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The infrared band intensities of crystalline n-alkanes and polyethylene decrease nonlinearly with increasing temperature. For all modes except the C-H stretches, the decrease is large and far exceeds that expected from density and refractive index effects. Similar anomalous decreases occur for the odd n-alkanes at their principal solid-solid (orthorhombic-to-hexagonal) phase transition. Further decreases occur in going to the liquid and gas phase. The intensities of the methylene bending and rocking fundamentals for the gas at 300 K are about I/, those of the crystalline solid at 77 K. A good correlation between the temperature coefficients of the intensity and of the lateral expansion is found for the crystal. This relation suggests that low-frequency modes play an important role in determining the temperature behavior of intensities. A mechanism involving low-frequency modes is proposed that appears to qualitatively explain our experimental results. The sensitivity of intensities to temperature and phase must be taken into account in infrared studies of poly(methy1ene) chain systems and in the transfer of observed and calculated gas-phase intensities to the condensed state. Similar temperature behavior is expected for Raman intensities and for other flexible chain molecules. IntroductionInfrared intensities are defined experimentally by 1(1)where b is the pathlength in the sample, p is the sample density, and A, is the absorbance (to the base e) at frequency v in wavenumbers. The measurement of infrared intensities has been largely confined to small molecules in the gas phase. Large molecules have received relatively little attention and the temperature behavior of their intensities even less. Yet, in an obvious way the temperature behavior of infrared intensities in the condensed state bears directly on the use of infrared spectroscopy for quantitative analysis and structural investigation in cases where temperature is a variable. The present work concerns the poly-(methylene) chain and thus is relevant to studies dealing with how temperature affects the structure of chain-molecule assemblies, such as lipid bilayers, micelles, and hydrocarbon polymers. Such studies have become common by virtue of modern FTIR instrumentation, but the question of how intensities are intrinsically affected by temperature has remained essentially unaddressed.The paucity of experimental studies on solids is probably partly accounted for by the expectation of only a small intrinsic temperature effect. A priori, such an expectation might seem well founded since the Einstein coefficients, which determine the intensity of a spectral line associated with the transition between two isolated energy levels, do not change with temperature. In the near-and mid-infrared regions, small intensity changes with temperature would be expected from the Boltzmann factors and from changes in the density of the sample. However, as we shall show, these factors fall far short of accounting for the large effects that are observed.The present study concern...

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The first application of the scanning tunneling microscope to determine a previously unknown adsorbate orientation is presented for naphthalene adsorbed on Pt(lll). Measurements in ultrahigh vacuum explore the details of molecular organization for both ordered and disordered systems and allow tentative assignment of the molecular binding site. Statistical analysis of the molecular packing within closepacked domains is inconsistent with the expected ordered structure for this system. Real-space imaging at submonolayer coverage reveals discrete molecular rotation among allowed orientations and translation between adjacent binding sites at room temperature.PACS numbers: 68.35.Bs, 61.16.Di, 68.55.Ln The ability of the scanning tunneling microscope (STM) to produce high-resolution images of molecule/ metal systems has been well demonstrated. 1_3 Molecular imaging has proven to be possible for those adsorbates which are highly localized, either due to packing constraints or to strong interactions between the molecules and the metal surface. For systems meeting this limitation, STM is on the verge of assuming its eventual role as a routine analytical probe of molecular adsorbate structure. This Letter describes the first application of the STM method to explore adsorbate organization in a system where unresolved questions remain, rather than verification of well-documented proposed structures.We describe the STM imaging of naphthalene adsorbed onto Pt(lll), which allows determination of the absolute molecular orientation, tentative assignment of the adsorbate binding site, and differentiation between ordered and disordered overlayers. Aspects of the molecular interactions which influence packing and domain formation are characterized. We observe novel molecular organization which meets the known lowenergy electron-diffraction (LEED) symmetry requirements for this system through pseudo-ordering rather than via the predicted perfect unit-cell structure. This observation has important implications for the utilization of LEED to infer structure in complex molecular systems. We report the first STM detection of molecular rotation among permitted orientations and discrete translation between adjacent binding sites for molecules not confined within close-packed domains. The influence of segregated impurities on both the molecular adsorption and the step formation on the metal substrate is also investigated for the first time.Previous investigations of naphthalene adsorbed on Pt(l 11) have revealed the existence of both ordered and disordered overlayer structures. 4-6 The disordered structure, resulting from room-temperature exposure to naphthalene vapor, yields only a diffuse, partially segmented j -order ring in LEED. The ordered overlayer is achieved either by maintaining the substrate at a tem-perature between 100 and 200 °C during the deposition or by annealing a disordered sample within this temperature range (molecular decomposition occurs above 200°C). 6 Auger electron spectroscopy (AES) and work-function measurement...

The scanning tunneling microscope has been used to distinguish among a series of related molecules on Pt(l 11) by details of their observed structures. The electronic isomers naphthalene and azulene can be recognized, as can three structural isomers of monomethylazulene, as well as dimethyl-and trimethylazulene. The number and position of substituents affect the diffusion rates, sticking coefficients, and orientations. A simple methodology based on extended Hiickel theory, including the substrate, generates electron and hole density plots which show very good agreement with the data.PACS numbers: 6I.16.Ch, 31.20.Pv, 68.35.Bs, 82.65.My Chemisorbed molecules on surfaces are relevant to many chemical processes, including catalysis, corrosion, and etching. The ability to view such processes on an atomic scale in real time promises great progress in understanding and improving these processes. The development of the scanning tunneling microscope (STM) has now made possible the imaging of molecules on surfaces with near-atomic resolution [1] enabling, for example, studies of adsorbate organization [2] and analysis of internal molecular structure [3l. Real-space observation of surface reactions has already been achieved for simple systems such as the dissociation of ammonia on Si(lll) [4], the oxidation of Cu(llO) [5], and the conversion of ethylene to ethylidyne on Pt(lll) [6]. More complex molecular reactions, however, will require an improved ability to distinguish subtle features of the adsorbate images in order to identify reactants, intermediates, and products. We have imaged a series of related molecules on Pt(l 11), including naphthalene, azulene, and a variety of methyl-substituted azulenes, to demonstrate the efficacy of the STM for recognizing individual species and for distinguishing among molecular isomers in mixed overlayers. These systematic observations have also assisted development of an image computation procedure based on extended Hiickel theory (EHT). The extremely good agreement between calculated and observed images indicates that this computational method can provide a predictive tool for STM molecular imaging. Analysis of the computed images suggests that the STM probes electronic states formed by mixing molecular and metal states near the Fermi level.The multichamber ultrahigh vacuum STM apparatus has been previously described in detail [7]. The Pt(l 11) crystal was cleaned by Ar-ion bombardment (500 eV) and flash annealed to 850°C. The sample was generally cooled for 45 min to 1 h to allow the sample to equilibrate to room temperature. Molecules were dosed through a leak valve from a gas manifold heated to main-tain the molecular vapor pressure near 1 torr. Further details of sample preparation are given elsewhere [2,8]. The constant current STM measurements were mostly performed with positive sample bias, ranging from 0.05 to 1.5 V, with tunneling currents of 0.5-4 nA, and with typical image acquisition times of 5-10 min. Low and high resolution molecular corrugations were approximately I ...

Articles you may be interested in A modification of the image dipole selection rules for surface Raman scatteringWe report the results of an experimental survey determining the contributing factors for Raman spectral activity of molecules adsorbed on metal surfaces. The systems investigated are benzene, pyrazine, and s-triazine physically adsorbed onto the ( 111) and (110) faces of single crystal silver. We consider the relative importance of three factors: symmetry reduction, image dipoles (metal optics), and quadrupole polarizability contributions. The experimental data may be explained satisfactorily by involving both the adsite geometry and image effects in the theoretical selection rules. Quadrupole interactions appear neither necessary nor sufficient to predict the proper surface spectra.

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