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 monitoring 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 stationary value at about 1x10(16) Cs+/cm(2) for Si and Al, 5x10(15) Cs+/cm(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 ionization 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 close to the ionization potential of Cs. Computer simulations of the Cs incorporation 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.