with the exciting wavelength and the polarization of the exciting fields. In a fractal cluster the individual oscillating elements are arranged in such a manner that they possess dilation symmetry so that magnification of portions of the cluster on a sufficiently broad range of length scales is self-similar. The localization results from...

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