AVS 45th International Symposium
    Surface Science Division Monday Sessions
       Session SS1-MoM

Paper SS1-MoM11
Scattered and Recoiled Ion Fractions From Cd- and S-Terminated CdS(0001) Surfaces

Monday, November 2, 1998, 11:40 am, Room 308

Session: Issues in Surface Electronic Structure
Presenter: M. Liu, University of Houston
Authors: L. Houssiau, University of Houston
M. Liu, University of Houston
J.W. Rabalais, University of Houston
J. Wolfgang, Rice University
P. Nordlander, Rice University
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Scattered and recoiled ion fractions from CdS(0001) and CdS(000-1) surfaces have been determined by means of time-of-flight scattering and recoiling (TOF-SARS) spectrometry. An acceleration tube placed between the sample and the CEM detector enabled separation of the ions and neutrals. By applying a positive (negative) high voltage on the tube, the negative (positive) ions are accelerated, giving rise to neutral and ion peaks in the TOF spectra. On the Cd-terminated surface, 4 keV Ne@super +@ ions scattered at an incident angle @alpha@ = 11° through a scattering angle @theta@ = 50° exhibit a pronounced angular dependence, i.e. the lowest ion fractions (Ne@super +@/Ne@super 0@) are observed along the close packed atomic rows <1000> and <1-100>. On the S-terminated surface however, the same experiment exhibits the opposite angular dependence, i.e. the highest ion fractions (Ne@super +@/Ne@super 0@) being now observed along the close packed rows. 4 keV Kr@super +@ ions were also used to study the S recoiled ion fractions. On the Cd-terminated surface, the lowest S@super -@ ion fractions are found along both the <1000> and <1-100> rows. On the S-terminated surface, the lowest S@super -@ ion fractions are found along the <1000> rows, but the highest ion fractions are found along the <1-100> rows. The final charge state of the ejected atoms is determined by electron tunneling between the surface and the atom as it leaves the surface. This process has been modeled using ab initio calculations of the potentials in the CdS surfaces. In conclusion, it is found that the ion fractions and their angular dependence are extremely sensitive to the nature of the surface terminating layer. This is important in determining the microscopic reactive sites on surfaces.