Aart W. Kleyn

Real-time observation of molecular motion on a surface

The laser-induced movement of CO molecules over a platinum surface was followed in real time by means of ultrafast vibrational spectroscopy. Because the CO molecules bound on different surface sites exhibit different C–O stretch vibrational frequencies, the site-to-site hopping, triggered by excitation with a laser pulse, can be determined from subpicosecond changes in the vibrational spectra. The unexpectedly fast motion—characterized by a 500-femtosecond time constant—reveals that a rotational motion of the CO molecules, rather than pure translation, is required for this diffusion process. This conclusion is corroborated by density functional theory calculations.

Dissociative and non-dissociative sticking of O2 at the Ag(111) surface

Abstract A molecular oxygen beam has been used to study the dissociative and molecular sticking probability at the Ag(111) surface as a function of incidence energy and angle, surface temperature and surface coverage. Two mechanisms are seen to play a role in the dissociative sticking: a direct and a precursor mediated one. Both are strongly activated by the incidence energy. The molecular chemisorption state is shown to be the precursor to dissociation. Results on oxygen desorption are also discussed.

Molecular beam apparatus to study interactions of oriented NO and surfaces

Abstract A molecular beam apparatus to study molecule-surface dynamics is described. A pulsed molecular beam is first state-selected by electrostatic focusing and subsequently oriented with either the N-end or with the O-end towards the surface before it interacts with a Pt(111) or Ag(111) surface. The theoretical analysis is given predicts the degree of orientation. The analysis is verified by measuring the electric field dependence of the orientational effect upon the surface scattering process. Good agreement is obtained.

Probing the (dual) repulsive wall in the interaction of O2, N2, and Ar with the Ag(111) surface

We have performed molecular beam scattering experiments of O2, N2, and Ar from the Ag(111) surface in the translational energy regime from 0.2 to 2.6 eV and at surface temperatures of 600 and 150 K. The experiments were carried out to probe the repulsive part in the particle–surface interaction potential. It is shown that the scattering dynamics of the systems N2–Ag(111) and Ar–Ag(111) is characteristic of physisorption systems. The scattering data for the system O2–Ag(111) shows that both the physisorption and chemisorption part of the interaction potential are probed in the experiment. For ‘‘normal incidence energies’’ exceeding a threshold energy, the barrier to the chemisorption part of the interaction potential can be crossed. An upper limit for this threshold is Eiu2009cos2u2009θi=0.3 eV. Although the chemisorption region is probed, the (dissociative and nondissociative) chemisorption probability remains negligible.

Direct inelastic scattering of oriented NO from Ag(111) and Pt(111)

A pulsed supersonic and cold oriented beam of NO molecules is incident upon the (111) face of clean Ag and Pt single crystal surfaces. The steric effect in the scattered density distributions is determined by a quadrupole mass spectrometer. It is found that the steric effect in the peak in the distribution of direct inelastically scattered molecules depends linearly on the reflection angle. In all circumstances O‐end collisions lead to scattering angles more inclined towards the surface than N‐end collisions. For the Pt(111) surface a much stronger steric effect is measured than for the Ag(111) surface. The steric effect seems to scale with the incident normal velocity. These strong steric effects can be explained by the larger trapping probability for the N‐end orientation and a leverage effect due to the high trapping probability.

Dissociative scattering of polyatomic ions from a liquid surface: CF3+ on a perfluoropolyether film

The dissociative scattering of CF3+ ions from a liquid insulating perfluorinated polyether surface was explored in the incident energy range 100–500 eV. We have measured the angular and energy distributions of the positive and negative fragment ions. The impulsive energy transfer to the liquid molecules and to internal energy of the scattered ions has been determined as a function of incoming energy and total scattering angle. The energy transfer in the ion/surface collision indicated scattering off the –CF3 groups of the liquid molecules. The dissociation of the CF3+ ions was due to a single impulsive collision with these groups. The parent ion was observed as well as CF2+, CF+, and C+ ions. The degree of dissociation depends only on the total scattering angle and the incoming energy, as in gas phase scattering. Remarkably, negatively charged F− fragment ions were also observed. These were formed in a process referred to as collisionally-induced “ion-pair” formation from neutrals leaving the surface. The...

Orientation dependence of rotational excitation in no scattering from Ag(111)

Abstract Scattering experiments with oriented NO beams on Ag(111) have been performed. The scattered molecules are detected by a resonantly enhanced multiphoton ionization. A dependence of the rotational rainbow upon the scattering angle has been observed. The first results of the steric effect measured for single rotational ( J ) levels of scattered molecules are presented. The J =18.5 state is produced preferentially when the O-end collides first with the surface. For the J =8.5 state we find that the N-down geometry is preferable. These results are in qualitative agreement with theory.

The interaction of oxygen with the Ag(110) surface

Results on the scattering and the dissociative sticking of molecules on the Ag(110) surface are presented. The dependence on the incidence energy and angle of the molecules is investigated by employing molecular beam techniques. Both the angle with respect to the surface normal and that with respect to the azimuthal direction on the surface are varied. The and azimuthal directions display a different corrugation as observed by the incident molecule. Dissociative sticking is observed to proceed via the molecular chemisorption state and is strongly enhanced by the incidence energy. An azimuthal dependence of the sticking is observed. Results on the desorption of from the Ag(110) surface are also given.

Dissociative adsorption of NO upon Al(111): Orientation dependent charge transfer and chemisorption reaction dynamics

The dissociative and abstractive chemisorption dynamics of NO on Al(111) were studied. A higher sticking probability for the N end-on of NO onto Al(111) was measured. In contrast, Auger electron experiments reveal stepped surfaces to be oxygen rich at low coverage after exposure to NO. Density functional theory calculations show (i) a few angstroms from the surface, an N end-on first collision geometry results in electronic structures consistent with charge transfer; (ii) there is stabilization on the surface for N end-on or side-on orientations; (iii) dissociation is enhanced by a partial or full flip of the molecule.

The adsorption of methanol on Ag(111) studied with TDS and XPS

Abstract The adsorption of methanol on Ag(111) is studied using thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). It is found that methanol adsorbs reversibly. The TDS spectra reveal three different states. These states can be assigned to molecules desorbing from the monolayer, the crystallised multilayers and the two amorphous multilayers which are located between the two other phases. This is the first time that these three states are seen by means of TDS. This multilayer behaviour of methanol most likely also applies to other metals. The sticking coefficient is found to be coverage independent from the submonolayer into the multilayer regime. The desorption energy is found to be 0.41 eV. XPS shows for both the oxygen and carbon a small shift to higher binding energies with increasing exposure.