Initial stages of cesium incorporation on keV-Cs+-irradiated surfaces: Positive-ion emission and work-function changes.

Abstract

The gradual incorporation of cesium into elemental samples of Al, Si, Nb, and Au bombarded with 5.5-keV ${\mathrm{Cs}}^{+}$ ions was investigated by monitoring the emission of sputtered positive ${\mathrm{Cs}}^{+}$ and molecular ions and the relative work-function changes (\ensuremath{\Delta}\ensuremath{\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 1\ifmmode\times\else\texttimes\fi{}${10}^{16}$ ${\mathrm{Cs}}^{+}$/ ${\mathrm{cm}}^{2}$ for Si and Al, 5\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{Cs}}^{+}$/${\mathrm{cm}}^{2}$ for Nb, and 1.5\ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{Cs}}^{+}$/${\mathrm{cm}}^{2}$ for Au. The corresponding \ensuremath{\Delta}\ensuremath{\Phi} shifts then amount to -1.3\ifmmode\pm\else\textpm\fi{}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 ${\mathrm{Cs}}^{+}$ 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 \ensuremath{\sim}2.5% for Au. These values scale inversely with these elements' sputtering yields. \textcopyright{} 1996 The American Physical Society.