W. Harbich

Mass-selected deposition of Ag7 on Pd (100): Evidence for implantation and strong fragmentation

We present a new experiment which is able to monitor mass-selected cluster depositions in red time using specular reflected thermal helium scattering. Ag/sub 7//sup +/ with an incident energy of 2.86 and of 13.6 eV/atom is deposited on Pd(100) at different surface temperatures. The measurement technique relies on the large atomic cross sections Sigma of adsorbates for diffuse helium scattering. An analysis of Sigma shows that heptamers become fragmented upon impact for both energies and that some of the cluster atoms are implanted into the surface layer.

Deposition of mass-selected clusters studied by thermal energy atom scattering and low-temperature scanning tunneling microscopy: An experimental setup

We present an experimental setup for the investigation of the processes occurring during the deposition of mass-selected clusters on a well-defined surface. The sample is analyzed in situ by two complementary methods: thermal energy atom scattering (TEAS) and scanning tunneling microscopy (STM). TEAS is used to study the dynamical processes during the deposition and to gather statistical information about the resulting structures on the surface. Subsequent STM measurements allow us to investigate the collision outcome on an atomic scale. The setup is highly versatile and guarantees ultra-high-vacuum conditions and cryogenic temperatures (≈30 K) of the sample at all times even during sample transfer. Clusters are produced in a CORDIS-type cluster source. A new compact multichannel effusive He source in combination with a new Wien-filter-based He detector are used for TEAS measurements. The new low-temperature STM allows measurements in a temperature range between 8 and 450 K. Atomic resolution on the Pt(11...

The fate of mass selected silver clusters deposited on Pd(100)

We present a new experiment to monitor ionized atom and mass selected cluster depositions in real time using specular reflected thermal helium scattering. Monomer Ag ions with an incident kinetic energy of E/sub dep/=20 eV and E/sub dep/=95 eV create surface defects on Pd(100) which affect the surface morphology even at 400 K surface temperature. The defect production is the dominant process responsible for the reflectivity decay at low surface temperatures. Kr/sup +/ ions deposited under identical conditions confirm this. Silver clusters (Ag/sub 7/) with an incident energy of 2.85 and of 13.5 eV/atom become severely fragmented upon impact, the remaining cluster-like features being implanted fragments, whose size increases with incident energy.

Neighbor driven mobility of silver adatoms on Pd(100) measured by thermal helium scattering

Thermal helium scattering is used to characterize the deposition of silver atoms on Pd (100) in the coverage range 0 to 10% of a monolayer, at surface temperature between 80 and 160 K, the latter being the threshold of adatom mobility. The attenuation of the specular helium intensity as a function of coverage cannot be interpreted by a statistical distribution which excludes any mobility of the deposited adatoms. The data are well simulated by a model implying that an adatom is not stable if it is separated from another adparticle by only one adsorption site, but it would rather move and form a dimer or enlarge a cluster. This behavior is confirmed by Monte Carlo simulations. The cross section of the silver adatom for diffuse helium scattering is measured to be 109 AA/sup 2/.

Mass selected cluster deposition in strongly or weakly interacting media

Mass selected cluster deposition is discussed with respect to three questions: what is the integrity of the clusters after deposition; are they uniform in terms of size and isomerisation; and finally what is the influence of the support on the clusters? The answer to these questions will depend on the strength of cluster media interaction and we have chosen two extreme cases the inert rare gases on one hand and metal surfaces on the other. Three examples are given: Raman studies as well as low-temperature STM give structural information on the deposited clusters, while other optical spectroscopies (absorption and fluorescence) yield information on the electronic structure of the clusters.

Surface diffusion of Ag on Pd(100) measured with specular helium scattering

We present a new method to determine surface diffusion constants using specular reflected thermal helium scattering. The specular reflected helium intensity is recorded in real time as a function of surface coverage of the adsorbate and surface temperature. The experiment is compared to a model calculation based on rate equations for the generation and destruction of adatoms and clusters. We find E/sub d1/=0.35+or-0.03 eV and nu /sub 01/=3*10/sup 9/s/sup -1/ as surface diffusion barrier and pre-exponential factor for the system Ag/Pd(100).

Chemisorption and macroscopic diffusion of NO on Pt(111)

Thermal energy helium scattering is used to measure the isothermal desorption of NO from Pt(111) in the temperature range 343-383 K. Taking into account the vibrational degrees of freedom to calculate the residence time of the molecules, the energy of adsorption is E/sub a/=-29.8+or-1 kcal/mol, and a repulsive interaction energy between molecules is found: phi =6.1+or-1 kcal/mol. Also, the macroscopic diffusion of NO on Pt(111) is measured for the first time between 313 and 358 K. The technique uses a helium beam to monitor the time dependence of the coverage on a well defined surface spot, when the adsorbed molecules diffuse outside this spot. The activation energy for diffusion at low coverage is E/sub D/=11.8+or-1.2 kcal/mol, and the pre-exponential factor is D/sub 0/=2.4*10/sup -3/ m/sup 2//s.

Size selected cluster deposition on well characterized surfaces: Agn/Pd(100)

We study the deposition of Ag ions, and size selected Ag/sub 7/ and Ag/sub 19/ cluster ions on Pd(100) at total kinetic energies of 20 eV and 95 eV using thermal energy atom scattering and molecular dynamics simulations. We find that at all energies Ag atoms are implanted into the substrate and Pd atoms are ejected in the adlayer. The experimental results in the case of Ag/sub 7/ can be understood in taking into account both implantation of Ag atoms and heavy fragmentation. In the case of Ag/sub 19/ the deposition leads at low temperature to non compact structures localized around the impact point. We propose a model in which morphology changes take place between 200 K and 300 K resulting in well separated compact structures formed of Ag and Pd adatoms.