Effects of molecular axis orientation of MeV diatomic projectiles on secondary ion emission from biomolecular targets

We report first experimental results on the orientation effect of fast diatomic molecular projectiles on secondary ion emission processes for 3.6-MeV C2+ traversing a self-supporting target of phenylalanine (Phe) film evaporated on a carbon foil. Coincidence measurements were performed on the image of fragments resulting from the Coulomb explosion of C2+ projectiles and the time-of-flight mass spectrometry of secondary ions emitted from the Phe film. We compared the secondary ion yield for C2+ projectiles with parallel and perpendicular orientations to the beam direction. The parallel orientation was found to enhance the secondary ion yield by a factor of approximately 1.1 compared with that for the perpendicular orientation. This enhancement corresponds to an increase in the average charge of Coulomb-exploded fragments. This finding implies that there is a linear correlation between the electronic energy deposition causing secondary ion production and the effective charge number of the incident molecular ions which interact with biomolecular targets.