Molecular beam study of non-adiabatic electron transfer in the Cl2+Ksurf surface reaction

Doctoral thesis, 2009

Halogen molecules impinging on alkali metal surfaces constitute model systems for studies of charge transfer, dissociation dynamics and non-adiabatic energy dissipation in molecule-surface reactions. This thesis reports emission of exo-electrons and photons from the surface as a result of strong non-adabaticity in the Cl2 gas on K$^{solid}$ reaction. The high electron affinity of chlorine and low work function of potassium results in unusually high yields of emitted particles. This thesis contains three parts: (i) The building of the experimental equipment, (ii) experiments using the equipment, (iii)) and theoretical and numerical modelling of the system . The first and major part of this work was the building of a molecular beam apparatus for scattering of halogens on evaporated alkali surfaces. A supersonic nozzle source plus a velocity selector provided Cl2 molecules from 460 m/s to 1360 m/s (0.08-0.68 eV) by He seeding. The scattering chamber has facilities for measurements of energy resolved photon and electron emission, negative ions, UPS spectra and sticking coefficient. In the second part experimental emission yields and energy distributions of exo-electrons and photons were measured as a function of Cl2 velocity and surface exposure. The exo-electron yield increases by a factor of 4 for the range of molecular velocities mentioned above, while the photon yield did not show any significant change. A small shift towards higher energy of both the exo-electrons and the photons was observed. >98% of the negative charge emitted in the reaction consists of electrons, but some Cl- ions were detected. The initial sticking coefficient was unity for all velocities. The model calculation, based on classical trajectory calculations, includes all the steps of the reaction, from an initial electron transfer from the surface to the molecule (harpooning) that causes dissociation of the molecule, to a second and final electron transfer that, with certain probability and branching ratio, causes emission of an electron or a photon. By comparing the results from the molecular beam experiments with the model calculations, we are able to propose a detailed picture of the dissociation dynamics and of the electron and photon emission processes.