Theory of the alkali-metal chemisorption on metal surfaces

Lanthanum modulated reaction pacemakers on a single catalytic nanoparticle

Promoters are important in catalysis, but the atomistic details of their function and particularly their role in reaction instabilities such as kinetic phase transitions and oscillations are often unknown. Employing hydrogen oxidation as probe reaction, a Rh nanotip for mimicking a single Rh nanoparticle and field electron microscopy for in situ monitoring, we demonstrate a La-mediated local catalytic effect. The oscillatory mode of the reaction provides a tool for studying the interplay between different types of reaction pacemakers, i.e., specific local surface atomic configurations that initiate kinetic transitions. The presence of La shifts the bistable reaction states, changes the oscillation pattern and deactivates one of two pacemaker types for the La-free surface. The observed effects originate from the La-enhanced oxygen activation on the catalyst. The experimental observations are corroborated by micro-kinetic model simulations comprising a system of 25 coupled oscillators.

Optical and spin-selective time-of-flight measurement of light-induced desorption of Rb from Fe[Formula: see text]O[Formula: see text] surfaces

Light-induced desorption of Rb atoms from a ferrimagnetic Fe[Formula: see text]O[Formula: see text](001) surface was studied using a spin-selective optical method, which provides information on the spin polarization, velocity distribution, and amount of the desorbed atoms. The results showed that the intensity of the desorption of Rb from Fe[Formula: see text]O[Formula: see text](001) induced by ultraviolet (UV) light irradiation was smaller than the detection limit at coverages lower than the threshold coverage at which the desorption rate began to increase. Moreover, the average magnetic quantum number of the desorbed atoms was smaller than that of electrons at the Fermi level of the Fe[Formula: see text]O[Formula: see text](001) surface. These indicate that the light-induced desorption of Rb from an Fe[Formula: see text]O[Formula: see text](001) surface occurs only in the high-coverage region in which the desorbing atoms are not in contact with the Fe[Formula: see text]O[Formula: see text] surface, and that the desorption does not involve spin transfer.

Coverage dependent adsorption sites in the K/Cu(100) system: A crystal truncation rod analysis

The analysis of the intensity along the diffraction rods normal to the crystal surface (crystal truncation rods) has been used to analyse the adsorption sites of potassium atoms adsorbed at T = 330 K on atomically clean Cu(100) under ultra high vacuum conditions. At potassium coverages below about Θ = 0.18 ML (1 ML = 1.53 × 1015 atoms cm−2), where no ordered adsorbate superstructure forms, the potassium atoms are adsorbed in the fourfold hollow sites of the Cu(100) substrate at d 1 = 2.1(4) Å above the first Cu layer corresponding to an effective potassium radius of r eff = 1.5(3) Å which is close to the ionic radius of 1.33 Å. Within about 1 –2% the substrate interlayer spacings, d 12 and d 23 are not affected.

At high coverage (Θ ≈ 0.30 ML) where an ordered incommensurate overlayer forms, the adsorption site is partially on-top. These results are in correspondence with the model of alkalimetal adsorption on metals supposing extensive charge transfer from potassium to the substrate at low coverage.

Electron-stimulated desorption of cesium atoms from cesium layers adsorbed on gold-covered tungsten

The electron-stimulated desorption (ESD) yields and energy distributions (ED) for neutral cesium atoms have been measured from cesium layers adsorbed on a gold-covered tungsten surface as a function of electron energy, gold film thickness, cesium coverage and substrate temperature. The measurements have been carried out using a time-of-flight method and surface ionization detector in the temperature range 160-300 K. A measurable ESD yield for Cs atoms is observed only after deposition of more than one monolayer of gold and cesium on a tungsten surface at a temperature T = 300 K, which is accompanied by the formation of a CsAu semiconductor film covered with a cesium atom monolayer. The Cs atom ESD yield as a function of incident electron energy has a resonant character and consists of two peaks, the appearance of which depends on both electron energy and substrate temperature. The first peak has an appearance threshold at an electron energy of 57 eV and a substrate temperature of 300 K that is due to Au 5p3/2 core level excitation in the substrate. The second peak appears at an electron energy of 24 eV and a substrate temperature of 160 K. It is associated with a Cs 5s core level excitation in the Cs adsorbed layer. The Au 5p3/2 level excitation corresponds to a single broad peak in the ED with a maximum at a kinetic energy of 0.45 eV at a substrate temperature T = 300 K, which is split into two peaks with maxima at kinetic energies of 0.36 and 0.45 eV at a substrate temperature of 160 K, associated with different Cs atom ESD channels. The Cs 5s level excitation leads to an ED for Cs atoms with a maximum at a kinetic energy of ∼ 0.57 eV which exists only at T < 240 K and low Cs concentrations. The mechanisms for all the Cs atom ESD channels are proposed and compared with the Na atom ESD channels in the Na-Au-W system.

Theory of nanoscale atomic lithography. An ab initio study of the interaction of "cold" Cs atoms with organthiols self-assembled monolayers on Au(111)

This paper deals with the microscopic mechanism of nanolithography of self-assembled monolayers (SAM) of alkanethiol molecules on Au(111) induced by the exposure of the film to a beam of "cold" Cs atoms. Density functional theory calculations have been carried out to elucidate the mechanism of interaction of the Cs atoms with the SAM. We found that the film damage occurs in two steps: the Cs atom penetrates the SAM and at a distance of 10-12 Angstrom from the surface donates one electron to Au, forming a Cs(+) cation which binds strongly to the surface and interacts with the polar head of the SR molecule. The thermal energy released in this process largely exceeds the energy required to stimulate the desorption of RS-SR disulfide molecules from the Au surface with consequent damage of the film. No chemical interaction occurs between Cs or Cs(+) and the hydrocarbon chain of the thiol molecule.

Surface femtochemistry: observation and quantum control of frustrated desorption of alkali atoms from noble metals

This review presents a case study of the direct, real-time observation of a surface photochemical reaction, namely the frustrated photodesorption of alkali atoms from noble metal surfaces. Charge transfer excitation of an electron from the metal substrate into an unoccupied resonance of the alkali atom instantaneously turns on the repulsive Coulomb force inducing the nuclear motion of both the adsorbate and substrate atoms. The incipient nuclear wave packet dynamics are documented for the case of Cs/Cu(111) through the accompanying change in the surface electronic structure. The intimate view of atoms attempting to escape the surface bond highlights the unique role of the substrate in the electronic and nuclear dynamics that ultimately determine the product yields. Moreover, slow dephasing of the coherent polarization is exploited to demonstrate the control of nuclear wave packets through the phase of the excitation light.