Adsorbate-substrate complex

Benzene adsorbs weakly on Cu and strongly on Ni. It is interesting to study how the differences in adsorption strength are reflected in the electronic structure of the adsorbate-substrate complexes as determined based on the XAS and XES spectra for benzene on Cu(l 10) and Ni(100) shown in Figures 2.39 and 2.40, respectively [83,90]. 

This approach has the potential to resolve the time evolution of reactions at the surface and to capture short-lived reaction intermediates. As illustrated in Figure 3.23, a typical pump-probe approach uses surface- and molecule-specific spectroscopies. An intense femtosecond laser pulse, the pump pulse, starts a reaction of adsorbed molecules at a surface. The resulting changes in the electronic or vibrational properties of the adsorbate-substrate complex are monitored at later times by a second ultrashort probe pulse. This probe beam can exploit a wide range of spectroscopic techniques, including IR spectroscopy, SHG and infrared reflection-adsorption spectroscopy (IRAS). 

The molecular orbitals of the adsorbate and the electronic band structure of the substrate may be complex and often poorly understood. So predicting interactions between the two is nontrivial and inexact. Stated generally, the adsorbate-substrate complex has a different electron distribution from the isolated components, resulting in a different cross section. While the details are usually complex and often undefined, at least one theory of the effects of adsorption on Raman cross section has yielded useful explanations and predictions. The theory attributes chemical enhancement of cross sections to charge transfer between adsorbate and substrate orbitals (or vice versa) and is generally known as charge-transfer theory (1,15,16). 

The complex excitation mechanism essentially states that the action spectrum of surface photochemistry should be referenced to the transitions of the adsorbate-substrate complex rather than the gas phase spectrum. As a first approximation the complex is taken as being analagous to the corresponding organometallic compound. Alternatively the perhaps known electronic structure of the adsorbate may be referenced to an electronically similar gas phase... 

The adsorbate substrate complex excitation mechanism predicts an action spectrum analagous to the absorption spectra of the complex. The direct mechanism predicts a photochemical action spectrum similar to that of the gas phase molecule. The final mechanism, dissociative electron attachment (DEA), suggests that the action spectrum should be referenced to the absorbance of the substrate, modified by the surface work function and electron attachment cross section of the adsorbate. The DEA mechanism appears to be of importance for many metal and semiconductor substrates, especially for the case of photochemistry induced by anomalously low energy radiation. 

It should not be concluded that the red shifted photolysis of adsorbates is solely a result of electron attachment. There are a few detailed studies of photolysis due to adsorbate-substrate complex excitation. So etal. studied photodesorption of NO from Ag(lll) and Cu(lll) by HREELS and MS. For irradiation at 436 nm the power absorbed by the substrate, calculated from the reflectivity, as a function of polarisation and 6inc adequately described the yield. Calculations for adsorbate excitation, ie the field intensity, were ambiguous. Certainly the field intensity projected onto an NO transition dipole normal to the surface could not fit the data. However for a dipole parallel to the surface the substrate absorption and adsorbate excitation mechanisms could not be distinguished. The action spectrum for NO on Cu(lll) was most revealing. For incident wavelengths between 450 and 600 nm the desorption yield was well fit by substrate absorption. However at 300 nm the yield was five times greater than expected from substrate absorption alone (assuming photolysis at 500 nm to be due solely to substrate excitation). Since NO itself has no... 

The photochemical behaviour of other halogenated systems such as phosgene have also been investigated on silver(lll) either as monolayers or multilayers. The UV irradiation of this system brings about C—Cl bond fission. The Cl remains chemisorbed to the surface while the CO is desorbed. Again, the data collected suggest that the mechanism involves excitation of an adsorbate/substrate complex. There is evidence that the silver has a catalytic effect with the onset of the reaction red-shifted by 2.6-2.8 eV from the gas... 

Restructuring occurs in order to maximize the bonding and stability of the adsorbate-substrate complex. Thus it is driven by thermodynamic forces and is most likely to occur when the stronger adsorbate-substrate bonds that form compensate for the weakening of bonds between the substrate atoms, an inevitable accompaniment to the chemisorption-induced restructuring process. 

Here G[A (aq)] is the free energy of the aqueous adsorbate-substrate complex, G[A"(aq)] is the free energy of the aqueous anion, and G( ) is the free energy of the substrate. The free energy of the isolated aqueous phase species can be computed via ... 

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