Vibration internal adsorbate

A. Otto, International Conference on Vibrations in Adsorbed Layers, Julich, 1978. A. Otto, Surf Sci. 92 (1980) 145. 

The second property to be examined involves vibrations of adsorbates in particular, the internal vibration of molecular adsorbates. Here, the standard, sometimes naive, interpretations of shifts of vibrational frequencies between a free and an adsorbed molecule are based upon changes in the occupation of bonding and antibonding orbitals of the adsorbate, llie ab initio studies show that other factors are important and cannot be neglected. 

The frequencies of vibrational motion in crystals and molecules fall into the infrared (IR) spectral range. IR radiation illuminating the sample surface can therefore excite either phonons in crystals or vibrations of adsorbed molecules. Their excitation is most efficient when the frequency of the IR radiation is close to the internal vibrational frequencies of the sample. As a result, the optical response is a maximum at resonance and decreases when the detuning from resonance increases. This feature allows one to determine the vibrational frequencies or to identify molecules present at the surface if their frequencies are known. 

Here is the number of vibrational degrees of freedom of a single adsorbate, which has a value of three for atomic adsorbates, but includes additional internal vibrations for adsorbed molecules. If the surface vibrational modes are insensitive to the presence of adsorbates, the surface terms cancel and the expression simplifies to ... 

Vibrational energy states are too well separated to contribute much to the entropy or the energy of small molecules at ordinary temperatures, but for higher temperatures this may not be so, and both internal entropy and energy changes may occur due to changes in vibrational levels on adsoiption. From a somewhat different point of view, it is clear that even in physical adsorption, adsorbate molecules should be polarized on the surface (see Section VI-8), and in chemisorption more drastic perturbations should occur. Thus internal bond energies of adsorbed molecules may be affected. 

Figure 7. Total internal reflection sum frequency generation (TIR-SFG) vibrational spectroscopy of high-pressure room temperature adsorption of carbon monoxide on PVP-protected Pt cube monolayers and calcined (373 K, 3h) monolayers [27], The infrared spectra demonstrate CO is adsorbed at atop sites, but is considerably red-shifted on the PVP-protected Pt cubes. After calcination, the atop frequency blueshifts to 2085 cm in good agreement with CO adsorption on Pt(l 0 0) at high coverages [28], (Reprinted from Ref [27], 2006, with permission from American Chemical Society.)...

The surface is assumed to consist of M adsorption sites. Suppose we consider the case in which TV of the sites are occupied that is, TV molecules are adsorbed. To write the partition function Q for the surface molecules, we must ask how these molecules differ from those in the gas phase (superscript g). Some of the internal degrees of freedom may be modified by the adsorption ((/, ,), but the most notable difference will be in the translational degrees of freedom. From three equivalent translational degrees of freedom, the adsorbed molecule goes to two highly restrained translational degrees of freedom (remember the adsorption is localized) and one vibrational degree of freedom normal to the surface(s) ... 

The internal vibrations of the gaseous and the adsorbed molecules may be taken to be equal. The adsorbed molecules, however, may have a vibration perpendicular to the surface which has taken the place of the lost translation (see above). We may, therefore, write... 

In this chapter, we review important concepts regarding vibrational spectroscopy with the STM. First, the basis of the technique will be introduced, together with some of the most relevant results produced up to date. It will be followed by a short description of experimental issues. The third section introduces theoretical approaches employed to simulate the vibrational excitation and detection processes. The theory provides a molecular-scale view of excitation processes, and can foresee the role of various parameters such as molecular symmetry, adsorption properties, or electronic structure of the adsorbate. Finally, we will describe current approaches to understand quenching dynamics via internal molecular pathways, leading to several kinds of molecular evolution. This has been named single-molecule chemistry. 

In Fig. 5 we compare HREEL spectra recorded after exposing the flat and stepped Ag surfaces at T = 105 K to small amounts of 02 dosed with E[ = 0.39 eY and at a crystal temperature T = 105 K. The angle of incidence was chosen normal to the crystal for Ag(l 0 0) and nearly normal to the (1 1 0) nanofacets for Ag(4 1 0) and Ag(2 1 0). HREEL spectra indicate that at this temperature only ad-molecules are observed on Ag(l 00), at least for small exposures. This is witnessed in the HREEL spectra by the loss at 81 meV [55], corresponding to the internal stretch motion of adsorbed 02, and by the absence of intensity in the frequency region of the O/Ag stretch, between 30 meV and 40 meV [62]. On Ag(4 1 0) partial dissociation occurs since two Ag/O stretch losses are present, at 32 meV and at 40meV, while the internal 02 vibration is visible at 84meV [96]. On Ag(2 1 0), on the contrary, only the low frequency losses are present, indicating that the admolecules are unstable [97]. Our first conclusion is therefore that open steps cause 02 dissociation and that this mechanism is very effective on Ag(2 1 0) and less efficient on Ag(4 1 0) where the terraces have a finite width. Also in this latter case,... 

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