Adsorbed diatomic molecules

Pdf 1111-CN. The usual bonding geometry for an adsorbed diatomic molecule is the end-on configuration where the molecular axis is perpendicular to the surface, as in the case of Ni 100)-C0 described above. This observation is consistent with the behaviour of CO, NO or N2 as ligands in co-ordination chemistry. By the same token we would perhaps expect a surface CN species also to be "terminally" bonded via the C atom as is normally found in cyano complexes. Surface vibrational spectroscopy has, however, indicated that surface CN formed by the decomposition of C2N2 on Pd and Cu surfaces is adsorbed in a lying-down configuration [16]. This result has since been confirmed by NEXAFS [17] and has led to a new consideration of the photoemission data from adsorbed CN [ 18]. 

Fig. 6. Schematic representation of the normal modes of an adsorbed diatomic molecule neglecting the surface structure, after Richardson and Bradshaw . In parentheses the experimentally measured values for CO in the ontop position on Pt(lll). (a) A frustrated translation (60 cm (b) A frustrated rotation (not yet detected), (c) The metal-molecule stretch (460cm ) . (d) The intramolecular stretch model (2100cm" ) . ...

The advantages of electron spectroscopy for the study of adsorbed diatomic molecules are illustrated by reference to the adsorption of carbon monoxide, nitrogen, nitric oxide, and oxygen on different metal surfaces. 

Thus, in this review we present the desorption phenomena focused on the rotational and translational motions of desorbed molecules. That is, we describe the DIET process stimulated by ultraviolet (UV) and visible nanosecond pulsed lasers for adsorbed diatomic molecules of NO and CO from surfaces. Non-thermal laser-induced desorption of NO and CO from metal surfaces occurs via two schemes of DIET and DIMET (desorption induced by multiple electronic transitions). DIET is induced by nanosecond-pulsed lasers and has been observed in the following systems NO from Pt(0 0 1) [4, 5],... 

The model described below is that previously given by White and Lassettre 22). The adsorbent is regarded as a plane-surfaced semiinfinite solid. The forces between the solid adsorbent and the adsorbed diatomic molecules are assumed to be centered at the positions of the component atoms of the molecule. The total interaction between the molecule and the surface is simply the sum of the atom-surface interactions (16, 22). The interaction potential for each atom of the adsorbed molecule is given by f zi) where Z is the distance of the i-th atom measured normal to the surface. Let the distance from the center of mass to the atoms of mass mi and m2, respectively be b and 2, as shown in Figure 1. The potential energy, V, of the adsorbed diatomic molecule is then... 

In order to obtain the energy levels of the adsorbed diatomic molecule, it is necessary to solve the Schroedinger equation for the Hamil-tonion (13). 

The extension of the ROZ formalism to confined molecular fluids has recently been carried out for adsorbed diatomic molecules [6] and dipolar fluids confined in hard sphere matrices [18, 19], In the case of ionic matrix, new features of the system have to be taken into account. On one hand, we have now a two component matrix (with positive and negative ions). This case was already considered in [14, 15] for the primitive model electrolyte adsorbed in an electroneutral charged matrix. On the other hand, we have to deal with two different temperatures the matrix temperature, (h (at which the ionic fluid is equilibrated before quenched) and the fluid temperature fi, at which the fluid is adsorbed in the solid matrix. As usual when dealing with molecular fluids one starts with an expansion of the correlation functions in terms of spherical harmonics as follows,... 

Fig. 2.18 The simplest model for the calculation of the vibrational frequency of an adsorbed diatomic molecule. The surface Is replaced by a single atom C connected with the molecule AB by an elastic force with force constant k.

Bradshaw, A., Woodruff, D., Davila, M., Asensio, M., Conesa, J., Gonzalez-Elipe, A. Is the frequency of the internal mode of an adsorbed diatomic molecule a reliable guide to its local adsorption site J. Electron. Spectrosc. Relat. Phenom. 64-65 (1993) 75. 

Transfer matrix calculations of the adsorbate chemical potential have been done for up to four sites (ontop, bridge, hollow, etc.) or four states per unit cell, and for 2-, 3-, and 4-body interactions up to fifth neighbor on primitive lattices. Here the various states can correspond to quite different physical systems. Thus a 3-state, 1-site system may be a two-component adsorbate, e.g., atoms and their diatomic molecules on the surface, for which the occupations on a site are no particles, an atom, or a molecule. On the other hand, the three states could correspond to a molecular species with two bond orientations, perpendicular and tilted, with respect to the surface. An -state system could also be an ( - 1) layer system with ontop stacking. The construction of the transfer matrices and associated numerical procedures are essentially the same for these systems, and such calculations are done routinely [33]. If there are two or more non-reacting (but interacting) species on the surface then the partial coverages depend on the chemical potentials specified for each species. 

Recently, a quantitative lateral interaction model for desorption kinetics has been suggested (103). It is based on a statistical derivation of a kinetic equation for the associative desorption of a heteronuclear diatomic molecule, taking into account lateral interactions between nearest-neighbor adatoms in the adsorbed layer. Thereby a link between structural and kinetic studies of chemisorption has been suggested. 

Note that a diatomic molecule in the gas phase has only one vibration, but as soon as it adsorbs on the surface it acquires several more modes, some of which may have quite low frequencies. The total partition function of vibration then becomes the product of the individual partition functions ... 

Perhaps the first evidence for the breakdown of the Born-Oppenheimer approximation for adsorbates at metal surfaces arose from the study of infrared reflection-absorption line-widths of adsorbates on metals, a topic that has been reviewed by Hoffmann.17 In the simplest case, one considers the mechanism of vibrational relaxation operative for a diatomic molecule that has absorbed an infrared photon exciting it to its first vibrationally-excited state. Although the interpretation of spectral line-broadening experiments is always fraught with problems associated with distinguishing... 

Table VI. Farther (1st and 3rd rows) and nearer (2nd and 4th rows) atom charges for diatomic molecules adsorbed perpendicularly to the surfaces in Fig. 35 (Ref. 52). 0 atom farther in CO and NO.

D. Marx, H. Wiechert, Ordering and phase transitions in adsorbed monolayers of diatomic-molecules, Adv. Chem. Phys. 95, 213 (1996). 

For diatomic molecules adsorbed by metal surfaces, the adsorption potentials utyj Vj) flx> as a rule, molecular axis perpendicular to the surface (0j=O) thus... 

If the adsorbing molecule dissociates into two or more fragments, each requiring a site, the fraction covered (coverage) differs from that given by equation 8.4-11. For example, consider the adsorption of a dissociating diatomic molecule, B2 ... 

Figure A.15 Energy diagram for the adsorption of a simple diatomic molecule on a d-metal. Chemisorption orbitals are constructed from both the bonding and the antibonding levels of the molecule. As the latter becomes partially occupied, the intramolecular bond of the adsorbate has been activated.

However, if the inhibitor is a diatomic molecule and adsorbed atomically,... 

Prior to 1970 our understanding of the bonding of diatomic molecules to surfaces, and in many cases the type of adsorption (i.e., molecular or dissociative) was almost entirely dependent on indirect experimental evidence. By this we mean that deductions were made on the basis of data obtained from monitoring the gas phase whether in the context of kinetic studies based on gas uptake or flash desorption, mass spectrometry, or isotopic exchange. The exception was the important information that had accrued from infrared studies of mainly adsorbed carbon monoxide, a molecule that lent itself very well to this approach owing to its comparatively large extinction coefficient. 

B.E.T. equation for multimolecular adsorption depend on the ability of a diatomic molecule to rotate on the surface. Kemball (8, 9) measured entropies of substances physically adsorbed on mercury and found a range of freedem from complete mobility for acetone to complete immobility with water. 

It is generally found that the activation energy for dissociation of simple diatomic molecules decrease when going left from the noble metals in the periodic table. This can be described most simply in terms of an increased interaction between the anti-bonding adsorbate states and the metal d-states. 

One structural parameter of potential interest in the adsorption of simple diatomic molecules, such as CO, NO and N2, is the intramolecular bondlength. Much of the motivation for studying such adsorbates is related to the way adsorption modifies the chemistry of these species as the basis for heterogeneous catalysis. Many such reactions involve scission of the intramolecular bond, and if the adsorption is of the... 

In the following, we will discuss a number of different adsorption systems that have been studied in particular using X-ray emission spectroscopy and valence band photoelectron spectroscopy coupled with DFT calculations. The systems are presented with a goal to obtain an overview of different interactions of adsorbates on surfaces. The main focus will be on bonding to transition metal surfaces, which is of relevance in many different applications in catalysis and electrochemistry. We have classified the interactions into five different groups with decreasing adsorption bond strength (1) radical chemisorption with a broken electron pair that is directly accessible for bond formation (2) interactions with unsaturated it electrons in diatomic molecules (3) interactions with unsaturated it electrons in hydrocarbons ... 

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