Lars Hellberg

Ballistic motion and abstraction in dimer dissociation at surfaces: Cl2 on K

Non-traditional dissociation dynamics at surfaces, with charge transfer and diabatic motion, is studied for the model system of Cl-2 molecules impinging on the metal K. Three-dimensional molecular dynamics on simple but physically reasonable potential-energy surfaces is used to investigate the final fates of the dissociation fragments (Cl, Cl-) of the molecular ion, Cl-2(-), that is formed by electron transfer (harpooning) to Cl-2 from the K surface. The model study demonstrates that both abstraction (emission of neutrals) and ballistic motion can result from dissociative chemisorption. Our qualitative reasoning has strong implications on other chemisorption systems, like O-2 on alkali and other metals and halogens on Si, e.g., a proposed explanation for ballistically moving O atoms on Al.

Water desorption from nanostructured graphite surfaces

Water interaction with nanostructured graphite surfaces is strongly dependent on the surface morphology. In this work, temperature programmed desorption (TPD) in combination with quadrupole mass spectrometry (QMS) has been used to study water ice desorption from a nanostructured graphite surface. This model surface was fabricated by hole-mask colloidal lithography (HCL) along with oxygen plasma etching and consists of a rough carbon surface covered by well defined structures of highly oriented pyrolytic graphite (HOPG). The results are compared with those from pristine HOPG and a rough (oxygen plasma etched) carbon surface without graphite nanostructures. The samples were characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The TPD experiments were conducted for H2O coverages obtained after exposures between 0.2 and 55 langmuir (L) and reveal a complex desorption behaviour. The spectra from the nanostructured surface show additional, coverage dependent desorption peaks. They are assigned to water bound in two-dimensional (2D) and three-dimensional (3D) hydrogen-bonded networks, defect-bound water, and to water intercalated into the graphite structures. The intercalation is more pronounced for the nanostructured graphite surface in comparison to HOPG surfaces because of a higher concentration of intersheet openings. From the TPD spectra, the desorption energies for water bound in 2D and 3D (multilayer) networks were determined to be 0.32 ± 0.06 and 0.41 ± 0.03 eV per molecule, respectively. An upper limit for the desorption energy for defect-bound water was estimated to be 1 eV per molecule.

Surface chemiluminescence of Cl2 on potassium

We have measured the initial (zero coverage) photon (1.5 ePhi, where ePhi is the work function of K (=2.25 eV), i.e. energies where electrons can also be emitted. (2) The photon spectrum is shifted towards shorter wavelengths at higher Cl, velocities. (3) The photon yield varies only weakly with increasing, Cl, velocities. The photon and exoelectron emission observations are generally consistent with the basic picture that both electrons and photons are created by energetic (non-adiabatic) transitions from valence band states of potassium to a Cl 3p(5) hole state shifted up to similar to4 eV below the Fermi energy. The third point above, the weak variation of the photon emission yield with v(CI2), is in stark contrast to the exoelectron results, where a strong yield increase with the molecular velocity is observed. Details of the velocity dependence of photon and electron emission are still not fully understood, and require advanced calculations