Electron spectroscopy in proton collisions with dry gas-phase uracil base

We have investigated the electron emission by the RNA uracil base $\left({\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2{\mathrm{O}}_2\right)$ due to collisions with protons in the $25\mathrm{keV}–100\mathrm{keV}$ energy range. Electron spectroscopy was performed at 35° with respect to the beam direction and absolute values for the double-differential cross section were obtained. Our results show preferential emission of low-energy electrons that are responsible for damage in biological material through dissociative electron attachment [Boudaïffa et al., Science 287, 1658 (2000)]. Experimental results are compared to calculations that used the classical trajectory Monte Carlo method, and a reasonable agreement is obtained.

Figure 1

(Color online) Schematic of the normalization procedure (see text). Elastic scattering, at right angle, in ${\mathrm{H}}_3^{+}$ $\left(25\mathrm{keV}\right)+\text{uracil}$ collisions. The scattered particles are identified by time-of-flight spectrometry and detected by a multichannel plates package.

Figure 2

(Color online) (a) Left: DDCS versus electronic energy. Three sets of data are presented for incident proton energies of 100, 50, and $25\mathrm{keV}$, in solid, dashed, and dotted lines, respectively. (b) Right: Comparison between CTMC computation in filled squares, and experimental data in solid line.

Figure 3

(Color online) (a) Left: The uracil molecule in the $XOY$ plane. The electrostatic potentials are displayed in the isodensity surface representation. The very electronegative oxygen atoms are particularly noticeable. For normalization purposes, the initial position of the projectile is contained within a $16\times 16{\mathrm{a.u.}}^2$ surface (see text). (b) Right: A typical trajectory.