Yield enhancement of molecular ions with MeV ion-induced electronic excitation

We performed secondary ion mass spectrometry for arginine thin films in the incident energy range from 10 keV to 2 MeV, in which electronic energy loss in the surface region increases with energy and exceeds the nuclear energy loss. The yield of protonated arginine increased with incident energy, and at 2 MeV it was approximately two orders of magnitude larger than at 10 keV, where the nuclear collision process is dominant. It was found that the molecular ion desorption is enhanced by the electronic excitation induced in the near-surface region. In addition, the yield ratios of the fragment ions to the protonated arginine decreased with incident energy, which indicates that the nuclear energy deposition significantly affects molecular fragmentation. These results demonstrate a soft desorption and ionization of biomolecules by the MeV ion-induced electronic excitation.