INITIAL-STAGES OF CESIUM INCORPORATION ON KEV-CS-IRRADIATED SURFACES - POSITIVE-ION EMISSION AND WORK-FUNCTION CHANGES()

The gradual incorporation of cesium into elemental samples of Al, Si, Nb, and Au bombarded with 5.5-keV Cs+ ions was investigated by monitor ing the emission of sputtered positive Cs+ and molecular ions and the relative work-function changes (Delta Phi) induced from the shifts of secondary-ion energy distributions. With increasing Cs fluence and Cs surface concentration the work function is reduced, and reaches a stat ionary value at about 1x10(16) Cs+/cm(2) for Si and Al, 5x10(15) Cs+/c m(2) for Nb, and 1.5x10(15) Cs+/cm(2) for Au. The corresponding Delta Phi shifts then amount to -1.3+/-0.1 eV for Al and Si, -0.9 eV for Nb, and -0.4 eV for Au. This lowering of the work function reduces the io nization probability of positive Cs+ ions by factors of about ten (Al) , seven (Si), and three (Nb). In agreement with the electron-tunneling model of secondary-ion formation, this reduced ionization is observed only when the work function falls below a limiting value which is clo se to the ionization potential of Cs. Computer simulations of the Cs i ncorporation process result in stationary Cs surface concentrations of 12 at % for Si, 10% for Al, 5.5% for Nb, and similar to 2.5% for Au. These values scale inversely with these elements' sputtering yields.