Electron-monocation collisions for protonated and sodiated single-stranded dinucleotides with various base compositions and sequences have been studied using an electrostatic storage ring equipped with a merging-electron-beam device. Resonant neutral-particle emissions have been observed at a collision energy of 4-5 eV. The strength of the resonant bumps changes depending on the number of sodium, as well as the DNA base composition and sequence, which mostly increases with an increase in number of sodium. The results were compared with theoretical structures generated and studied by molecular mechanics and semiempirical quantum-chemistry calculations. It is deduced that the rate correlates with various base-base interactions. (C) 2008 Elsevier B.V. All rights reserved.