Surface chemistry under high pressure of gaseous reactants is often different from surface chemistry at low pressure. For example, many surface chemical reactions proceed readily under high pressure conditions typical of a commercial, heterogeneous catalytic reaction but appear not to proceed at low pressures (<10-4 torr), despite favorable thermodynamics. The different chemistry and, in particular, the lack of reactivity at the low pressures where ultrahigh vacuum surface science techniques are operable is known loosely as the pressure gap and casts doubt on the relevance of UHV surface chemistry to high pressure processes such as catalysis, chemical vapor deposition and etching reactions. Much of our research has been and remains directed toward understanding the physical basis for this pressure gap. We have found four previously unrecognized phenomena responsible for or plausibly responsible for differences in surface chemistry at high and low pressure: translational activation, collision induced activation, collision induced desorption and collision induced absorption.

We have demonstrated that high resolution electron energy loss spectroscopy, a technique previously believed to be surface sensitive only, can be used to detect the vibrations of species buried in the bulk of the metal. Specifically, we have characterized a H atom embedded in the Ni bulk by a single 800 cm-1 loss feature. This feature, for which an impact scattering mechanism is operable, is identified as an interstitial Ni-H vibration by the similarity of the dependence of its intensity on electron impact energy to the dependence of the electron inelastic mean free path on electron energy. This study, which is described in Phys. Rev. Lett. 67, 927 (1991), represents the first conclusive detection of subsurface vibrations of absorbed species by HREELS.

Knowledge of these microscopic origins for the pressure gap has allowed us to develop a scheme to bypass the high pressure requirement simply by raising the energy of the incident molecule or collisionally inducing dissociation. We have used the former trick to synthesize and identify spectroscopically, by high resolution electron energy loss spectroscopy, adsorbed CH3 radicals, benzene from CH4 over a Ni(111) catalyst, bulk H in Ni, and demonstrated the hydrogenation of adsorbed methyl, ethylene and acetylene by bulk H.

We have discovered that a Au/Ni(111) surface alloy, with Au coverages up to 0.3 ML, efficiently catalyzes the oxidation of CO at low temperature (J. Am. Chem. Soc. 128, 1800 (2006)). Clearly, the substitution of a small percentage of Ni atoms on the Ni(111) surface by Au atoms has dramatically changed the chemistry of Ni. The oxidation of CO on Ni has never been observed under ultrahigh vacuum laboratory conditions, presumably because both the oxygen atom and the CO are too strongly bound, and hence the barrier to their reaction is too large. The introduction of gold into the Ni lattice serves to weaken sufficiently the binding of oxygen and CO to the surface, thereby allowing the reaction to proceed.

We have demonstrated (Phys. Rev. Lett. 74, 2603 (1995)) the abstraction of a F atom from a F2 molecule incident on a Si(100)2x1 surface by detection of the scattered, complementary F atom in a molecular beam-surface scattering apparatus (described in "Molecular Beams: Probes of the Dynamics of Reactions on Surfaces," Physical Methods of Chemistry, 2nd ed., Wiley, 1993). The Si dangling bonds abstract one of the F atoms while the complementary F atom scatters into the gas phase. No Si-Si dimer bonds or Si-Si lattice bonds are broken. Because the detection of reactive radicals such as F atoms is experimentally difficult, this mechanism has gone undetected in numerous previous studies of the interaction of fluorine and fluorine containing molecules with Si. It may be an additional source of radicals that are presently unaccounted for in kinetic models of semiconductor etching. The dynamics of atom abstraction are analyzed to reveal an “attractive” interaction potential between F2 and the Si dangling bond with a transition state that is not constrained geometrically. This result demonstrates (J. Chem. Phys. 111, 3679 (1999)) that the available computed potential energy surfaces for the interaction between F2 and Si are inadequate.

The reaction of thermal energy F2 with Si(100) ceases after the dangling bonds are saturated at one monolayer of coverage. This lack of additional reactivity precludes the build up of a sufficient layer of fluorine to produce the volatile etch product SiF4 and therefore the use of F2 as an etchant of Si. However, we have shown that if the kinetic energy of F2 is increased above a threshold value of 3.8 kcal/mol, the dissociation probability of F2 on a fluorinated surface increases linearly with the normal component of the kinetic energy, up to a value of 3.6x10–3 at 13 kcal/mol. The relatively small effect of translational energy implies a late barrier in the potential energy surface for the interaction of F2 with the Si-Si bonds. Information regarding the specific Si-Si site, Si–Si dimer bonds (bonds between two Si atoms on the surface) or Si–Si lattice bonds (bonds between a Si atom on the surface and a Si atom in the second layer), at which the translationally activated reaction occurs is obtained from He diffraction measurements. These measurements show that the reaction does not occur preferentially at the Si–Si dimer bonds, which are the weakest Si–Si bonds in the system. Reaction at Si–Si lattice bonds also occurs, leading to disorder of the Si surface and near surface as soon as the coverage increases beyond 1 ML. (J. Phys. Chem. B 105, 486 (2001))

We find that the interaction of low energy XeF2 with Si(100) is strikingly identical to that of F2 up to a coverage of one monolayer. The XeF2 dissociatively chemisorbs via atom abstraction with high probability solely on the Si dangling bonds. No reaction of XeF2 with the Si-Si dimer or Si-Si lattice bonds is observed, resulting in an ordered fluorine layer at one monolayer of fluorine coverage. The critical observation of this study is that despite the large exothermicity resulting from the dissociative chemisorption of XeF2 or F2, the order of the surface is not destroyed in either system. However, after saturation of the dangling bonds at 1 ML coverage, F2 ceases to react with the surface whereas XeF2 continues to deposit fluorine by reacting with the Si-Si dimer bonds and the Si-Si lattice bonds. The order is destroyed as a result of the continued fluorine deposition, and ultimately, etching occurs by the formation of volatile SiF4. It is clear that the presence of disorder is not a precondition for etching to begin, in contrast to previous beliefs. Clearly, XeF2 begins to react with the Si-Si bonds in the presence of an ordered overlayer of fluorine. It is the reaction of XeF2 with these Si-Si bonds that leads to the disorder of the surface periodicity rather than the disorder induced by the exothermicity release that leads to the reaction of XeF2 with the Si-Si bonds. (J. Phys. Chem. B 106, 8399 (2002))

As mentioned above, we find that XeF2, a linear molecule, interacts with Si(100) by atom abstraction as evidenced by detection, with a triply differentially pumped, rotatable mass spectrometer in a molecular-beam surface scattering UHV apparatus, of the scattered, complementary XeF fragment. (Phys. Rev. Lett. 92, 188302 (2004)) More importantly, we observe that some scattered XeF is sufficiently excited internally to dissociate in the gas phase before reaching the detector, producing a F and a Xe atom. The F and Xe atoms are shown conclusively to result from dissociation of gas phase XeF by demonstrating that the measured velocity distributions of F, Xe and XeF conserve momentum and energy and that the number of scattered F atoms equals that of Xe. Specifically, conservation of energy, momentum and mass accurately predict the angle-resolved Xe atom TOF spectra based on knowledge of the angle-resolved TOF spectra of the scattered F atom and XeF fragment. This excellent agreement resulting from a two body treatment of this system implies dissociation of XeF in the gas phase, unperturbed by the presence of a third body, the nearby surface. This experiment shows for the first time that a product of a surface chemical reaction can undergo dissociation in the gas phase as a result of partitioning of the exothermicity of a surface reaction. Knowledge and inclusion of atom abstraction and gas phase dissociation of a surface reaction product are critical to the development of accurate kinetic models for semiconductor etching, heterogeneous catalysis and chemical vapor deposition.

During our studies of the CO adsorption dynamics, we became aware of confusion in the literature regarding the binding site of CO on Ni(111) despite the fact that this system had been studied spectroscopically over a dozen times. Through our studies of the vibrational spectra as a function of surface temperature (in contrast to the typical measurements carried out as a function of coverage), we discovered that the population of molecules adsorbed on the bridge and atop sites was determined by a rapid exchange of the molecules between these sites and therefore by a straightforward equilibrium distribution. A study of the equilibrium constant as a function of surface temperature provided a value for the binding energy difference between the bridge and atop sites. We showed that the presence of this equilibrium and its sensitivity to temperature and coverage are the origins of the conflicting reports in the literature for the CO binding sites. We have also shown that the concept of sequential site filling as a mechanism by which molecules populate sites is thermodynamically naive and mechanistically simplistic and is an accidental consequence of the convenience of measuring spectra as a function of coverage rather than reflective of the microscopic physics of the system (J. Chem. Phys. 84, 1876 (1986)).

The effects of translational energy on the molecular chemisorption of CO on Ni(111) have revealed intriguing dynamics of the chemisorption process. Through a series of exhaustive measurements of the initial adsorption probability, CO mobilities, and vibrational spectra as a function of the CO incident energy, evidence emerged for a CO precursor molecule to the molecularly chemisorbed CO molecule at translational energies less than 4 kcal/mol and an effective barrier to chemisorption directly from the gas phase (J. Chem. Phys. 84, 6488 (1986)). We have followed up on these experiments by building a liquid helium cryostat which enables us to cool the crystal from 1000 K to 8 K in 5 minutes with the goal of trapping the precursor molecule on a clean surface so that it could be spectroscopically identified (Surf. Sci. 195, 77 (1988)). However, temperatures as low as 8 K were not sufficient to trap the elusive precursor molecule. This result implies that a dynamical bottleneck between the chemisorption and precursor states is responsible for the long lifetime of the precursor molecule proposed to explain our earlier results. Further information on the nature of precursor molecule awaits the development of a means to detect spectroscopically a submonolayer coverage of precursors with at least submicrosecond time resolution. The saga of the precursor is far from finished, and the challenges presented here are very similar to those for detection of transition states in gas phase reactions. Nevertheless, these studies have proven quite thought-provoking about the unexpectedly complex pathway of a gas phase molecule to a molecularly chemisorbed species.