Direct Experimental Observation of a New Mechanism for Sputtering of Solids by a Large Polyatomic Projectile: Velocity-Correlated Cluster Emission

We have measured kinetic energy distributions of ${\mathrm{Ta}}_n{\mathrm{C}}_n^{+}$ ($n=1–10$) and ${\mathrm{Ag}}_n^{+}$ ($n=1–9$) cluster ions sputtered off Ta and Ag targets, following impact of ${\mathrm{C}}_{60}^{-}$ at 14 keV kinetic energy. A gradual increase of the most probable kinetic energies with increased size of the emitted cluster was observed (nearly the same velocity for all $n$ values). This behavior is in sharp contrast to that reported for cluster emission induced by the impact of a monoatomic projectile. Our observation is in good agreement with a mechanism based on the new concept of a superhot moving precursor as the source of the emitted clusters.

Figure 1

(a) Measured KEDs of ${\mathrm{Ta}}_n{\mathrm{C}}_n^{+}(n=1–10)$ cluster ions sputtered from TaC layer by ${\mathrm{C}}_{60}^{-}$ impact. Insert shows a representative part of the mass spectrum. (b) Kinetic energy distributions of ${\mathrm{Ta}}_n^{+}(n=1–9)$ cluster ions sputtered from a tantalum target by ${\mathrm{Cs}}^{+}$ impact.

Figure 2

Analysis of the measured ${\mathrm{Ta}}_n{\mathrm{C}}_n^{+}$ KEDs based on a fit to a shifted Maxwell distribution [Eq. (2)]. (a) Measured KEDs are given by solid and empty circles. Shifted Maxwell distributions fitted with a single pair of parameters ($ϵ=1.16\mathrm{eV}$, $kT=1.25\mathrm{eV}$) are given by the solid lines. (b) Measured and calculated $E_{\mathrm{m}\mathrm{p}}$ values vs cluster size $n$. The measured values are given by solid circles. The solid line is given by Eq. (3), the dashed line by Eq. (4), and the dash-dotted line is $E_{\mathrm{m}\mathrm{p}}$ in the $T=0$ limit.

Figure 3

Measured and calculated KEDs of ${\mathrm{Ag}}_n^{+}$($n=1–9$) cluster ions sputtered from a silver target by ${\mathrm{C}}_{60}^{-}$ impact. (a) Measured KEDs are given by solid and empty circles. Shifted Maxwell distributions [see Eq. (2)] fitted with a single pair of parameters ($ϵ=0.40\mathrm{eV}$,$kT=1.13\mathrm{eV}$) are given by the solid lines. (b) Measured and calculated $E_{\mathrm{m}\mathrm{p}}$ values vs cluster size $n$. The measured values are given by solid circles. The solid line is given by Eq. (3), the dashed line by Eq. (4), and the dash-dotted line is $E_{\mathrm{m}\mathrm{p}}$ in the $T=0$ limit.