Band bending in semiconductors: chemical and physical consequences at surfaces and interfaces.

Electron Spectroscopy Using Metastable Atoms as Probes for Solid Surfaces.

Electronic excitations by chemical reactions on metal surfaces

Structural studies of alkali metal adsorption and coadsorption on metal surfaces

Gaining insight into the nature and dynamics of the transition state is the essence of mechanistic investigations of chemical reactions1, yet the fleeting configuration when existing chemical bonds dissociate while new ones form is extremely difficult to examine directly2. Adiabatic potential-energy surfaces—usually derived using quantum chemical methods3 that assume mutually independent nuclear and electronic motion4—quantify the fundamental forces between atoms involved in reaction and thus provide accurate descriptions of a reacting system as it moves through its transition state5,6. This approach, widely tested for gas-phase reactions7, is now also commonly applied to chemical reactions at metal surfaces8. There is, however, some evidence calling into question the correctness of this theoretical approach for surface reactions: electronic excitation upon highly exothermic chemisorption has been observed9, and indirect evidence suggests that large-amplitude vibrations of reactant molecules can excite electrons at metal surfaces10,11. Here we report the detection of ‘hot’ electrons leaving a metal surface as vibrationally highly excited NO molecules collide with it. Electron emission only occurs once the vibrational energy exceeds the surface work function, and is at least 10,000 times more efficient than the emissions seen in similar systems where large-amplitude vibrations were not involved12,13,14,15,16,17,18. These observations unambiguously demonstrate the direct conversion of vibrational to electronic excitation, thus questioning one of the basic assumptions currently used in theoretical approaches to describing bond-dissociation at metal surfaces.

Conversion of large-amplitude vibration to electron excitation at a metal surface

Exposure of Cs surfaces to ${\mathrm{O}}_{2}$ causes ejection of ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ ions with low yields (\ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}8}$ per incident ${\mathrm{O}}_{2}$ molecule) during the first stages of dissociative chemisorption (followed by exoelectron emission at higher exposures), although the work function of the surface exceeds the electron affinity of O and the energetics of the overall reaction is almost zero. A mechanism is proposed whereafter the release of ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ is a consequence of strong repulsion in ${\mathrm{O}}_{2}^{2\mathrm{\ensuremath{-}}}$ species intermediately formed in front of the surface.

O- escape during the oxidation of cesium.

During oxidation of thin Cs films, a nonadiabatic surface reaction manifests itself in the emission of electrons. This effect was investigated in detail by combining measurements of the current and of energy distributions of these exoelectrons with studies on the electronic properties of the surface by means of ultraviolet photoelectron spectroscopy and metastable deexcitation spectroscopy. Exoelectron emission occurs via Auger deexcitation of the empty state derived from the O2 affinity level. This process is confined to the stage Cs2O2→CsO2 in which resonance ionization of the affinity level of the impinging O2 molecule upon crossing the Fermi level EF is efficiently suppressed due to the absence of metallic states near EF. A kinetic model based on the successive steps involved in the oxidation of Cs is developed which describes qualitatively well all the experimental findings.

Exoelectron emission during oxidation of Cs films

Oxidation of alkali metals is found to be accompanied by emission of electrons (\"exoelectrons\") indicating the possibility for a nonadiabatic channel which enables transformation of the reaction enthalpy into electronic excitation [1,2]. The subsequent deexcitation step is thought to be an Auger deexcitation process in which an empty hole state on the oxygen species with an energy below the Fermi level (representing the electronic excitation and being derived from the aSnity level of the interacting particle) is filled by electronic transition from the solid whereby the energy is released to another electron which eventually may escape into the vacuum where it may be detected [3]. Previous studies with the oxidation of thick (i.e., bulk) Cs films revealed that electron emission was essentially confined to the last step of oxidation, namely, formally the transformation of Cs202 into Csoq. A pronounced inIIuence of the sample temperature suggested that a thermally activated surface process is involved and it was speculated that it creates an activated site on the surface which upon collision of an incoming 02 molecule gives rise to exoelectron emission [4]. Although a kinetic model was proposed which was able to reproduce the recorded kinetics rather satisfactorily, the actual nature of the thermally activated step remained unclear. Subsequent experiments on the oxidation of Na

Emission of exoelectrons during oxidation of Cs via thermal activation of a metastable O2- surface species.

The first stages of oxidation of thin Li films (leading to the formation of ${\mathrm{Li}}_{2}$O) were found to be accompanied by emission of ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ ions as well as electrons, reflecting the participation of electronically highly excited states in this reaction. A model is proposed and supported by the results of local-spin-density-approximation cluster calculations whereby electron transfer from the metal onto the impinging ${\mathrm{O}}_{2}$ molecule leads to formation of a transient ${\mathrm{O}}_{2}$ $^{2\mathrm{\ensuremath{-}}}$ species. This species dissociates without a noticeable activation barrier and there is a finite (but rather low) probability that one of the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ fragments formed near the surface is ejected into the gas phase. The ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ species at the Li surface forms, on the other hand, a hole state, which subsequently transforms into the ${\mathrm{O}}^{2\mathrm{\ensuremath{-}}}$ ground state. For excitations larger than the work function, the energy associated with its decay (g2 eV) may be released in an Auger process associated with electron emission. The yield of light emission was found to be below the detection limit of about ${10}^{\mathrm{\ensuremath{-}}10}$ photons per reacting ${\mathrm{O}}_{2}$ molecule and indicates a short lifetime (100 fs) of the ${\mathrm{O}}^{\mathrm{\ensuremath{-}}}$ species at a Li surface.

Nonadiabatic processes during the oxidation of Li layers

Energy distributions of electrons emitted from alkali-metal surfaces by impact of metastable He atoms reveal that there is a high probability for transformation of singlet atoms (excitation energy ${\mathit{E}}^{\mathrm{*}}$=20.6 eV) into triplet atoms (${\mathit{E}}^{\mathrm{*}}$=19.8 eV) prior to deexcitation into the ground state. The conversion probability (as expressed by the ratio R of the intensities of valence-band emission due to triplet and singlet ${\mathrm{He}}^{\mathrm{*}}$ deexcitation, respectively) increases with increasing alkali-metal coverage on a Ru(0001) substrate, and in turn decreases with increasing oxygen exposure at a fixed alkali coverage. These findings indicate that R is a qualitative measure for the degree of ``metallization'' of the adlayer. R also increases with temperature due to broadening of the nearest-neighbor distribution whereby, on the average, a larger part of the adlayer becomes metalliclike. For Cs overlayers exhibiting work functions 2 eV the mechanism of deexcitation changes and may proceed via ${\mathrm{He}}^{\mathrm{*}\mathrm{\ensuremath{-}}}$ (1${\mathit{s}}^{1}$2${\mathit{s}}^{2}$) formation as reflected by the R data as well as by the widths of the electron spectra.

R. Grobecker

Papers

O- escape during the oxidation of cesium.

Exoelectron emission during oxidation of Cs films

Emission of exoelectrons during oxidation of Cs via thermal activation of a metastable O2- surface species.

Nonadiabatic processes during the oxidation of Li layers

Singlet-to-triplet conversion of metastable He atoms at alkali-metal overlayers.