John William Gadzuk

Laser-excited hot-electron induced desorption: A theoretical model applied to NO/Pt(111)

Abstract We present a theoretical model for stimulated desorption due to the interaction of energetic substrate carriers with molecular adsorbates. The model is based on the premise that optically excited hot electrons scatter into an unoccupied valence electron resonance of the adsorbate, thus forming a temporary negative molecular ion which then experiences an enhanced attraction towards the substrate. Neutralization of the ion returns the adsorbed molecule to one of the continuum states of the molecule/substrate potential energy surface, possibly in an internally excited state. The consequences of such a model are worked out using semiclassical wave packet dynamics which, in the short time limit relevant to the present situation, can be brought to an analytic realization. The model provides considerable insight into recent experiments on the laser-induced desorption of NO from Pt(111).

Resonance-assisted, hot-electron-induced desorption

The phenomenon of resonance-assisted hot-electron-induced desorption of adsorbed atoms or molecules is considered in terms of a semi-classical Gaussian wavepacket model. Inelastic scattering of hot electrons, forming temporary negative ion adsorbate states, gives rise to enhanced energy transfer between the hot electron and the adsorption complex. The numerical implications of the wavepacket model, which follows in the spirit of past theory of resonance Raman processes, suggests a very simple and intuitive analytic representation of the resonance desorption process providing a clear connection between the three independent timescales of the process and observable quantities such as desorption probabilities and rates. The timescales are identified as the vibrational period associated with the negative-ion-surface bond, that portion of the vibrational period within which desorption is possible, and the resonance lifetime of the intermediate negative ion state.

Surface molecules and chemisorption: I. Adatom density of states

Abstract A useful picture of chemisorption on metal surfaces is one in which a localized molecule is formed between the adatom and its nearest neighbor substrate atoms. The interaction responsible for the molecule formation is treated as the coupling between the adsorbate state and a group orbital formed from a linear combination of atomic orbitals on the substrate atoms. Within the surface molecule picture, level width and level shift functions, given by Newns modification of the Anderson theory, have been calculated and the resulting adatom density of states function has been obtained. This has been done for model systems in which the substrate is either a free electron metal or a tightbinding p-band metal and the adsorbate is s or p like. The results show how it is possible to simultaneously have narrow virtual levels due to chemisorption (∼ 1 eV) which previously implied weak interactions and also high binding energies (≳ 3 eV) as are observed experimentally.

Nonequilibrium Theory of Scanning Tunneling Spectroscopy via Adsorbate Resonances: Nonmagnetic and Kondo Impurities

We report on a fully nonequilibrium theory of scanning tunneling microscopy (STM) through resonances induced by impurity atoms adsorbed on metal surfaces. The theory takes into account the effect of tunneling current and finite bias on the system, and is valid for arbitrary intra-adsorbate electron correlation strength. It is thus applicable to recent STM experiments on Kondo impurities. We discuss the finite-temperature effects and the consequences of atomic scale resolution of the STM for the spectral property of such systems. We find that the tip position affects the resonance line shapes in two ways. As a function of the distance from the surface, the line shapes vary due to the different extents of the adsorbate and metal wave functions into the vacuum. However, we do not expect large variations in line shapes unless tunneling into the tightly bound adsorbate states is considerable, or nonequilibrium effects are significant. As a function of the lateral tip position, line shapes should not change significantly on length scales of

On the dissociation of diatomic molecules at metal surfaces

{R}_{\ensuremath{\parallel}}l~10\AA{}

Photoinduced desorption in NO/Pt: a time-dependent quantum mechanical study

under typical experimental conditions when the electrons tunnel into the perturbed bulk conduction states hybridized with the outer shell

Hot-electron femtochemistry at surfaces: on the role of multiple electron processes in desorption

\mathrm{sp}

A soluble relaxation model for core level spectroscopy on adsorbed atoms

adsorbate orbitals. Tunneling into surfaces states on (111) surfaces of noble metals should be important for an observation of resonance at larger distances (g10 \AA{}), and oscillatory variations in the line shape should develop. This long-range behavior was not resolved in recent experiments with Kondo impurities. The temperature dependence of the Kondo resonance cannot be deduced directly from the differential conductance, as the thermal broadening of the tip Fermi surface produces qualitatively similar effects of comparable and larger magnitudes. A careful deconvolution is necessary to extract the temperature dependence of the Kondo resonance. The finite-bias current-induced nonequilibrium effects in tunneling through Kondo impurities should produce a characteristic broadening of the resonance in the case of strong hybridization of the discrete state with the STM tip.

High resolution photoemission study of condensed layers of nitrogen and carbon monoxide

Abstract The role of activation barriers in the process of dissociative adsorption of diatomic molecules on metal surfaces is considered in terms of the topology of the diabatic potential energy surfaces of the interacting system. The location of possible barriers on the potential energy surface determines the dependence of the dissociative threshold on either the normal translational or total energy content of the incident molecules.

Angular distributions of electrons photoemitted from chemisorbed atoms

Abstract We have carried out a TDQM study of photoinduced desorption in NO/Pt by involving hot electron transfer and allowing different residence times ( τ R ) on the excited state potential-energy surface. The desorption probability ( P des ) is dependent on τ R , but it is clear from the results that the substantial P des can be accounted for by assuming τ R is in the range 20–60 fs. The study also accounts for vibrational excitation in the desorbed NO.