Alkali Films Research Articles

Enhancement of hydrolysis in alkali ultrathin layers on metal substrates in the presence of electron confinement

Vibrational spectroscopy has been used to study the interaction at room temperature of H 2O with ultrathin alkali films grown on Cu(1 1 1), Ni(1 1 1) and Ag/Ni(1 1 1). We find strongly enhanced dissociation efficiency in the monolayer regime of Na/Cu(1 1 1), indicative of an active role of the existing Na-derived quantum wells in hydrolysis. No water adsorption has been observed for alkalis (Na, K) adsorbed on Ni(1 1 1), very likely due to the absence of electron confinement in these systems. On the other hand, partial water dissociation has been revealed in K-doped silver thin layers on Ni(1 1 1), in the presence of Ag 5sp-derived quantum wells.

Collective Excitations in Nanoscale Thin Alkali Films: Na/Cu(111)

The relationship between electron quantum confinement and the energy dispersion of the surface plasmon in nanoscale thin Na layers adsorbed on Cu(111) at room temperature have been studied by high resolution electron energy loss spectroscopy. Screening effects due to electron quantum confinement occurring in this system cause the lowering of the surface plasmon frequency and, moreover, make the energy range of its dispersion curve larger than in thick alkali films. Landau damping of the plasma excitation was unexpectedly very efficient at small momenta. The dispersion curve of the Na surface plasmon was found to depend on the primary electron beam energy. Multipole surface plasmon at 4.70 eV was observed only for higher impinging energies.

Induced spin currents in alkali films

In sandwiches of FeK and FeCs, the conduction electrons in the alkali metals have a large mean-free path. The experiments suggest that the specular reflection for spin-up and -down electrons is different at the interface, yielding a spin current in the alkali film. The spin current is detected by the anomalous Hall effect of Pb surface impurities.

Interference effects in the optical second harmonic generation from ultrathin alkali films

Interference effects are shown to strongly modulate the transmission second harmonic signal (fundamental wavelength 1067 nm) from rough alkali island films grown on insulating substrates if one varies the angle of incidence. Depending on growth conditions and substrate thickness, the measured second harmonic dependencies can be interpreted in terms of interference between frontside and rearside adsorbed islands or by taking into account the morphology of the adsorbed alkali islands. By the use of different polarization combinations of both pump and reflected second harmonic wave we obtain accurate values of the ratios of the relevant nonlinear optical coefficients.

Unrelaxed processes in photostimulable energy transfer to the dopant in thin films of alkali halides

In thin films of alkali halides the primary process of defect production takes place in the near vicinity of the dopant ion in a ps-time range forming a close pair {F, dopant hole centre} due to capturing of an unrelaxed H-centre by the dopant ion. Only in nanostructures of doped alkali halides the unrelaxed H-centres are able to escape from the surface producing in the grains of nanostructure an interacting F-centre chain (or a channel). It is concluded that specific interaction of unrelaxed H-centres and electrons with the F-centre chain leads to a temperature-independent exciton and electron transport via the chain at any distances. The systems are available as active highly sensitive storage media for fast and multifunctional optical data processing in miniaturised optoelectronic and photonic devices (including optical chips).

Theoretical investigation of epitaxial deformation and the hcp-bcc transition of alkali metals

The deformation of alkali metals K, Rb, and Cs under epitaxial deformation is studied via the ab initio pseudopotential plane wave method using the local-density approximation. Under loading from the stable fcc phase, metastable states along directions @001#, @111#, and @201# are identified. One metastable state, presented at direction @201#, has a very low symmetry in contrast to the planes @001# and @201#. Our results show that the softening direction and sequences of growth is significantly affected by the existence of the metastable states and magnitude of the energy barrier. The resulting softening sequences from soft to hard are @201#, @110#, @001#, and @111# under biaxial compression and @001#, @111#, @201#, and @110# under biaxial tension. An orthorhombic deformation path is used to investigate the fact, that the structure of the alkali films K and Cs evolve from the quasihexagonal structure into the ~110!-oriented bcc structure, observed by experiments. @S0163-1829~99!06741-7#

Unified behavior of alkali core-level binding-energy shifts induced byspmetals

Thin overlayers of Na, K. Rb, and Cs on different sp-metal substrates have been investigated using photo-electron spectroscopy. The alkali core levels show clearly resolved binding-energy shifts between the surface layer, the intermediate layer(s), and the interface layer. The magnitude of these shifts depends on sp metal and on alkali metal. The layer-resolved core-level binding-energy shifts are well reproduced by models based on a thermodynamical description. For three-layer alkali films the core-level binding energy of the intermediate layer is found to exhibit a small but significant shift between different sp-metal substrates. A simple relationship between the core-level binding-energy shift for the interface layer and the difference in rs value between the sp substrate and the adsorbate is shown to exist. (Less)

Adsorbate-adsorbate interactions in Na covered Mg surfaces

We present a study on alkali cluster formation induced by ion bombardment, the information being obtained by Collisionally Excited Autoionization Electron Spectroscopy (CEAES). We show that a uniform alkali film on a metal surface can be easily converted in large patches by low energy ion bombardment. The observed phenomenon is coverage dependent, in fact, for alkali density below a critical value the adlayer is uniformly removed. Beyond this critical value a rearrangement mechanism leads to the formation of extended adatom aggregates revealed by the splitting in two components of both Ne and Na autoionization lines. A Mg atomic structure is also revealed in the spectrum, and, according to our interpretation model, no splitting is observed for this peak due to slow excited atoms.

XPS and UPS study of the oxidation of a stage 1 Cs-graphite intercalation compound: Comparison with alkali films deposited on substrates

The surface of a Cs-Graphite Intercalation Compound (stage 1) has been studied by UPS and XPS under controlled oxygen atmosphere. The evolution of this surface is compared with that of usual Cs thin films. In spite of the presence of half a Cs monolayer before the oxidation, the surface of a GIC does not behave like a submonolayer Cs film due to the diffusion of alkali from the inner inter-layers onto the surface. More generally, the differences between Cs films and Cs-GIC surface are discussed, in particular the nature and structure of the oxide overlayer.

Alkali core level binding energy shifts from buried interfaces between alkali films and metallic substrates with different surface index

Core level binding energy shifts from approximately three layers of different alkali metals deposited on Pd(111), Pd(100), Rh (111), and Rh (110) are presented in order to demonstrate how the packing of the substrate surface affects the layer-resolved alkali core level binding energy shifts. It is found that a more open surface induces a larger alkali core level binding energy shift. It is shown that this behaviour can be explained as a consequence of the higher coordination of the alkali metal atom to the substrate on a more open surface than on a more closed packed surface.

Alkali core-level binding-energy shifts in alkali/4d-metal interface systems.

Photoemission from different alkali core levels (Na 2p, K 3p, Rb 4p, and Cs 4d) has been studied for thin alkali films deposited on four different 4d metal surfaces: Mo(110), Rh(111), Rh(110), and Pd(100). Alkali atoms at the interface, the bulk, and at the surface of the adsorbed alkali film are found to have different core-level binding energies. It is found that the core-level binding-energy shifts of the alkali metals induced by the 4d-metal substrate increase with increasing atomic number of the alkali metal and with increasing atomic number of the 4d metal. Thermodynamical quantities such as interface segregation energies and adhesion energies are deduced from the layer-resolved shifts. Estimates of the experimental binding-energy shifts are given using semiempirical calculations, and certain complications in doing that are discussed.

Electron loss spectra from thin alkali films on Al(111)

Angle resolved inelastic electron scattering has been used to measure the energy and dispersion of the electronic excitations for Na, K, and Cs thin films on Al(111) as a function of thickness. The energy loss spectrum for a two layer film is dominated by a collective excitation similar to that seen for a thick film. This ‘‘surface plasmon’’ mode for a two‐layer film exhibits a negative dispersion even larger than that observed for the thick film, even though the damping is appreciably larger for the thin film. The loss spectra in the submonolayer region are fundamentally different, apparently dominated by single particle excitations. The transition from a single particle to a collective excitation occurs quite abruptly as the second layer begins to form.

AES and EELS study of alkali-metal adsorption kinetics on either cleaved or sputtered GaAs and InP (110) surfaces

A detailed study at room temperature of the interfaces between thin alkali films and cleaved or sputtered processed (110) surfaces of GaAs and InP has been performed using Auger and electron energy loss spectroscopy. Attention has been focused on the interface growth kinetics, and on subsurface intermixing and segregation processes in the presence of different metals (Cs, Na and K) and substrates. The most relevant features in the interface formation are very similar in all cases. Alkali adatoms partially diffuse in the semiconductor subsurface. For K and Na on GaAs, this intermixing also results in an As outdiffusion. On sputter-prepared substrates, the interface formation rate is lower, the As outdiffusion on the contrary is larger and occurs also in the Cs/GaAs case. A comparison with high and low electronegativity metals on III–V semiconductors is made and the results are discussed. As far as interfacial reaction and products are concerned, we found that cations and anions are predominantly involved in the chemical bonding with the adatoms in GaAs and in InP, respectively.

Photoemission study of alkali/GaAs(110) interfaces

A detailed core-level photoemission study of interfaces between thin alkali films andn-orp-type GaAs (110) formed at different substrate temperatures 85 K and 300 K) is reported. All the interfaces grown at 85 K (with Na, K, Rb, and Cs) were found to be non-reactive, while at 300 K, the interface with Na is reactive and that with Cs remains non-reactive. In case of the non-reactive interfaces, a strong band bending of ≊1.0 eV is observed forp-GaAs at alkali coverages as low as θ≊0.01 monolayers, but practically none forn-GaAs. This striking asymmetry in band bending is interpreted as a consequence of the donor character of the alkali atoms. On the other hand, an approximately symmetric band bending at low coverages is observed for the reactive interfaces of Na withn- andp-GaAs and assigned to defect states. For high alkali coverages (θ>2 monolayers), the final band bending is characterizeds by the same Fermilevel position forn- andp-GaAs, independent of the reactivity of the interface, and assigned to metal-induced gap states. Furthermore, systematic trends along the alkali series in Fermi-level position ionization energy, plasmon-loss features, and layer-dependent binding-energy shifts of alkali core levels are discussed.

The nature, role and fate of surface active sites in Li/MgO oxidative coupling catalysts

Drastic changes in nature and surface composition of lithium promoted magnesia are shown to occur in the course of the oxidative coupling of methane, together with changes in kinetics. Thus, the fresh material with a large and heterogeneous surface promotes the total oxidation of methane into CO, CO 2 while the aged catalyst, sintered, partially decarbonated and coated by an alkali film leads preferentially to dimerization products. On the basis of the above features, the nature, role and fate of the active sites for methane conversion are discussed, aiming at possible catalyst improvements.

A kinetic study on the corrodability of anodic oxide films formed on molybdenum in NaOH solutions

The corrodability of anodic oxide films formed on molybdenum in NaOH solutions was studied using impedance and potential measurements. The corrosion rate was found to increase with increase of alkali concentration, film thickness and temperature and was nearly independent of the rate of oxide formation. The dissolution process was found to involve a valency change from Mo(IV) to Mo(VI) where it seemed, from cathodic polarization, that no electron transfer through the oxide film to/from the metal surface was involved during the dissolution process. In concentrated NaOH solutions ([OH−]≧9 M), the dissolution process appeared to follow zero-order kinetics.

Reflection spectra and optical parameters of alkali nitrates at 3?23 eV

Electronic absorption spectra have been recorded for thin films of alkali nitrates at 5-23 eV [I]. The UV and Si observed at 4 and 6 eV have been interpreted as n § ~* and ~ § ~* transitions [2, 3] in the NO~ group (Fig. i). The absorption bands in the region of i0 ($2) and 12 ($3) eV have been assigned [i] to charge-transfer transitions due to excitation of n and ~ electrons to a cation level in the corresponding alkali-metal atom. It is considered that the bond between the alkali cation and the NO~ group is purely ionic, which may be due to the ionization potentials of the alkali metals (Table i) [4] being substantially less than the value of about 13.6 eV for the oxygen atom. In that case, it is assumed that the nature of the cation should not affect the mutual disposition of the n and ~ levels in the valency band and consequently the energy difference between the $3 and S= transitions. Within the errors of experiment, the $3 -$2 difference was found as constant [i]. In this model, the position of the cation level should move downwards in energy in accordance with the ionization potential of the alkali metal in the series Cs < Rb < K < Na (Table i). However, the results obtained in that paper conflict with this conclusion, and it is therefore necessary to refine the model for the electronic structures of the alkali nitrates. Also, there are difficulties in operating with thin films, so it is necessary to check the results of [i]. For this purpose one can use the reflection spectra of the crystals. At present, data are available only for NaNOs [5], so one cannot perform a final identification of the electronic states of the nitrates in the high-energy region. Also, the literature lacks experimental data on the position of the absorption band in the region of 4 eV.

Search for superconductivity in vapour quenched alkaline earth metals and lanthanides

Recent papers by Reale [1–3] report superconducting transitions in vapour quenched films of several alkali-, alkaline earth-, and rare earth metals which were not previously known to be superconducting. We report here a vain effort to verify that Mg, Ba, Ce, Pr, Nd or Yb are superconducting above 0.3 K.

Characteristic energy losses of injected low energy electrons in ionic solids

Characteristic energy losses of 0–18 V electrons have been measured for thin films of alkali and alkaline earth fluorides as well as NaBr, RbBr and RbCl. In addition to losses in the optical region there are also several losses at lower energy for each system, some of which may be due to triplet exciton states. Results for the fluorides, in particular, can be correlated with each other and with optical data.

The role of K-shell ionization in the formation of F centers in alkali halides at 78°K III. KCl

Characteristic energy losses of 0–18 V electrons have been measured for thin films of alkali and alkaline earth fluorides as well as NaBr, RbBr and RbCl. In addition to losses in the optical region there are also several losses at lower energy for each system, some of which may be due to triplet exciton states. Results for the fluorides, in particular, can be correlated with each other and with optical data.