Surface-adsorbed carbon monoxide, stretching

The surface properties of these nano-objects match those of metal nano crystals prepared in ultrahigh vacuum, for example the C - O stretch of adsorbed carbon monoxide or the magnetic properties of cobalt particles embedded in PVP. This demonstrates the clean character of the surface of these particles and its availabihty for reactivity studies. 

IRES Versus Other Reflection Vibrational Spectroscopies. In order to achieve a sensitivity sufficient to detect absorption due to molecules at submonolayer coverages, some sort of modulation technique is highly desirable. Two candidates for modulation are the wavelength and the polarization state of the incident light. The former has been successfully applied to single crystal studies by Pritchard and co-workers (5j, while the latter is the basis of the Toronto ellipsometric spectrometer and of the technique employed by Bradshaw and coworkers (6) and by Overend and co-workers (7). The two different techniques achieve comparable sensitivities, which for the C-0 stretching mode of adsorbed carbon monoxide amounts to detection of less than 0.01 monolayer. Sensitivity, of course, is very much a function of resolution, scan rate, and surface cleanliness. 

The red shift of the CO stretch vibrational frequency due to the static interaction between the neighboring adsorbed COs seems to be abnormal, as their repulsive, static dipole-dipole interaction would push electron charge away from the negative charged CO, mostly from the anti-bonding 27t orbitals, into the metal surface, leading to a strengthen CO bond and, therefore, a blue frequency shift. It is found that this red shift of the CO stretch vibrational frequency is caused mainly by the interaction between the Cu 4sp band electrons and the 27t orbitals of the adsorbed carbon monoxides, as discussed below. 

What is interesting in our calculated results is that a negative chemical shift of the stretch vibrational frequency of carbon monoxide adsorbed on the Cu(lOO) surface is obtained from the first principle DFT calculation. It comes from the abnormal negative frequency shift due to the static interaction between the adsorbed carbon monoxides. 

However, the quantum treatment of these processes increases the role of the energy transfer from the model heat bath to the reaction co-ordinate, resulting in thermal desorption of excited carbon monoxide adsorbates. The other frustrated translational or rotational modes may also play an intermediate role in energy transfer from excited carbon monoxide stretching mode into the adsorption bond mode (CO-surface). These interactions are important because direct coupling between the earbon monoxide internal stretching mode and the frustrated translational mode, responsible for desorption, is very weak. Taking into account these interactions, this leads to drastic acceleration of desorption [5, 49]. 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

Now, theoretical calculation methods of sufficient accuracy may fill the lack of quantitative information concerning so elusive species. On the other hand, the use of a monocoordinated complex as being the simplest molecular model to simulate a chemisorption phenomenon on a metallic surface, for instance the chimisorption of carbon monoxide on iron or nickel [16,17,18] enables to predict the shifts of the CO stretching vibration of the adsorbed species. Similar effects observed with cyanide anions CN on a cathode of platinum, silver or gold, using non-linear optics techniques can be rationalized by computing the CN vibration mode of the corresponding triatomic systems [19,20,21]. 

One important catalytic reaction cycle which starts from a primary gas mixture of carbon monoxide and hydrogen is the Eischer—Tropsch synthesis. Depending on the reaction parameters (temperature of the catalytic surface, gas pressure and composition of the gas mixture) a great variety of aliphatic, aromatic and even oxygen-containing compounds can be obtained. The understanding of reaction mechanisms in terms of the appearance of intermediates on the surface, their structure and symmetry, is of fundamental interest for the development of well-defined reaction pathways. The frequency of the C—H stretching Raman band is a measure of the state of hybridization of the adsorbed molecule. 

The coverage dependence of the chemical shift and the coupling shift of the stretch vibration of carbon monoxides adsorbed on a Cu(lOO) surface are studied based on cluster model calculations using density functional theory (DFT) with local density approximation. It is found that the Cu 4sp conduction band electrons dominate the interaction with the CO 27t orbitals, leading to an abnormal, red chemical shift. 

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