Surface chemiluminescence of Cl2 on potassium

We have measured the initial (zero coverage) photon (1.5<hν<3.5 eV) and electron emission from the reaction of a Cl2 molecular beam with a potassium surface for a range of molecular velocities (460<vCl2<1350 m/s). Three distinct features are observed: (1) A majority of the photons are emitted with hν>eΦ, where eΦ 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 Cl2 velocities. (3) The photon yield varies only weakly with increasing Cl2 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 3p5 hole state, shifted up to ∼4 eV below the Fermi energy. The third point above, the weak variation of the photon emission yield with vCl2 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.