Regular Threshold-Energy Increase with Charge for Neutral-Particle Emission in Collisions of Electrons with Oligonucleotide Anions

Electron-DNA anion collisions were studied using an electrostatic storage ring with a merging electron-beam technique. The rate of neutral particles emitted in collisions started to increase from definite threshold energies, which increased regularly with ion charges in steps of about 10 eV. These threshold energies were almost independent of the length and sequence of DNA, but depended strongly on the ion charges. Neutral particles came from breaks of DNAs, rather than electron detachment. The step of the threshold energy increase approximately agreed with the plasmon excitation energy. It is deduced that plasmon excitation is closely related to the reaction mechanism.

Figure 1

Experimental setup for an electrostatic storage ring with a merging electron beam device and a neutral beam detector.

Figure 2

Neutral particle production rate as a function of the relative energies ($E_r$) in collisions between electrons and deprotonated DNA anions. $E_r$ is given by $\sqrt{E_r}=\sqrt{E_e}-\sqrt{(m_e/m_i)E_i}$, where $E_i$ is the ion energy, and $m_e$ and $m_i$ are electron and ion masses, respectively. The charge states and lengths are given in the figure. The sequences are (a) TG, (b) ATG, (c) ATGC, (d) ATGCTG, (e),(f) ATGCATGC, (g) ATGCATGCTG, (h) ATGCATGCATGC, and (i) ATGCATGCATGCAT.