A. Priebe

The correlation between film thickness and adsorbate line shape in surface enhanced infrared absorption

Various experimental results on surface enhanced infrared absorption reveal asymmetric line shapes. Whereas the order of magnitude of the enhancement can be understood from electromagnetic field enhancement the unusual line shape remains without satisfactory explanation. An interaction with electron-hole pairs would lead to an asymmetric line but this should be restricted to the first monolayer. However, asymmetry is also observed for vibrations at larger distances from the metal-film surface. Here we show strongly asymmetric lines and their enhancement as a consequence of the interaction of adsorbate vibrations with surface plasmons of metal islands. Both the effects and also the baseline change can be estimated by a proper application of well established effective-media models.

Enhanced Infrared Absorption of SERS‐Active Lines of Ethylene on Cu

Vibrational modes of adsorbates on rough metal films can show surface enhanced Raman scattering (SERS) as well as surface enhanced infrared (IR) absorption (SEIRA). Both effects are a consequence of field enhancement and of a so-called chemical effect. Our experiments show that chemical contributions to SERS and to SEIRA from ethylene on Cu may have the same origin. We investigated ethylene adsorbed on ultrathin Cu films at about 100 K and below with IR transmission spectroscopy. At exposures up to 2 L the IR spectra show lines only at the positions of the very intense SERS E-bands. At higher exposures lines at the frequencies of the infrared active vibrations of ethylene molecules develop. All lines are stronger than one expects from molecular IR activities. Co-adsorption experiments performed with ethylene and CO show that E-lines in SEIRA do not occur after pre-coverage of Cu with CO.

An IR-transmission spectroscopical study of the influence of substrate surface defects on the morphology and the electronic structure of ultrathin Fe grown on MgO(001)

Abstract IR broad-band properties are sensitive to electronic transport and charge localization. This allows to investigate the electronic structure of ultrathin metal films by means of IR-transmission spectroscopy. As this electronic structure is determined by the interfaces of the film, the dynamic conductivity also reflects its morphology. We performed in-situ transmission spectroscopy in the middle infrared (MIR) during the evaporation of Fe on MgO(001) at room temperature in ultrahigh vacuum. We varied the quality of the substrate surface by using MgO(001) cleaved in air and cleaved in ultrahigh vacuum. The difference in the thicknesses for the onset of continuous transport properties of the growing films is well detected by their different IR-spectroscopical behavior. For continuous films, we calculate the thickness dependent scattering of electrons and the thickness dependent effective IR-optical oscillator strength from their IR-optical data. For MgO cleaved in air compared with cleavage in ultrahigh vacuum, the continuous thin film model becomes relevant at a smaller thickness, i.e. the substrate is covered faster in this case.

The enhanced infrared absorption on different types of metal nano-particle films

The surface enhanced infrared absorption (SEIRA) of adsorbates on nano-structured metal films is widely exploited in chemistry and biology, but the basic effects are only partially understood. Our IR transmittance studies of thin metal films under ultra-high vacuum (UHV) conditions showed two types of SEIRA lines. The systems investigated were C/sub 2/H/sub 4/ and C/sub 2/H/sub 6/, respectively, on cold-deposited Cu films, on transparent substrates (MgO, KBr) and on Cu films prepared at room temperature on the same substrates. For the room-temperature films only the IR-active vibrations of the adsorbates give SEIRA lines, which can be explained by interaction of the molecules with the electromagnetic field. On cold-deposited films also Raman-active lines of C/sub 2/H/sub 4/ appear in the IR-transmittance spectra. These lines saturate already below monolayer coverage. For C/sub 2/H/sub 6/ Raman bands like in SERS of C/sub 2/H/sub 6/ on cold-deposited silver are not observed. The appearance of the Raman lines of centrosymmetric molecules in the IR spectra is a first-layer effect and should be due to an indirect excitation mechanism involving metal electrons. For this mechanism roughness on an atomic scale and an adsorbate /spl pi/* state are important.

An IR spectroscopic study of silicon oxynitride films

Correlated to the ever shrinking dimension of the nowadays electronic devices and since the electronic factories try to stick at the silicon technology as long as possible new problems arise. One of these problems is the breakdown of the gate-oxide function of silicon dioxide layers at the necessarily small thickness of few nanometers and below. The main reasons of the failure of such silicon oxide are tunneling of charge carriers and diffusion of dopands through the ultrathin layer. The disadvantage of the SiO/sub 2/ may be overcome by inserting nitrogen atoms into the silicon-dioxide network. To do this with sufficient lateral homogeneity is a nowadays challenge for deposition and control. We report on an infrared spectroscopic study of silicon oxynitride films with respect to the lateral variation of the chemical composition on a silicon wafer.