Tuning Charge Transfer in Ion–Surface Collisions at Hyperthermal Energies

Abstract

Charge exchange in ion-surface collisions may be influenced by surface adsorbates to alter the charge state of the scattered projectiles. We show here that the positive-ion yield, observed during ion scattering on metal surfaces at low incident energies, is greatly enhanced by adsorbing electronegative species onto the surface. Specifically, when beams of N(+) and O(+) ions are scattered off of clean Au surfaces at hyperthermal energies, no positive ions are observed exiting. Partial adsorption of F atoms on the Au surface, however, leads to the appearance of positively charged primary ions scattering off of Au, a direct result of the increase in the Au work function. The inelastic energy losses for positive-ion exits are slightly larger than the corresponding ionization energies of the respective N and O atoms, which suggest that the detected positive ions are formed by surface reionization during the hard collision event.

Dynamic nitroxyl formation in the ammonia oxidation on platinum via Eley–Rideal reactions

“…The dramatic intensity enhancement is primarily a result of the significant lowering of the Pt surface work function 26 due to the large dipole moment of ammonia: indeed, the work function of Pt(111) has been shown to decrease by B3 eV upon ammonia adsorption, albeit at 100 K. 31 A low work function favours negative ion formation during ion-surface charge exchange. 26 The competition for surface adsorption sites upon NH 3 exposure reduces O-atom coverage and suppresses O 2 À formation. Interestingly, the dynamic O 2 À peak at an exit energy of B50 eV virtually disappears upon exposure to NH 3 , while a slow peak appears at an exit energy of B10 eV (Fig.…”

“…The scattering apparatus and associated ion beam-line have been described in detail elsewhere. [25][26][27] Isotopically-pure reactive beams of O + and O 2 + , extracted from an inductively-coupled plasma and mass-filtered, are directed onto a polycrystalline Pt surface (4N purity, ESPI) which is pre-cleaned by sputtering in situ with an Ar + gun. Beam energy is controlled by varying externally the plasma bias with respect to ground.…”

Direct Hydrogenation of Dinitrogen and Dioxygen via Eley–Rideal Reactions

“…All experiments were carried out in a custom-built ultrahigh vacuum (UHV) scattering apparatus, equipped with a low energy ion beamline connected to an inductively coupled plasma ion source. [12,[36][37] Molecular N2 + and O2 + ions were mass selected from the plasma discharge, operated at 5mTorr and ~500W, with a feed of N2/Ar/Ne and O2/Ar/Ne, respectively. Typical beam current for N2 + was 10-27 µA, and for O2 + was 7-17 µA, over a spot ~3 mm in diameter.…”

Direct Hydrogenation of Dinitrogen and Dioxygen via Eley–Rideal Reactions

“…All experiments were carried out in a custom‐built ultra‐high vacuum (UHV) scattering apparatus, equipped with a low energy ion beamline connected to an inductively coupled plasma ion source . Molecular N 2 + and O 2 + ions were mass selected from the plasma discharge, operated at 5 mTorr and 500 W, with a feed of N 2 /Ar/Ne and O 2 /Ar/Ne, respectively.…”