Energetics of $M{\mathrm{Cs}}_n^{+}$ molecular ions emitted from ${\mathrm{Cs}}^{+}$ irradiated surfaces

Kinetic energy distributions of $M{\mathrm{Cs}}_n^{+}$ ($n=0$,1,2,3, …, etc.) molecular ions ($M$—target element of interest and ${\mathrm{Cs}}^{+}$—primary ion) emitted in the secondary ion mass spectroscopy process have been studied for various metallic samples (Al, Cu, and Ag) under varying ${\mathrm{Cs}}^{+}$ impact ion energies (1–5 keV). From the peak shifts of the energy distributions it is argued that the surface binding energy as well as the work function changes with the change in the surface Cs concentration arising out of the variation in the impact ion energy. The dependence of the measured integrated yields of the ions ($M^{+}$, $M{\mathrm{Cs}}^{+}$, and $M{\mathrm{Cs}}_2^{+}$ for individual elements) on the estimated work functions has been found to be exponential. Results have been interpreted in terms of the relative changes in the instantaneous local work function of the sputtered zone. The present study is strongly supportive of the suggested formation mechanism of a $M{\mathrm{Cs}}_2^{+}$ molecular ion via a combination of $M^{-}$ and two ${\mathrm{Cs}}^{+}$ ions in the close proximity of the Cs-covered surface.

Figure 1

Energy distributions of $M^{+}$, $M{\mathrm{Cs}}^{+}$, and $M{\mathrm{Cs}}_2^{+}$ ions at a bombarding energy of 2 keV for the elements (a) Al, (b) Cu, and (c) Ag.

Figure 2

Energy distributions of (a) ${\mathrm{Al}}^{+}$, (b) $\mathrm{Al}{\mathrm{Cs}}^{+}$, and (c) $\mathrm{Al}{\mathrm{Cs}}_2^{+}$ ions for different bombarding energies.

Figure 3

Range calculations (obtained from TRIM) for Al, Cu, and Ag targets under ${\mathrm{Cs}}^{+}$ bombardment as a function of impact energy.

Figure 4

Work function change $\left(\mathrm{\Delta }\mathrm{\varphi }\right)$ as a function of impact energy for ${\mathrm{Al}}^{+}$, $\mathrm{Al}{\mathrm{Cs}}^{+}$, and $\mathrm{Al}{\mathrm{Cs}}_2^{+}$.

Figure 5

Integrated intensities vs bombarding energies for (a) $M{\mathrm{Cs}}^{+}$ and (b) $M{\mathrm{Cs}}_2^{+}$ ions