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 H{sup +}, H{sub 2}{sup +}, and H{sub 3}{sup +} (0.3--1.2 MeV/amu), and C and Al have been bombarded with C{sup +}, O{sup +}, and CO{sup +} (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 ({ital v}{sub {ital p}}{approx}{ital v{ital o}}) and yield enhancements at higher velocities ({ital v}{sub {ital p}}{much gt}{ital v{ital 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 {ital v}{sub 0}.