Lars Diekhöner

Exchange Interaction between Single Magnetic Adatoms

The magnetic coupling between single Co atoms adsorbed on a copper surface is determined by probing the Kondo resonance using low-temperature scanning tunneling spectroscopy. The Kondo resonance, which is due to magnetic correlation effects between the spin of a magnetic adatom and the conduction electrons of the substrate, is modified in a characteristic way by the coupling of the neighboring adatom spins. Increasing the interatomic distance of a Cobalt dimer from 2.56 to 8.1 A we follow the oscillatory transition from ferromagnetic to antiferromagnetic coupling. Adding a third atom to the antiferromagnetically coupled dimer results in the formation of a collective correlated state.

Kondo temperature of magnetic impurities at surfaces

Based on the experimental observation that only the close vicinity of a magnetic impurity at metal surfaces determines its Kondo behavior, we introduce a simple model which explains the Kondo temperatures observed for cobalt adatoms at the (111) and (100) surfaces of Cu, Ag, and Au. Excellent agreement between the model and scanning tunneling spectroscopy experiments is demonstrated. The Kondo temperature is shown to depend on the occupation of the d level determined by the hybridization between the adatom and the substrate with a minimum around single occupancy.

Indirect evidence for strong nonadiabatic coupling in N2 associative desorption from and dissociative adsorption on Ru(0001)

This paper reports the simultaneous internal state and translational energy resolved associative desorption flux of N2 from Ru(0001) using two different experimental approaches. Both experiments show that the nascent N2 is formed with little vibrational excitation and that the total excitation in all N2 degrees of freedom accounts for only 13 of the barrier energy. Roughly 23 of the energy necessary to surmount the barrier is lost to the surface in desorption. This behavior, as well as the unusual behavior noted previously in direct measurements of dissociative adsorption, both imply strong vibrational quenching in reactive trajectories passing over the high exit channel (vibrational) barrier. Adiabatic quasiclassical dynamical calculations based on the ab initio potential energy surface and various models of coupling to the lattice are not qualitatively consistent with N2 vibrational damping to phonons. However, including a strong nonadiabatic coupling of the vibrational coordinate to electron–hole pairs...

Quantum Coherence of Image-Potential States

The quantum dynamics of the two-dimensional image-potential states in front of the Cu(100) surface is measured by scanning tunneling microscopy and spectroscopy. The dispersion relation and the momentum resolved phase-relaxation time of the first image-potential state are determined from the quantum interference patterns in the local density of states at step edges. It is demonstrated that the tip-induced Stark shift does not affect the motion of the electrons parallel to the surface.

N2 dissociative adsorption on Ru(0001): The role of energy loss

New molecular beam experiments on the dissociation probability S0 for N2 on Ru(0001) are presented. These are in general agreement with prior measurements and exhibit very unusual behavior; a very slow increase of S0 with incident kinetic energy E and the fact that S0 is still only ∼10−3 at incident energies considerably above the barrier. A simple dynamical model is developed to describe this unusual sticking behavior. The key aspect is that there is considerable energy loss Δ from E upon initial impact with the surface (principally to the lattice) and only E−Δ is then available to surmount the activation barrier in the exit channel. Using experimentally measured values of Δ from scattering experiments gives good qualitative agreement of this model with the measured S0. One implication of the strong energy loss is that there is an apparent violation of detailed balance when comparing only the reactive fluxes of activated adsorption and associative desorption.

CH4 dissociation on Ru(0001): A view from both sides of the barrier

This paper reports measurements of both dissociative adsorption on and associative desorption from CH4 on Ru(0001). We consider the former a view of dissociation from the front side of the barrier, while the latter is considered as a view of dissociation from the back side of the barrier. A combination of both previous and new molecular beam measurements of dissociative adsorption shows that S0 depends on all experimental variables (E, Tn, Ts and isotope) in a manner similar to other close-packed transition metals. The interpretation of this behavior in terms of a theoretical description of the dissociation is discussed critically, with special emphasis on insights from new theoretical studies. The energy-resolved desorption flux Df(E,Ts) is obtained in associative desorption experiments using the technique of laser assisted associative desorption (LAAD). Measurements at several Ts allow both a direct determination of the adiabatic barrier V*(0) and considerable insight into the dynamics of dissociation. ...

Dynamics of ammonia decomposition on Ru(0001)

Using supersonic molecular beam techniques we have investigated the dissociative adsorption of NH3 on a Ru(0001) surface. At high incident energies, the dissociation increases substantially due to a direct breaking of the N–H bond on impact with the surface. For low incident translational energies, the dissociation depends on surface temperature Ts in an unusual manner, peaking sharply around 400 K. Increasing the surface defect density by low-fluence Ar+ sputtering strongly enhances the dissociation probability while preserving the overall Ts-dependence. We interpret the low incident energy behavior as due to a mechanism in which a molecular precursor must undergo diffusion to defects before dissociating. At the lowest surface temperatures, dissociation is limited by the diffusion of the reaction products away from the defects in order to reactivate them. A kinetic model based on this mechanism is developed which is in good agreement with all experimental observations.

State resolved inelastic scattering of N2 from Ru(0001)

Detailed measurements of state resolved inelastic scattering of N2 from Ru(0001) are reported for a wide range of initial energies (0–3 eV) and angles of incidence. The ion time-of-flight resonantly enhanced multiphoton ionization (REMPI) detection scheme developed here and used with cw molecular beams simultaneously measures the internal quantum state and translational energy normal to the sample surface. Doppler broadening of the REMPI spectrum of scattered particles yields the dispersion in scattering out of plane. The results are qualitatively similar to inelastic N2 scattering studies for a wide variety of other metal surfaces; i.e., no observable vibrational excitation, weak rotational excitation described as a Boltzmann distribution, strong surface excitation depending upon the incident normal energy, and an anticorrelation between rotational and surface excitation. The absence of any vibrational excitation at E≈3 eV is inconsistent with adiabatic model dynamics based on the ab initio potential-ene...

Laser assisted associative desorption of N2 and CO from Ru(0001)

An experimental technique, laser assisted associative desorption (LAAD), is described for determining adiabatic barriers to activated dissociation at the gas-surface interface, as well as some aspects of the dynamics of associative desorption. The basis of this technique is to use a laser induced temperature jump (T-jump) at the surface to induce associative desorption and to measure the translational energy distribution of the desorbing molecules. The highest translational energies observed in desorption are a lower bound to the adiabatic barrier and the shapes of the translational energy distributions provide information on the dynamics. Implementation of the experimental technique is described in detail and unique advantages and possible limitations of the technique are discussed. The application of this technique to very high barrier surface processes is described; associative desorption of N2 from Ru(0001) and CO formed by C+O and C2+O on Ru(0001). N2 barriers to dissociation increases strongly with ...

Observation of metastable atomic nitrogen adsorbed on Ru(0001)

Exposing a Ru(0001) surface to an atomic beam of N produces a series of different states of atomic N adsorbed on the surface. For low atom doses, well-known low coverage states are produced, but for higher atom doses, several previously unknown higher coverage states are sequentially filled. These states exhibit well defined temperature programmed desorption (TPD) peaks which shift to considerably lower temperatures with N coverage. The highest N coverage obtainable is almost 1 ML N/Ru. Recent density functional calculations demonstrate that the N–Ru energy decreases significantly with N coverage, and in fact predict that N adsorbate states are not thermodynamically stable relative to associative desorption at high coverage. The observed high coverage states must, therefore, be metastable with lifetimes determined by the height of the barrier between gas phase N2 and the adsorbed atomic states. Simple analysis of the TPD features in combination with the theoretical adsorption energies allows us to estimat...