Surface vibration overview

Interest in the vibrational spectra of adsorbed molecules is at least 40 years old. The past ten years have seen the development of many novel techniques for determining the vibrational spectra of adsorbed species and this symposium brings together a state-of-the-art survey of these techniques. In one s ethusiasm for the recent advances made in any subject there is a tendency to forget the parent technique and its steady contribution to our knowledge. In this case, the parent is simple transmission infrared spectroscopy. This paper, therefore, is an attempt to briefly present an overview of some of the developments which have occurred in the application of transmission infrared spectroscopy to surface studies with emphasis upon results generated in the past 10 years. For more detailed information on work published prior to 1967 the reader is referred to three texts which have appeared on this subject (1-3). 

The identification of species adsorbed on surfaces has preoccupied chemists and physicists for many years. Of all the techniques used to determine the structure of molecules, interpretation of the vibrational spectrum probably occupies first place. This is also true for adsorbed molecules, and identification of the vibrational modes of chemisorbed and physisorbed species has contributed greatly to our understanding of both the underlying surface and the adsorbed molecules. The most common method for determining the vibrational modes of a molecule is by direct observation of adsorptions in the infrared region of the spectrum. Surface spectroscopy is no exception and by far the largest number of publications in the literature refer to the infrared spectroscopy of adsorbed molecules. Up to this time, the main approach has been the use of conventional transmission IR and work in this area up to 1967 has been summarized in three books. The first chapter in this volume, by Hair, presents a necessarily brief overview of this work with emphasis upon some of the developments that have occurred since 1967. 

To get an overview of the relevant potential energy surfaces, several stationary points of each system were calculated with the 6-311+G basis set and the B3LYP functional using the Gaussian package of programs [5]. For the structural identification of the expected species it was also necessary to obtain the calculated vibrational spectra. 

A key requirement for in-situ spectroscopic methods in these systems is surface specificity. At Uquid/Uquid junctions, separating interfacial signals from the overwhelmingly large bulk responses in linear spectroscopy is not a trivial issue. On the other hand, non-Unear spectroscopy is a powerful tool for investigating the properties of adsorbed species, but the success of this approach is closely linked to the choice of appropriate probe molecules (besides the remarkably sensitivity of sum frequency generation on vibrational modes of water at interfaces). This chapter presents an overview of linear and non-linear optical methods recently employed in the study of electrified liquid/liquid interfaces. Most of the discussion will be concentrated on the junctions between two bulk liquids under potentio-static control, although many of these approaches are commonly employed to study liquid/air, phospholipid bilayers, and molecular soft interfaces. 

Several recent overviews of principles and applications of Raman, FTIR, and HREELS spectroscopies are available in the literature [35-37, 124]. The use of all major surface and interface vibrational spectroscopies in adhesion studies has recently been reviewed [38]. Infrared spectroscopy is undoubtedly the most widely applied spectroscopic technique of all methods described in this chapter because so many different forms of the technique have been developed, each with its own specific applicability. Common to all vibrational techniques is the capability to detect functional groups, in contrast to the techniques discussed in Sec. III.A, which detect primarily elements. The techniques discussed here all are based in principle on the same mechanism, namely, when infrared radiation (or low-energy electrons as in HREELS) interacts with a sample, groups of atoms, not single elements, absorb energy at characteristic vibrations (frequencies). These absorptions are mainly used for qualitative identification of functional groups in the sample, but quantitative determinations are possible in many cases. 

Here, the PES of the anion lies dose to, but below, the neutral surface, and the lowest vibrational levels of the dipole-bound anion are located just below the corresponding vibrational levels of the neutral. These long-lived VFRs appear as narrow features below the vibrational thresholds in elastic or vibrationally inelastic electron scattering cross sections (Hotop et al. 2003). VFRs may also decay through the dissociative electron attachment mechanism. For an excellent, detailed overview, see reviews by Schulz (1973a, b) and Hotop et al. (2003). 

In this paper we discuss intermolecular potential energy surfaces and vibrational predissociation of van der Waals molecules. The discussion is meant to give an overview rather than to provide a review of all the experiments and theories of van der Waals molecules. Comprehensive reviews may be found elsewhere.Moreover the present discussion is biased toward a view of van der Waals molecules that we have been developing over the years at Indiana University. 

In this chapter, we provided an overview of the heavy atom substitution effects of the constituent ions in some ILs on the static properties such as liquid density, shear viscosity, and surface tension, along with the effects on the interionic vibrational dynamics. With respect to the static properties, we can summarize the heavy atom substitution effects in the ILs as follows. 

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