Translation, hindered, adsorbed molecules

The appearance of additional peaks in the monolayer spectrum suggests the existence of surface vibratory modes associated with rotations and translations of the free molecule hindered by adsorption. To identify these modes, it is necessary to perform normal mode calculations of the vibrational spectrum of the adsorbed molecule. These calculations are also of interest because of the sensitivity of the frequency and intensity of the surface vibratory modes to the molecular orientation and the location and strength of its bonds to the substrate. 

The term spectroscopy implies questioning molecules about their eigenvalues of molecular motion. In the case of free molecules these are usually the eigenvalues of free rotation, vibration or electronic motion, which changes for adsorbed molecules to more or less hindered or perturbed motions, including frustrated translation. 

A considerable element of the model is the assumption connected with the possibility of the kinetic motion of adsorbed molecules. When the motion of molecules in the z direction is restricted but molecules are able to move freely in the (x,y) plane, the process is classified as mobile adsorption. However, if the lateral translation is also hindered, the process is classified as localized adsorption. The motion of admolecules is controlled by the energetic topography of the surface, molecular interactions, and thermal energies. The adsorbed molecule is considered as localized on a surface when it is held at the bottom of a potential well with a depth that is much greater than its thermal energy. Except for extreme cases, adsorption is neither frilly localized nor frilly mobile and can be termed partially mobile [8]. Because temperature strongly affects the behavior of the system, adsorption may be localized at low temperatures and become mobile at high temperatures. 

Considerable changes in the intramolecular vibrations of the adsorbed molecule result from adsorption. The first is the transformation of some of the translation and rotational degrees of freedom into vibrational modes, the so-called frustrated translation and rotational modes. The frustrated translation mode perpendicular to the oxide surface corresponds to the vibration of the new adsorption bond. Depending on the geometry and the interactions in the adsorption complex, this bond is formed between one or more atoms from the adsorbate and the surface. The simplest case is one-end adsorption of the molecule, i.e., only one of the atoms is bonded to a surface atom (see Scheme 4) and the frequency of the frustrated translation is equal to the frequency of the adsorption bond. When two or more atoms are connected in the adsorption bond, it is convenient to consider the frustrated translation as the center of mass oscillation of the whole molecule. The frustrated rotational modes usually have a lower frequency, but they are important in the interactions with neighboring surface atoms, in surface diffusion, and in lateral interactions with other adsorbate molecules. In addition to these new vibrational modes, some of the gas-phase vibrations are also changed due to the hindering of the solid surface. 

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