Negative ion formation in near grazing surface scattering of hyperthermal neutral C60: Image charge effects

Abstract

Formation of negatively charged C 60Ϫ was observed in the near-grazing scattering of hyperthermal ͑10-50 eV͒ neutral C 60 0 from a carbon-covered ͑monolayer graphite͒ nickel surface. Under these conditions the normal energy component of the scattered particle ͑0.3-1.5 eV͒ is of the same order of magnitude as the attractive potential well. We have measured both energy and angle distributions of the scattered C 60Ϫ and studied the electron transfer process by comparing the energy and angular distributions of the scattered negatively charged and neutral C 60 as a function of primary energy. The shifts observed between the angular and energy distributions maxima of the neutral and negative ion could be analyzed and explained in terms of image charge effects on the outgoing trajectory ͑deflection͒ and exit energy ͑retardation͒ of the C 60Ϫ . The angular deflection analysis yield image charge barrier of 0.28Ϯ0.02 eV corresponding to a rather large ''ion formation'' critical distance of 12.9Ϯ0.9 Å. We believe that this is the first direct observation of image charge effects in molecule/cluster-surface scattering.

Relative yield ofC60−in hyperthermal surface scattering of neutralCBekkerman1, Tsipinyuk2, Kolodney3 2000Phys. Rev. BSelf Cite21020View full textAdd to dashboardCite

“…The observed distributions are very similar, except for a small width difference for some E 0 values. This small effect is probably apparatus related, 31 and has no importance with regard to the study presented here. These results for ϭ135°are in good agreement with former observations ͑at ϭ45°) of small retardation effect on the C 60 Ϫ scattered energy ͑as compared with C 60 0 ) due to image charge attraction.…”

“…These results for ϭ135°are in good agreement with former observations ͑at ϭ45°) of small retardation effect on the C 60 Ϫ scattered energy ͑as compared with C 60 0 ) due to image charge attraction. 31 The expected small shift of ϳ0.3 eV is practically nondiscernible here.…”

“…33 The molecular-beam-surfacescattering apparatus is described in detail elsewhere. 28,31 The kinetic energy E 0 of the C 60 molecules is measured ͑after ionization͒ by a 90°electrostatic energy analyzer which is mounted on the beam line. The kinetic energy was mainly varied by tuning the nozzle backing pressure, i.e., controlling the velocity slip between the helium carrier gas and the C 60 seed molecules.…”

“…Surface preparation and analysis were described before. 28,31 This surface will be denoted as Ni/C from this point on. The scattering configuration is that of a rotating surface and a fixed detector with a scattering angle ϭ180Ϫ( i ϩ r ), where i is the incidence angle and r is the reflection angle, both measured from the normal to the surface.…”

“…30 Very recently we reported on negative-ion formation (C 60 Ϫ ) in the hyperthermal (E 0 ϭ10-50 eV) scattering of neutral C 60 molecules from a fully carbonized nickel surface ͑covered with a monolayer of graphite͒. 31 We have focused on measurements of image charge effects on energy and angular distributions of the scattered C 60 Ϫ , and by comparing with the corresponding distributions of scattered neutral C 60 ͑denoted C 60 0 throughout this paper͒ we have extracted an effective ''ion formation'' distance Z C ͑between the C 60 center and the surface͒ of 13Ϯ1 Å. This distance is defined as a critical distance only in relation with the simple model used for analysis.…”