Electron emission in grazing ion-surface collisions

Author

Ronaldo Minniti

Date of Award

12-1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Physics

Major Professor

Stuart B. Elston

Abstract

An experimental setup has been developed to study electron emission produced in grazing incidence ion-surface collisions. Incoming C^q_in+ (q_in = 1,2,3) ions with energies of approximately 0.5 MeV/amu, produced by the EN Tandem Van de Graaff Facility at the Oak Ridge National Laboratory, were directed towards a silicon(lGO) surface at grazing angles of incidence between 0.1° and 0.8°. Absolute measurements of electron emission spectra were obtained by performing coincidence measurements between electrons emitted in the interaction and ions scattered from the silicon(lOO) surface in the specularly reflected direction and in a given outgoing charge state. The electrons emitted from the ion-surface interaction were measured using a 160° spherical sector electron energy analyzer employing a position sensitive detector. Such arrangement permitted simultaneous measurement of the energy and angular distributions of emitted electrons. An electrostatic analyzer in combination with a movable electron multiplier detector was used to measure charge state and angular distributions of ions exiting the collision. Convoy electrons produced in ion-surface collisions result from electron capture and loss to low-lying continuum states of the projectile and constitute a prominent feature in the electron emission spectrum. The present measurements confirm pre-viously reported surface effects on convoy emission , mainly the shift in angle and energy of the convoy peak. A value for the yield of convoy electrons, restricted to within the full-width at half-maximum of the convoy peak, has been determined from the measured absolute electron emission spectra to be ~ 0.4 electrons per ion, in the present projectile velocity regime. Study of the projectile velocity dependence of the convoy electron yields allowed addressing the origin of convoy emission in ion-surface collisions. It has been found that convoy electrons result from capture of valence as well as core electrons in the silicon surface. This result is in contrast with previous conclusions derived from measurements using SnTe surfaces in that convoy emission originates mainly from electron capture from Sn and Te core states. The present results thereby suggest that convoy emission in ion-surface collisions is sensitive to the target species.