A. Otto

Excitation of nonradiative surface plasma waves in silver by the method of frustrated total reflection

A new method of exciting nonradiative surface plasma waves (SPW) on smooth surfaces, causing also a new phenomena in total reflexion, is described. Since the phase velocity of the SPW at a metal-vacuum surface is smaller than the velocity of light in vacuum, these waves cannot be excited by light striking the surface, provided that this is perfectly smooth.However, if a prism is brought near to the metal vacuum-interface, the SPW can be excited optically by the evanescent wave present in total reflection. The excitation is seen as a strong decrease in reflection for the transverse magnetic light and for a special angle of incidence. The method allows of an accurate evaluation of the dispersion of these waves. The experimental results on a silver-vacuum surface are compared with the theory of metal optics and are found to agree within the errors of the optical constants.

Surface-enhanced Raman scattering

On the basis of different types of experiments, the authors develop implicitly the model of surface-enhanced Raman scattering (SERS) of adsorbates on metal surfaces. The long-range enhancement by resonances of the macroscopic laser and Stokes field is separated quantitatively from the metal electron-mediated resonance Raman effect. The latter mechanism proceeds by increased electron-photon coupling at an atomically rough surface and by temporary charge transfer to orbitals of the adsorbates. This model can account for the chemical specificity and vibrational selectivity of SERS and (partly) for the SERS specificity of the various metals.

Theory of First Layer and Single Molecule Surface Enhanced Raman Scattering (SERS)

The first layer SERS effect is usually ascribed to dynamical charge transfer (DCT) between metal and absorbate, following a model by Persson. Experiments show that there is no first layer SERS effect at smooth noble metal surfaces, in spite of the surface resistance caused by adsorbates, which is also caused by DCT according to Persson. On the other hand, both SERS and SEIRA (surface enhanced infrared absorption) of ethylene on cold-deposited Cu display at low coverages exactly the same Raman active modes, which can be only understood by DCT. The apparent difference is attributed to special events of double scattering by DCT and atomic scale surface roughness, which prevent dephasing of the optical currents in the electromagnetic resonances. The double scattering model explains the observed change of the SERS background and SERS phonon spectra during annealing of the atomic scale roughness. Single molecule SERS is assigned to molecules within narrow and small contact zones of metal particle. The vibrations are effectively excited by the optical currents flowing through the molecule, without becoming dephased. “Blinking” (sudden changes in intensity and band positions) is assigned to thermally activated atomic scale jumps of the molecule, frozen out at low temperatures in agreement with experimental observations.

Surface enhanced Raman spectra of oxygen adsorbed on silver

Abstract Silver films deposited in ultra high vacuum at 120 K and exposed to relatively low doses (up to 1000 L) of molecular oxygen display Raman vibrational bands at 697 and 1053 cm −1 , assigned to nominally peroxidic and superoxidic dioxygen. Oxygen exposure during operation of the ion getter pump leads to further Raman bands which might indicate further adsorption states of oxygen at rough silver surfaces.

On the nature of “SERS active sites”

Abstract Active sites of short range enhancement in surface enhanced Raman scattering (SERS) of adsorbates on cold deposited, porous silver films have been envisioned either as “cavity sites with local electromagnetic resonances” or as “sites of atomic scale roughness with photon driven charge transfer between metal and adsorbate”. Whereas SERS of CO and N2 is observed at low coverages, the Raman signal of physisorbed O2 is below the noise level, even at monolayer coverage of the “internal surface” or “filling of the pores”. On the other hand, SERS of ethylene and pyridine is quenched by some percent of a monolayer of oxygen. The Raman signal of multilayer condensed O2 is stronger for a “compact” film than for a porous film. These observations cannot be reconciled with the first hypothesis, but they are consistent with the second.

Model of electronically enhanced Raman scattering from adsorbates on cold-deposited silver

Abstract We propose an extension of the charge transfer model of surface enhanced Raman scattering (SERS) at silver surfaces. Within an incoherent approximation we include propagating hot electrons, created or annihilated by increased photon-electron coupling at disordered (internal) surfaces. The hot electrons are inelastically scattered by the adsorbates. In “shape resonances” of the free molecule the corresponding cross section is 14 orders of magnitude larger than the ordinary Raman cross section. We list results related to photon-electron coupling and inelastic scattering of electrons by molecules and discuss open problems.

Raman spectroscopy of pyridine adsorbed on single crystal copper electrodes

The attenuated total reflection (ATR) method was applied to obtain the first layer enhancement in the case of pyridine adsorbed on Cu(100), (111) and (110) and a (110) vicinal surface, the values being 3, 19, 40 and 83 respectively. By comparison of the Raman bands and the enhancement at the Cu(111) electrode with UHV measurements, these bands are assigned to species adsorbed at surface defects of unknown structure. The strong potential dependence of the Raman signal can be explained by a photon-driven charge-transfer process. An electronic Raman scattering process causes the background signal, but the face specificity is not yet understood. The stability of the surface structure with the variation of the electrode potential was shown in the case of the (110) vicinal surface.

Surface resistivity and vibrational damping in adsorbed layers

Abstract We derive a simple relation between the change in dc resistivity Δϱ of a thin metallic film due to adsorption of molecules on the film surface and the electron—hole damping (lifetime τ) of the parallel frustrated translations of the adsorbates. From the measured Δϱ for several different adsorbate systems, we deduce the corresponding τ which ranges from ∼ 10 −12 for chemisorption systems to ∼ 10 −9 s for physisorption systems. The result of model calculations for the damping of parallel frustrated translations are presented for three limiting cases of the adsorbate-substrate bond, namely for covalent, ionic, and van der Waals bond.

On the quantitative measurement of the roughness spectrum of silver films

Abstract We report measurements of the scattered light intensity, in order to evaluate the absolute amplitude of the spatial surface roughness spectrum. Detailed investigations were performed, to test whether the first order scattering theory was applicable. This was found to be the case for a silver film surface, whose rms roughness was 3–4 A in the range of spatial frequencies 0.5–1.5 × 10 5 cm −1 . The roughness spectrum was evaluated consistently by 3 different methods. For a silver surface, which was about 10 times rougher, strong higher order scattering effects were observed, e.g. a decrease of the phase velocity of the surface plasmon. Contributions to the scattered light from interior roughness were negligible.

On the contribution of classical electromagnetic field enhancement to raman scattering from adsorbates on coldly deposited silver films

At silver films, deposited at temperatures of about 10 K, the Raman signal of the most tightly bound CO (less than 20% of a monolayer) is about 7×10 6 fold enhanced. In contrast to IR spectroscopy, no Raman signal of physiorbed CO (more than 80% of an adsorbed monolayer) was observable. The Raman signals of the C−H and C−C stretch modes of C 2 H 6 saturate at a coverage of about 40% and 4% of a monolayer, respectively, and are 6×10 4 , respectively 1.9×10 4 fold enhanced. These differences cannot be explained by “classical electromagnetic enhancement” (CEME). Precoverage with far less than a monolayer of oxygen quenches the first layer signal This is no classical electromagnetic effect as demonstrated by reflection spectroscopy. There is no indication for CEME from coldly deposited (10 K) Ag films.