L. Walldén

Photon and electron emission during halogen adsorption on sodium

Abstract The adsorption of chlorine, bromine and iodine on UHV evaporated sodium films is accompanied by the emission of photons and electrons. Although the emission probabilities are small (∼10 −7 and ∼10 −5 per adsorbed molecule respectively), the effect provides a sensitive means of monitoring the adsorption process. For moderate exposures, the photon and electron yields are proportional to the pressure. At high exposures, due to the formation of a halide film on the surface, the photon yield decreases rapidly, eventually becoming proportional to the square root of the pressure. The high sensitivity allows use of the light signal to measure diffusion through the halide layer. The spectral distributions of the observed photons are characterized by wide (∼1 eV) emission bands in the range 4000–8000 A. The spectra are influenced by surface composition, as demonstrated most clearly in the Na + I 2 spectrum, where a peak occurs on the high energy side of the main band when the NaI film starts to form. The origin of the spectra, their comparison with gas phase chemiluminesccnce and a model of the reaction kinetics are discussed.

Energy shifts of a Cu(111) surface band upon adsorption of cesium and oxygen

Abstract Angle resolved photoelectron energy spectra show that a Cu(111) surface band shifts rigidly to lower energy upon cesium adsorption and to higher energy upon exposure of a Cs covered Cu(111) surface to oxygen. For Cu(111)/Cs the energy shift at low coverage corresponds to a transfer of 0.5 electrons per adsorbed Cs atom to the surface state. We associate the shift upon oxygen exposure with the formation on the Cu(111) surface of cesiumoxide molecules which are interacting weakly with the substrate.

Surface chemiluminescence in the Ksolid + Cl2gas reaction

Abstract Light emitted during the surface reaction between chlorine gas and potassium metal films has been spectrally investigated in the 200–900 nm range. Two distinct features appear in the spectra. Firstly a characteristic broad band peaking at 440 nm and with a low λ cut off at around 330 nm. originating in an electron transfer from the potassium valence band to an empty hole state in chlorine. Secondly two sharp lines are present at 766.5 and 769.9 nm respectively. Responsible for this doublet is K ∗ emitted from the surface during the chemical reaction and subsequently deexcited via the 4p → 4s transition. A comparison with sodium chlorine spectra reveals a blucshift of the emission band and a dramatic increase of photon yield for the present case. The model of Norskov, Newns and Lundqvist explains this behaviour: Since the Cl affinity level eventually shifts below the K valence band bottom as the Cl 2 develops a bond with the surface, non-radiative resonance tunneling of valence band electrons will diminish. The longer lifetime of the hole produces both the blueshift of the spectrum and an increase in the overall photon yield. Negative particles originating from the surface reaction have been observed for both systems with an increase of the yield for the present K+Cl 2 case.

Surface state energy shifts by molecular adsorption : CO on clean and Na covered Cu(111)

Angle resolved photoelectron energy spectra recorded in the near UV show that the surface state 0.4 eV below EF of Cu(111) shifts to higher energy upon CO adsorption. The surface band related emission intensity is reduced by the adsorption at a rate suggesting that each adsorbed molecule wipes out the surface state over an area corresponding to seven surface layer Cu atoms. The surface state energy shift is not as closely related to the adsorbate induced workfunction change as found theoretically and experimentally for alkali adsorbates. The influence on the shift of the workfunction and initial energy of the surface state is studied by preadsorbing Na on the Cu(111) surface.

Sodium induced structure in UPS spectra of Cu(111)/Na

Abstract UPS Spectra of sodium monolayers evaporated onto Cu(111) are presented. We interpret sodium induced structure below the Cu d-band as due to photoemitted 3d electrons which have experienced an energy loss by exciting a sodium monolayer plasmon. An experimental difficulty is that even small amounts (

Reduced stretch vibration frequency for CO on Na covered Cu(100)

When Na is coadsorbed with CO on Cu(100), EELS show a peak at around 0.17 eV loss energy in addition to the peak at 0.26 eV which is characteristic of CO on Na-free Cu(100). The low energy peak is ascribed to CO molecules adsorbed near Na atoms. The appearance of both the shifted and unshifted loss peaks demonstrates the local nature of the alkali-CO interaction for the present system.

Dilution of the electron gas in a Cs monolayer on Cu(111) by adsorption of oxygen

Abstract The characteristic loss energy of plasmons in a close packed Cs p(2 × 2) monolayer on Cu(111) is found to decrease gradually upon increasing the amount of oxygen adsorbed, the oxygen appearing to act as a thinner of the electron gas in the overlayer. We observe no dispersion for the monolayer plasmon.

The reactivity of adsorbed Na atoms probed by coadsorbed CH3OH

The chemical properties of sodium atoms adsorbed on a Cu(111) surface have been probed by coadsorbed methanol. Methoxide, stoichiometrically close to CH3ONa2, is identified as a product by photoemission spectroscopy. Sodium atoms are found to react in the same manner at all submonolayer coverages, i.e. independent of atomic charge. Films of closely packed Na atoms — the first sp-metal substrate probed by methanol — are slightly less reactive. Hartree-Fock-Slater cluster calculations as well as observed energy shifts of a Cu electronic surface state indicate a transfer of charge from the copper substrate to the methoxide group. The desorption temperature for a monolayer of methanol molecules from a Cu(111) surface is enhanced by the presence of low sodium concentrations (120 of a monolayer). CH3OH is found to adsorb associatively on the uncovered Cu(111) surface.

Some vibrational properties of clean and water vapour exposed Cu(111)/Na,K

Abstract When Na (K) is adsorbed on Cu(111) and the coverage is below half of a full monolayer electron energy loss spectra show a peak at 21 meV (13 meV) ascribed to the excitation of Na (K) vibrations perpendicular to the surface. From the intensity of the loss peak at low alkali metal coverage a dynamic charge of approximately 0.5e (0.6e) is obtained assuming dipolar scattering. The value agrees well with theoretical estimates obtained from the dependence of the induced dipole moment on the distance from the substrate surface and provides strong support for the description of the adsorbate as partly ionic at low monolayer coverages. If the Na covered Cu(111) surface is exposed to water vapour there is a rapid uptake only if there is more than 10% of a full monolayer of Na on the surface. The existence of a Na precoverage threshold is revealed by the exposure induced vibrational losses and work function changes. Above threshold there is, upon H 2 O exposure, an increase of the work function and strong changes in the low loss energy region of the electron energy loss spectra with peaks appearing at 18 meV and 36 meV loss energy.

Overlayer states for Na on Si(1 0 0)

Abstract Overlayer states formed by valence elctrons, which are reflected back and forth between the vacuum and the substrate interfaces of an adsorbed film, are observed by photoemission at low photon energy for a metal (Na) adsorbed on a cooled Si(1 0 0) substrate.