Incident Energy Dependence of the Molecular Orientation Effect of MeV C-2(+) Projectiles in Secondary-Ion Emission Processes

We study the effects of the incident energy and molecular axis orientation of diatomic cluster projectiles on secondary-ion emission. C-2(+) projectiles with energies of 2.0-3.6MeV were incident on a self-supporting target consisting of a glycine layer deposited on carbon foil. The molecular axis orientation of C-2(+) projectiles was determined by the foil-induced Coulomb explosion imaging method. Simultaneously, the positive secondary-ions emitted in the beam direction were measured with a linear time-of-flight mass spectrometer. We compared the secondary-ion yields for parallel orientation (the angle theta between the molecular axis and the beam direction is 0-30 degrees) and perpendicular orientation (theta = 60-90 degrees). We found that the orientation effect is negligible at low incident energies but significant at high incident energies. The parallel orientation resulted in a higher secondary-ion yield compared with that of the perpendicular orientation. This effect results from the track overlap created by cluster constituent atoms. Correlation analysis of the experimental results showed that a scaling model can explain the incident energy and orientation-dependent secondary-ion yield considering both the nonlinear electronic stopping power dependence of secondary-ion yield and the track overlap effect.