Local spectra of self-affine clusters of silver colloid particles recorded with subwavelength resolution by near-field spectroscopy are reported. Spectra were also simulated computationally. The observed and calculated near-field spectra consist of several resonances with highly location-dependent frequencies. The most highly resolved of these resonances correspond to individual surface plasmon ͑SP͒ normal modes. All of these features are only observable in the near field. Both theory and experiment also show that when excited by light in the SP region of the spectrum, the field-intensity distribution in the near field is very heterogeneous with most of the excitation concentrated in ''hot spots'' on the cluster surface that are strongly excitationwavelength dependent. This field-intensity localization provides a rationale for recently reported surfaceenhanced Raman enhancements in excess of

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The surface-enhanced Raman scattering spectra of a number of dye and colorless molecules adsorbed on deposited coloidal silver films were systematically studied as a function of power and position using a Raman microscope. The anti-Stokes portions of the spectra of the dyes reproducibly show line intensities much greater than what is expected on the basis of the equilibrium population of the excited vibrational states, even at the lowest incident light intensities used. This behavior was observed previously and attributed to optical pumping of vibrationally excited states of the molecules by unusually intense surface-enhanced Raman transitions, [Phys. Rev. Lett. 76, 2444 (1996)] suggesting either uncommonly large Raman cross-sections or very intense local field strengths exceeding those encountered in the most powerful currently available lasers. Based on this work, however, we ascribe the apparently large anti-Stokes intensities primarily to a difference in the Stokes and anti-Stokes Raman cross-sections resulting from resonance or pre-resonance Raman processes in the adsorbate–surface complex rather than to strongly nonequilibrium populations in the molecular vibrational states. Finally, we observed no significant inhomogeneity in the Raman enhancement in the images of the deposited silver coloid samples down to spatial resolutions of ∼1 μm.

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...