Experimental study of molecular and cluster effects in secondary electron emission

We have measured the ion-induced secondary-electron-emission (SEE) yields in forward and backward directions from thin sputter-cleaned foils in ultrahigh vacuum. C, Al, Ti, Ni, and Cu have been bombarded with ${\mathrm{H}}^{+}$, ${\mathrm{H}}_2^{+}$, and ${\mathrm{H}}_3^{+}$ (0.3–1.2 MeV/amu), and C and Al have been bombarded with ${\mathrm{C}}^{+}$, ${\mathrm{O}}^{+}$, and ${\mathrm{CO}}^{+}$ (15–85 keV/amu). The yields induced by molecular and cluster ions are compared to those induced by the corresponding isotachic monoatomic projectiles. We observe molecular effects as yield reductions at low projectile velocities (${\mathit{v}}_{\mathit{p}}$≊${\mathit{v}}_{\mathit{o}}$) and yield enhancements at higher velocities (${\mathit{v}}_{\mathit{p}}$≫${\mathit{v}}_{\mathit{o}}$). The results are discussed in the framework of the extended kinetic-emission model by Sternglass and the energy-loss model for clusters by Brandt and Ritchie. The velocity dependence of the molecular effect in SEE follows the velocity dependence of the molecular effect in Brandt’s energy-loss calculations. Thus it seems that the energy loss is also proportional to SE yields for molecular projectiles at velocities around and above the Bohr velocity ${\mathit{v}}_0$.