**4. DEA to gaseous DNA subunits and radiosensitizers**

## **4.1. DNA bases**

absolute cross section for Cl*‾*production at the peak values increases with decreasing thickness of the Kr film. When CH3Cl*‾* is formed too close to the metal substrate, the additional electron

**Figure 2.** CH3Cl*‾* formation and dissociation by electrons of 0–2.5 eV incident on submonolayer amounts of CH3Cl physisorbed on a multilayer film of Kr. (a) Variation of the charging coefficient of the film As due to CH3Cl*‾* dissocia‐ tion. (b) Variation with film thickness of: (■) the amplitude of the maximum in the charging cross section (*μ*); (‐·‐·) the amplitude of the maximum in *μ* calculated with the *R*‐matrix method [37]; (•) variation of the energy of maximum in *μ*

In the condensed phase, TMAs differ from their gas‐phase counterparts, in the following ways: (1) the electron energies required for their formation are usually lower by 0.5–1.5 eV, dependent

and As; and (‐‐‐) a parametric fit of this maximum using the image charge model [38].

transfers to the metal, and *μ* sharply decreases.

186 Radiation Effects in Materials

A large number of DEA studies have been performed on gas‐phase DNA bases and their derivatives over the last two decades [2, 39]. Briefly, DEA is the resonant process that involves the LEE capture by a molecule (AB) to produce gaseous TMAs ((AB)‾), described in Section 2, which then dissociate into an anion (A‾) and a neutral radical or radicals (B•), according to the following reaction:

$$\text{e}^- + AB \leftrightarrow \left(AB\right)^- \rightarrow A^- + B^\* \tag{5}$$

In general, the low‐energy resonances in nucleobases are present either at subexcitation (< 3 eV) energies or in the energy range 5–12 eV [39]. The yield function for the DEA processes for thymine resulting in multiple fragment formation is shown in **Figure 3**. To analyze the formation of the negative ions, yield functions were usually recorded by scanning the incident electron energy, while potential voltages applied to the quadrupole mass spectrometer were set for a given ion mass. The ion yields were detected by a channeltorn and plotted as a function of the incident electron energy.

The high‐energy resonances lead to transient anion fragmentation via opening of the ring structure, while the low‐energy resonances are primarily due to loss of one or two neutral hydrogen, which maintains the ring structure.

The DEA yield functions for nucleobases and their related compounds show a remarkable feature that can be recognized as a common phenomenon, that is, site selectivity [40–42]. By tuning the energy of the incoming electron, it is possible to control the location of the bond cleavage. That is, a specific chemical bond in a molecule can be targeted by electrons followed by fragmentation. As an illustration of this site selectivity in nucleobases, DEA to thymine with deuterated and methylated substitutions is described. This phenomenon was observed for other nucleobases and their derivatives, for example, adenine [43] and hypoxanthine [44].

**Figure 3.** (a) Total ion yield as a function of electron energy for DEA process of thymine. (b) Formation of dehydrogen‐ ated anions from thymine (black curve), methylated thymine at N3 site (red curve), methylated thymine at N1 site (blue curve), and thymidine (gray area) [45]. Molecular structure of thymine with numbering and atom labeling.

At subexcitation energies, DEA leads to thymine dehydrogenation by loss of a neutral hydrogen atom [40, 42]. This reaction can be expressed as follows:

$$e^- + T \to T^- \to T^-\_{-H} + H^\* \tag{6}$$

where T<sup>−</sup> is the TMA of thymine (T) and T−<sup>H</sup> <sup>−</sup> is the closed‐shell anion formed by the ejection of a neutral hydrogen radical <*i* >H• < / *i* >. This dehydrogenation process depends on the site from which the H atom is removed. Experimental studies with partially deuterated thymine, in which the deuterium is at either nitrogen or carbon sites, showed that hydrogen loss occurs exclusively from the N sites. H loss from the C sites is thermodynamically accessible within this energy range, but has not been observed experimentally. Moreover, in employing methylated thymine and uracil, it has been shown that by adjusting the electron energy, the loss of H can be made even site‐selective with respect to the N1 and N3 positions. Although 1 eV electrons induce H loss at the N1 position (N1‐H), the process can be switched at 1.8 eV to N3‐H (**Figure 3b**). These results have significant consequences for the molecular mechanism of DNA strand breaks induced by LEEs. Within DNA, the N1 position of thymine is coupled with the sugar moiety and thus forms thymidine, which is one of the nucleosides. Because the shapes of the signals from thymine and the more complex thymidine resemble each other, it can be concluded that H abstraction in thymidine predominantly occurs at the thymine moiety and, more precisely, at the N3 position (**Figure 3b**) [45].

In addition to the detection of anions and the energies at which they are formed, much effort has been expended to matching particular types of DEA process to specific resonant peaks observed in DEA ion yields. In the case of the most abundant anion formed for all nucleobases, it has been proposed that these resonant peaks can be assigned to vibrational Feshbach resonances (VFRs) [46, 47]. VFRs usually occur at low energies, when vibrational levels of the transient anion lie below the corresponding vibrational states of the neutral, and are more expected in greatly polarized molecules with very large dipole moment, which leads to a long‐ range attractive interaction. They may serve as a gateway for dissociation at low energies if they are coupled with a dissociative valence state. This can be the case for the formation of dehydrogenated ions from nucleobases, where resonances arise from coupling between the dipole bound state and the transient anion state associated with the occupation of the lowest *σ*\* orbital. Recently, the nucleobase fragmentation of N–H bonds induced by LEEs was studied by employing the CASPT2//CASSCF computational approach [48]. These calculations showed that the two lowest lying π\* states can be determined at energies below 1.0 eV and above 2.0 eV for pyrimidines, whereas for purines, this energy gap between the two anionic states was less pronounced. These calculations also suggested the possibility of coexistence of dipole‐ bound and valence‐bound processes in the low‐electron energy range.

Further to the observations of site selectivity in DEA processes leading to single‐bond cleavage within a nucleobase, site selectivity also occurred in multiple‐bond cleavage. As in the cases of both dehydrogenation of nucleobases and its complementary channels, which resulted in the Hˉ formation, site selectivity was demonstrated when multiple‐bond cleavage was involved, for example, for the formation of NCO‾ from thymine and its derivatives [49]. This anionic fragment was formed in a sequential decay reaction, in which the dehydrogenated anionic nucleobase acts as an intermediate product. In this case, the remarkable resonances, which were observed for dehydrogenation and for H reaction channels in nucleobases, were preserved for the subsequent decay reaction, leading to the formation of NCO‾ as the final product.

In general, the total cross sections for DEA to nucleobases exhibited comparable magnitudes in the energy range for TMA formation [50]. However, these cross sections were up to 10 times smaller than those for the formation of single‐strand breaks, while the cross sections for sugar and phosphate group analogs (see Section 4.2) were even smaller in magnitude.
