Eur. Phys. J. D (2015) 69: 3 DOI: 10.1140/epjd/e2014-50435-5

THE EUROPEAN PHYSICAL JOURNAL D

Regular Article

Fragmentation of uracil after electron capture by doubly charged ions Vadim V. Afrosimov1 , Alexei A. Basalaev1,a , Oleg S. Vasyutinskii1,2 , Michael N. Panov1, and Oleg V. Smirnov1,3 1 2 3

Ioffe Institute, 26 Polytekhnicheskaya, 194021 St. Petersburg, Russia St Petersburg Polytechnic University, 29 Polytekhnicheskaya, 195251 St. Petersburg, Russia St.Petersburg Academic university, Khlopina 8, 195220 St. Petersburg, Russia Received 10 June 2014 / Received in final form 15 August 2014 c EDP Sciences, Societ` Published online 8 January 2015 –  a Italiana di Fisica, Springer-Verlag 2015 Abstract. Single electron capture from molecular uracil (C4 H4 N2 O2 ) by doubly-charged He2+ , C2+ , and O2+ atomic ions followed by the fragmentation of uracil ions has been studied using time-of-flight mass-spectrometry at the collision velocity range 0.13–0.65 a.u. The uracil ion fragmentation mechanism has been clarified by the arrival time correlation analysis for all ion-fragments produced in a single collision. Electron capture with ionization cross section has been determined from experiment. As shown, single electron capture in reaction of uracil with C2+ leads to high fragmentation probability of the resulting uracil ion. The role of electron core rearrangement in the C2+ and O2+ atomic ions is discussed.

1 Introduction The interaction of nucleobases (NB) with relatively high energy electrons, photons and ions in the gas phase was the subject of many studies stimulated by the importance of understanding the mechanisms of DNA damage upon exposure to radiation. Some of these studies have resulted in important data on the relative molecular fragmentation cross sections initiated by electron impact ionization [1,2]. At electron energies near the maximum of the ionization cross section (Eel = 50−100 eV) the main reaction channel for all five NB leads to the production of non-dissociated molecular ions, however, contribution from dissociative ionization can also be significant. At electron kinetic energies close to the fragmentation threshold the ionization cross sections show strong energy dependence [3,4]. There is practically no data in the literature on the absolute ionization cross sections for the interaction of NB with electrons. To the best of our knowledge this data is only available for cytosine and thymine in the electron kinetic energy range from the ionization threshold up to 200 eV [5,6]. Mass-spectrometric studies of NB photoionization by vacuum ultraviolet (VUV) radiation have been carried out by Jochims et al. and by Li et al. [7,8]. The investigation of the ionization cross sections as a function of the UV radiation energy and the electron kinetic energy allows for determination of ionization potentials (IP) and ionfragments appearance energies (AE). Important information on the NB electronic structure has also been obtained from the analysis of ejected electron spectra observed a

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in photoionization [9–11] and electron ionization [12,13] experiments. The theoretical studies of the collision interaction between NB and ions performed so far dealt only with total cross sections and did not consider molecular ion fragmentations. Combining the classical treatment of the overbarrier transitions and the statistical approach within the frame of the classical trajectory Monte-Carlo calculations (CTMC-COB), Abbas et al. [14] calculated the cross sections of single electron capture, electron capture with ionization, double electron capture, ionization, and double ionization for the He2+ + Ade and He2+ + Cyt systems in the collision velocity range 1−11 a.u. The same method was used for calculation of the electron loss cross sections (single electron capture combined with single ionization) by all five NBs interacting with H+ , He2+ , and C6+ atomic ions in the relative velocity range from 0.6 to 20 a.u [15]. The ionization cross sections for uracil (Ura) colliding with multiply charged carbon and oxygen atomic ions were calculated using the CTMC-COB method and several quantum mechanical approximations: the continuum distorted wave-eikonal initial state (CDW-EIS), eikonal initial-state (EIS), and the first Born approximation in the collision velocity range 0.5−16 a.u. [16]. The CTMC-COB and CDW-EIS calculations gave satisfactory agreement with experimental results. The experimental studies of interaction of NB with ions were performed in several different directions. The determination of absolute ionization cross sections of adenine and uracil by fast protons (Vp = 1−9 a.u.) using electron spectroscopy without analysis of possible fragmentation channels was reported by Moretto-Capelle and Le Padellec [17] and by Iriki et al. [18–20]. The cross sections of direct ionization and electron capture in

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collisions between fast protons and adenine, thymine, cytosine, and uracil molecules in the relative velocities of Vp = 1−2.5 a.u. were determined and analyzed by Tabet et al. [21–23]. Both processes result in production of the same groups of fragments, however the electron capture has larger fragmentation probability than the direct ionization. Tribedi et al. [24] measured the fragmentation cross sections of uracil ions produced in collisions of uracil with fast nuclei of C6+ , O8+ , and F9+ at the relative velocity range Vp = 9−16 a.u. They reported that the relative cross sections of production of various ion-fragments are practically independent of the collision velocity while the total ionization cross sections decrease slowly with the collision velocity and have large values (for instance, σion ∼ 6 × 10−15 cm2 for F9+ ). Agnihotri et al. [16] determined the absolute total ionization cross sections of uracil in collisions with fast (Vp = 0.5−16 a.u.) multiply charged carbon and oxygen ions. Relative fragmentation cross sections determined at several collision energies were found to be similar to the ionization cross sections of uracil molecules colliding with electrons at 70 eV. Bernard et al. [25] studied multi-electron capture by Ar8+ ions from adenine at the collision energy of 56 keV (Vp = 0.24 a.u.). The number of electrons captured was determined as a function of the final projectile charge state and the number of electrons ejected in each collision. Schlath¨ olter et al. [26–29] studied the interaction of slow Cz+ (z = 1−6), He2+ , N2+ and O2+ ions at the velocity range Vp = 0.1−0.7 a.u. with uracil and thymine molecules using a chopped incident ion beam and time-offlight (TOF) ion detection. The authors observed strong dependence of the ion-fragment production on the collision velocity, on the target molecule electronic structure, and on the projectile charge and atomic number. Schlath¨ olter et al. [26–29] also reported the predominant role of the uracil ion fragmentation in the C2+ + Ura reaction. These results were supported theoretically by Bacchus-Montabonel et al. [30] who performed quantum mechanical calculations of the electron capture by multiply charged ions Cz+ (z = 2−4) from uracil at the collision velocity range of 0.1–0.7 a.u. and showed that the single electron capture cross section without fragmentation of the molecular ion Ura+ is very small and does not exceed 10−17 cm2 . However, the ion-fragment spectra detected by Schlath¨ olter et al. [26–29] were formed by the projectile ions produced in all possible final charge states. Also, the relative contribution of the processes of single electron capture and electron capture with ionization was not considered. The main aim of this work was to obtain new information on the interaction dynamics of doubly charged He2+ , C2+ , and O2+ ions colliding with Ura molecules using TOF detection of the molecular ions produced in the processes:  (1a) A+ + Ura+∗ 2+ A + Ura → 2+∗ + − A + Ura +e , (1b) where A = He, C, or O atoms.

Eur. Phys. J. D (2015) 69: 3

Reaction (1a) is electron capture and reaction (1b) is electron capture with ionization. The excited singly and doubly charged uracil ions Ura+∗ and Ura2+∗ in (1a) and (1b) can then dissociate into charged and neutral molecular fragments:  Ura+∗ → Fr+ Fr0i (2a) 1 + i  + Fr0i , (2b) Ura2+∗ → Fr+ 2 + Fr3 + i

where Fr0i are neutral fragments that were not detected in our experiment. Relative contribution from the single electron capture (1a) and electron capture with ionization (1b) and contributions from all fragmentation channels in reaction (1a) have been determined. The fragmentation mechanisms were studied by arrival time correlation analysis of the molecular ion-fragments produced in a single collision using a multi-hit mode when both ionic fragments produced in dissociation of a doubly charged molecular uracil were detected. This allowed us for separation of (1a) and (1b) processes by single-hit versus double-hit events. As shown, fragmentation of the molecular ion is the major channel of the single electron capture by C2+ . This result agrees perfectly with previous studies [26,28].

2 Experimental The experimental method used was already described in detail elsewhere [31]. Briefly, a collimated monokinetic beam of doubly charged He2+ , C2+ , or O2+ ions with kinetic energy in the range 6–30 keV crossed an effusive beam of uracil molecules. A homogeneous electric field was applied to the interaction area to extract the molecular ions produced and direct them into a time-of-flight (TOF) mass-analyzer. After passing the electron optics system of the mass-analyzer the molecular ions acquired the kinetic energy of 2.5 × q keV where q is the ion-fragment charge and then additionally accelerated up to the 14 × q keV energy and detected by an ion counter. The additional acceleration was used to provide high and practically uniform registration efficiency of the ions with various masses and charges. The charge of the projectile past the interaction area was determined by an electrostatic analyzer. The electrostatic analyzer detected all projectiles within the scattering angle of θ = ± 1.5 degrees. Electric pulses produced by the analyzer detector were used for synchronisation of all other detection electronics. The TOF mass-spectra were taken in a multi-hit mode when both ionic fragments produced in dissociation of a doubly charged molecular uracil (see Eq. (2b)) were detected. The design of the TOF massspectrometer ion-optics provided practically complete ion collection with initial kinetic energy below 9 eV and allowed for separation of (1a) and (1b) processes by singlehit versus double-hit events. The effusion beam of uracil molecules was formed by evaporating uracil crystals in an oven at 160–170 ◦ C.

Eur. Phys. J. D (2015) 69: 3

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N3 O7

σ 0221 σ 0121

Ura+

8

0.4

4

2

5

N1

6

0.3

O2++ Ura 0.2

0.1

He2++

Ura

×0.5

C2++ Ura 0

10

20

30

40

50

60

70

80

90

0 0.1 100

110

120

0.2

0.3

0.4

0.5

0.6

0.7

Vp (au)

m(u)

Fig. 1. Mass-spectra formed via single electron capture from uracil molecules by doubly charged C2+ , He2+ and O2+ ions.

The ratio of the integrated peaks in the detected massspectrum was almost independent of the oven temperature which was fixed to an accuracy of 1 ◦ C. For control of the mass-spectrum signal background the molecular flux from the oven could be closed providing the possibility to detect “background + effect” and “pure background” signals separately without changing the temperature in the interaction chamber. The uracil crystals were purified and dehydrated before the experiment by heating the molecular sample in vacuum at the temperature 110 ◦ C during several days under regular monitoring of the molecular flux mass-spectrum and background.

3 Experimental results and discussion 3.1 Electron capture and electron capture with ionization Typical experimental mass-spectra are given in Figure 1. Each of the three spectra in Figure 1 contains a peak of the singly charged uracil ions and mutual ion-fragment peaks produced due to dissociation of uracil molecular ions. The uracil ions were formed due to single electron capture from the uracil molecules by the doubly charged ions He2+ , C2+ and O2+ with almost the same velocities: Vp (He2+ ) = 0.30 au, Vp (C2+ ) = 0.27 au, Vp (O2+ ) = 0.28 au. The insert in Figure 1 shows the structure of the most common uracil tautomer. The relative concentration of other uracil tautomers in the gas phase at the temperatures up to 325 ◦ C is less than 1% [11,32]. The mass-spectra presented in Figure 1 relate to possible fragmentation reactions via process (1a). The sum of integrals of all peaks in Figure 1 is proportional to the to21 tal cross section of the single electron capture σ01 , where the first and the second superscript indices 2 and 1 denote the initial and final charge state of the ion-projectile, respectively, while the first and the second subscript indices 0 and 1 denote the initial charge of the uracil

Fig. 2. Relative cross sections of the electron capture with 21 21 ionization (σ02 /σ01 ). x − He2+ ,  − C2+ , ◦ − N2+ , ∇ − O2+ .

molecule and the final charge of the transition state of the uracil cation, respectively. As can be seen from Figure 1 the fragmentation spectrum depends dramatically on the type of incident atomic ion. Note that all mass spectra in Figure 1 relate to the fragmentation processes resulting from the production of singly charged molecular ions. No doubly charged fragments were detected in the experiment which means that the loss of two electrons with high probability results in dissociation of the Ura2+∗ ion into two singly charged fragments, see equation (2b). Projectile velocities are Vp (He2+ ) = 0.30 au, Vp (C2+ ) = 0.27 au, Vp (O2+ ) = 0.28 au. All spectra are normalized to the sum of all peak integrals assuming that the single electron capture cross section is the same for each of the three reactions. The intensities of the peaks in the He2+ + Ura mass spectrum in the mass range 100–120 u are multiplied by 0.5. The insert shows the structure of the most common uracil tautomer. Using the technique of almost complete collection of the ion-fragments (see Ref. [31] for details) the relative cross section of the electron capture with ionization 21 21 (σ02 /σ01 ) (see Eq. (1b)) was determined as a function of the collision velocity. The result obtained is given in Figure 2. As can be seen in Figure 2 the cross section of the electron capture with ionization (1b) is about one fourth of the single electron capture cross section (1a) and increases slowly with collision velocity. Also, Figure 2 clearly shows that the ratios of the process (1b) and (1a) for C, N, and O are similar, while the He ones differ distinctly. This result can be compared with that reported previously by Schlath¨ olter et al. [26,28] who introduced the relative fragmentation cross section: σfZ = 1 − (Yura /Ytotal ), where Yura is the uracil peak integral, Ytotal is the sum of all peak integrals, and Z is the projectile charge. In 21 our notation, Yura = Cσ01 (Ura+ ) is the peak integral of 21 21 non-dissociated Ura+ in Figure 1, Ytotal ≈ C(σ01 + 2σ02 ), and C is the proportionality constant. The expression for Ytotal is an approximation because the value of Ytotal experimentally determined in references [26,28] contained

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Eur. Phys. J. D (2015) 69: 3 100

1.0

Eel=70eV

σ % σ 0121

σ 2f

σ % σ ion

0.9 10

0.8

0.7

1

0.6 0.1

0.2

0.3

0.4

0.5

0.6

Vp (au)

Fig. 3. Relative cross sections of fragmentation σf2 (Eq. (3)). σf2 (–) present work data x – He2+ ,  – C2+ , ◦ – N2+ , ∇ – O2+ , σf2 – available data of [26,28]  (– – –) – C2+ , • – N2+ ,  – O2+ .

not only all peak integrals in reactions (1a) and (1b), but also contributions from double electron capture and double electron capture with ionization which were not determined in this work. The expression for the relative fragmentation cross section σf2 given by Schlath¨ olter et al. [26,28] written in this paper notation is presented in equation (3): σf2

0.3

0.4

0.5

0.6

0.7

21 Yura σ01 (Ura+ ) =1− ≈ 1 − 21 21 . Ytot σ01 + 2σ02

(3)

olter The experimental σf2 values reported by Schlath¨ et al. [26,28] and determined in this work are presented in Figure 3 for C, N, O and He as function of the collision velocity. As can be seen from Figure 3, our experimental data is in good agreement with the C, N, and O data reported by Schlath¨ olter et al. [26,28]. Note that unlike Figure 2, O data in Figure 3 looks different from the C and N data. This different behavior simply deals with dif21 ferent values of σ01 (Ura+ ) in equation (3) for the reactions above (see Figs. 4–6). 3.2 The fragmentation of singly charged uracil ions As shown in Figure 1 electron capture resulting in the production of a singly charged molecular ion (1a) is often followed by fragmentation into a charged molecular 0 fragment Fr+ 1 and neutral fragments Fri (see Eq. (2a)). Depending on their masses, the detected ions (see Fig. 1) can be separated into seven groups localized at the masses 112, 67–70, 50–56, 38–44, 26–29, 12–16 and 1, where the first group represents non-dissociative channel Ura+ while all other groups represent ion-fragments where the third and the sixth groups give only a minor contribution. The mass-spectra determined in this work demonstrate the relationship (Ura(113 + 114)/Ura(112)) = 6.2 ± 0.5% which is in a good agreement with the isotope ratio 5.84%

0.7

Vp (au)

Fig. 4. Relative cross sections of the reactions giving the major contribution to the total fragmentation cross section in the single electron capture process for He2+ + Ura collisions. Isolated symbols in the right represent relative cross sections of the fragmentation for electron impact ionization [1,2].  – Ura+ ,  – 69, ◦ – 68,  – 42, Δ – 41, x – 40,  – 28, • – H+ .

tabulated in reference [1] and supports the reliability of our procedure for the mass-spectra evaluation. Relative cross sections of the processes giving the major contribution to the reaction He2+ + Ura → He+ + Ura+∗

(4)

as a function of the collision velocity are given in Figure 4. The relative cross sections in Figure 4 represent the data for 8 out of all 37 reaction channels observed in our experiment, providing about 83% of the total single electron capture cross section. As can be seen from Figure 4, the largest contribution to reaction (4) is given by the nondissociated molecular ions C4 H4 N2 O+ 2 related to masses 112−114. The relative cross sections for single electron capture by O2+ ions in the reaction O2+ + Ura → O+ + Ura+∗

(5)

are presented in Figure 5. In general they behave similarly to the He2+ cross sections in Figure 4, however, the O2+ cross sections in most of the fragmentation channels are somehow larger than those in the He2+ case (4) and the total cross section of the eight fragmentation channels discussed above form about 70% of the single electron capture cross section. The mechanism of single electron capture by C2+ ions in the reaction C2+ + Ura → C+ + Ura+∗

(6)

differs distinctly from the He2+ and O2+ cases manifesting the major role of the Ura+∗ ion fragmentation. As shown in Figure 6, the contribution from the nondissociative channel Ura+∗ is relatively small and increases with the collision velocity. The cross sections of eight fragmentation channels presented in Figure 6 give about 68% of the total reaction (6) cross section.

Eur. Phys. J. D (2015) 69: 3

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100

σ % σ 0121

10

1

0.10

0.15

0.20

0.25

0.30

Vp (au)

Fig. 5. Relative cross sections of the reactions giving the major contribution to the total fragmentation cross section in the single electron capture for O2+ + Ura collisions. − Ura+ ,  – 69, ◦ – 68,  – 42, Δ – 41, x – 40,  – 28, •– H+ .

N3–C4, or C2–N3 and C4–C5 (see the insert in Fig. 1). As shown by calculation [34], the second alternative is the most energetically probable. The molecular fragment with mass 42 can be associated either with the NCO+ ion which probably needs preliminary tautomerization of the parent molecular ion, or with C2 H2 O+ . However, the latter case needs two bonds to be broken (N3–C4 and C5–C6, see Fig. 1) and the transition of an H-atom from the neutral to the charged fragment. The ion fragment C2 H2 O+ can be also produced via a sequential fragmentation, for instance Ura+ → C3 H3 NO+ → C2 H2 O+ . Zhou et al. [34] considered five possible fragmentation channels leading to the production of the C2 H2 O+ fragment. The molecular fragment with mass 28 can be associated either with CO+ or with HCNH+ , with production of the latter ion being energetically more favorable. 3.3 Multi-electron processes following the electron capture

20

σ % σ 0121

15

10

5

0

0.15

0.20

0.25

0.30

0.35

Vp (au)

Fig. 6. Relative cross sections of the reactions giving the major contribution to the total fragmentation cross section in the single electron capture for C2+ + Ura collisions.  – Ura+ ,  – 69, ◦ – 68,  – 42, Δ – 41, x – 40,  – 28, •– H+ .

Due to the large number of atoms with comparable masses in a molecular fragment, different fragments can have the same mass. The detailed theoretical analysis of the Ura+∗ fragmentation channels based on ab initio calculations of the electronic structure was performed by Weinacht et al. [33,34]. Assignment of the detected fragments can be done using the results of Weinacht et al. [33,34] and assuming that the most probable fragmentation channels require minimum number of broken bonds and minimum number of atoms transferring between the neutral and ionic fragments. The molecular fragment with mass 69 can be associated with the C3 H3 NO+ ion which is produced by abstraction of the neutral fragment HNCO (m = 43) from the parent molecular ion. This process can occur by breaking the bond pairs C6–N1 and C2–N3, or N1–C2 and

The mass-spectrum of the ion-fragments produced under ionization of uracil molecules by electrons at 70 eV kinetic energy [1,2] contains 58 peaks, with many of them having very small intensity, so the contribution from the total fragmentation cross section of the eight channels mentioned above reaches 86% of the total cross section. There are many similarities between the mass spectra of the ions produced by electron impact and by the electron capture by He2+ ions shown in Figure 4. Note that similarity between the fragmentation cross sections observed via electron ionization, ionization by fast multi-charged ions [16], and via electron capture by Ar6+ ions [35] has already been discussed. The essential difference between the fragmentation mechanisms after single electron capture by C2+ ions on one side and by He2+ , O2+ ions and electrons on the other side can be due to larger uracil ion excitation energy in the former case. Similar excitation of the uracil ion likely occurs in the interaction of uracil molecules with fast protons [21]. As shown in reference [21] relative fragmentation cross sections of the Ura+∗ ions are almost independent of the ionization mechanism and of the proton velocity in the range Vp = 0.9−2.4 au in both ionization and single electron capture processes. The main fragmentation channel in the fast proton excitation reaction studied in reference [21] consisted of the ions with masses 38−44 and gave the relative contribution of about 40%, while in the single electron capture by C2+ reaction studied in this work, the average contribution of the same ions was found to be 43 ± 3% (see Fig. 6). The second largest cross section channel in reference [21] consisted of the ions with masses 25–29 and gave a relative contribution of 22%, while in this work under single electron capture by C2+ condition the same channel gave a contribution of 25.6 ± 0.6% (see Fig. 6). Electron capture during the interaction of multiply charged ions with atoms is usually described by the multistate Landau-Zener model [36] containing the exothermic

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Eur. Phys. J. D (2015) 69: 3

Table 1. Theoretical and experimental energies Eb of the outer-valence vertical ionization transitions. 1 Ionic state Ura+ (1) Ura+ (2) Ura+ (3) Ura+ (4) Ura+ (5) Ura+ (6) Ura+ (7) Ura+ (8) Ura+ (9) a

2 Eb eV [33] theory 9.56 10.00 10.53 10.97 12.51 13.50 13.80 14.23

3 Eb eV [9] exp. 9.46 10.08 10.44 10.9 12.53 13.5 14.1 14.5 15.4

4 ΔEba eV 0 0.62 0.98 1.44 3.07 4.04 4.64 5.04 5.94

ΔEb = Eb (Ura+ (k)) − Eb (Ura+ (1)).

energy defect Q as a main parameter. The same model under some restrictions can be used for description of the electron capture in collisions of multiply charged ions with molecules. For determination of the parameter Q one should know the initial and final electronic energy of the incident ion1 and the energy of uracil ion Ura+ (n) final states n which can be estimated from the upper laying molecular orbital (MO) energies given in references [15,26,33]. The calculated values of three lowest excited uracil-ion MO agree well with experimental band maxima in uracil photoelectron spectra [9]. This means that the excited states can be satisfactorily described in terms of 1-hole configurations. However, it looks unlikely that highly excited uracil ions can also be described within the same approximation. For determination of the electron capture energy defects Q in this paper, experimental binding energies given in column 3 in Table 1 that are in agreement with theoretical values [15,26,33] have been used. Corresponding molecular ion excitation energies ΔEb are given in column 4 in Table 1. The estimated energy defects Q are given in Table 2 for several possible single electron capture channels by He2+ from uracil. As previously described [26,36–38], the electron capture occurs mostly due to Landau-Zener nonadiabatic transitions in the vicinity of the quasi-crossing points of potential curves, Rc (au) ≈ (1/Q (au)). In addition, for relatively low energy collisions studied in this paper the electron capture cross sections can be large only if the quasi-crossing point lays at internuclear distances of about Rc ∼ 3−10 au, within the “reaction window” [38,39]. Such internuclear distances correspond to energy defects Q ≈ −9 ÷ −3 eV. As can be seen from Table 2 the electron capture to the helium ground state ion He+ (n =1) occurs at very large energy defect values, Q ≈ 40 eV, resulting in a very small collision impact parameter. Therefore, the electron capture cross section in this case must be small. 1

http://physics.nist.gov/PhysRefData/ASD/ levels_form.html

As shown in Table 2 the electron capture to the helium ion excited state He+ (n = 2) occurs in the “reaction window” area, however it can result only in lowly excited uracil ions produced with energy not enough for Ura+∗ dissociation. Indeed, according to the experimental results the production of the fragment with mass 69 requires between 1.3 eV [4,40] and 1.8 eV [7], while the production of other observed fragments requires even more energy: between 3 and 5 eV [4,7,34,40]. Therefore, in this case the ion-fragments can be produced only in the reaction channels out of the “reaction window” having relatively small cross section compared with the reaction of capture without fragmentation. This conclusion is in perfect agreement with our experimental results presented in Figure 4. Two factors are essential for the analysis of the single electron capture by C2+ and O2+ ions from uracil. Firstly, one should take into account that the initial doubly charged projectiles can be either in the ground, or in metastable electronic states. Secondly, the single electron capture can occur via rearrangement of the projectile ion core [38,39,41–44] even if the produced molecular ions are not excited. The energy defects related to several possible electron capture channels form uracil by C2+ ions are given in Table 3. Only the final states of the collision partners formed in the vicinity of the interaction distances Rc which fit the “reaction window” are presented in Table 3. As can be seen from Table 3 the collisions involving ground state carbon ions C2+ (2s21 S) can fit the “reaction window” resulting in fragmentation of the uracil ions in the reaction: C+ (2s2 2p2 P) + Ura+ (8) and C+ (2s2 2p 2 P) + Ura+ (9). However, due to the small value of the interaction distance Rc the fragmentation cross section cannot be large. The channel resulting in excited carbon ions C+ (2s2 3s 2 S) is even less probable because in this case the quasi-crossing point is too far away and out of the “reaction window” region. The channel resulting in excited carbon ions C+ (2s2p2 2 L) occurs within the “reaction window”, however it cannot produce highly excited uracil ions Ura+ (n). Therefore, the coreconserving reaction channel involving ground state carbon ions seems unlikely given the crossing distances, while the core-changing channel mostly leads to non-dissociative capture. The reactions which are accompanied by the ion core rearrangement are written in bold in Tables 3 and 4. As can be seen from Table 3 the fragmentation of uracil ions observed in the experiment, can be explained by the influence of metastable C2+ (2s2p 3 P) ions with the excitation energy of ΔE = 6.5 eV1 which exist in the carbon projectile beam [38,41–43]. Almost every reaction which involves the metastable projectiles C2+ (2s2p 3 P) and fits the “reaction window” is not accompanied by the carbon ion core rearrangement and mostly results in the production of excited uracil ions Ura+ (n) with n > 4. The analysis of single electron capture by O2+ ions from uracil was done assuming that the initial oxygen projectiles in the beam can exist in the ground state O2+ (2s2 2p2 3 P) and two metastable excited electronic states [45]: O2+ (2s2 2p2 1 D) with ΔE = 2.51 eV and O2+ (2s2 2p2 1 S) with ΔE = 5.35 eV1 . The estimated

Eur. Phys. J. D (2015) 69: 3

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Table 2. The energy defect Q of the process He2+ + Ura → He+ (n) + Ura+ (k ). Initial state

Final state He (n = 1) + Ura+ (1) . . . Ura+ (9) He+ (n = 2) + Ura+ (1) . . . Ura+ (4) He+ (n = 3) + Ura+ (1) +

He2+ + Ura

Q (eV) −44.95 . . . −39.02 −4.14 . . . −2.70 +3.41

Rc (au) <1 6.6−10

Table 3. The energy defect Q of the exothermic process C2+ + Ura → C+ + Ura+ (k ). Final state a

Q (eV)

Rc (au)

C (2s 2p P) + Ura (8) . . . Ura (9) C+ (2s2p2 2 D) + Ura+ (1) . . . Ura+ (5) C+ (2s2p2 2 S) + Ura+ (1) . . . Ura+ (2) C+ ( 2s2 3s 2 S) + Ura+ (1)

−9.88 . . . −8.98 −5.63 . . . − 4.19 −2.96 . . . − 2.34 −0.48

2.8 . . . 3.0 4.8 . . . 10.6 9.2 . . . 11.6 56

C+ (2s2p2 2 D) + Ura+ (5) . . . Ura+ (9) C+ (2s2p2 2 S) + Ura+ (5) . . . Ura+ (9) C+ (2s2p2 2 P) + Ura+ (1) . . . Ura+ (8) C+ (2p3 4 S) + Ura+ (1) . . . Ura+ (4) C+ (2s2p(3 P)3s 2 P) + Ura+ (1)

−9.07 . . . −6.20 −9.46 . . . −3.52 −7.71 . . . −2.67 −3.82 . . . − 2.38 −0.72

3.0 . . . 4.4 2.9 . . . 7.7 3.5 . . . 10.2 7.1 . . . 11.4 37.8

Initial state +

2+

C

(2s

21

S) + Ura

C2+ (2s2p3 P) + Ura

a

2

2

+

+

The reactions which are accompanied by the ion core rearrangement are written in bold. Table 4. The energy defect Q of the exothermic process O2+ + Ura → O+ + Ura+ (k). Initial state O2+ (2s2 2p2 3 P) + U

O

2+

O

2

(2s 2p

2+

2

21

(2s 2p

D) + U

21

S) + U

Final state O+ (2s2 2p3 2 P) + Ura+ (9) O+ (2s2p4 4 P) + Ura+ (4) . . . Ura+ (9) O+ (2s2p4 2 D) + Ura+ (1) . . . Ura+ (4) O+ (2s2 2p2 (3 P) 3s4 P) + Ura+ (1)

Q (eV) −17.22 −9.36 . . . − 4.86 −5.08 . . . − 3.64 −2.69

Rc (au) ∼1.5 2.9 − 5.6 5.4 − 7.5 10.1

O+ (2s2p4 2 D) + Ura+ (1) . . . Ura+ (7) O+ (2s2p4 2 S) + Ura+ (1) . . . Ura+ (3) O+ (2s2 2p2 (1 D) 3s2 D) + Ura+ (1)

−7.59 . . . − 2.95 −3.91 . . . − 2.93 −2.51

5.4 − 9.0 7.0 − 9.3 10.8

O+ (2s2p4 2 D) + Ura+ (1) . . . Ura+ (9) O+ (2s2p4 2 S) + Ura+ (1) . . . Ura+ (6) O+ (2s2p4 2 P) + Ura+ (1) . . . Ura+ (4) O+ (2s2 2p2 (1 S) 3s2 D) + Ura+ (1)

−10.44 . . . − 4.50 −6.75 . . . − 2.71 −4.66 . . . − 3.22 −2.42

2.6 − 6.0 4.0 − 10.0 5.8 − 8.4 11.2

energy defects Q are given in Table 4. As can be seen from Table 4 single electron capture which fits the “reaction window” proceeds with simultaneous oxygen ion core rearrangement and results in the large number of the Ura+ (n) final states. As described above, the lower excited uracil ions cannot result in molecular fragmentation like in the He2+ case and the highly excited uracil ions most probably result in the following fragmentation, which is more typical for the C2+ case.

4 Conclusions The mechanisms of single electron capture by doubly charged ions He2+ , C2+ , O2+ from uracil have been studied in the collision velocity range 0.13–0.65 au. Relative cross sections of the electron capture with ionization have been determined. As shown, the fragmentation of uracil ions produced via the interaction with doubly charged ions is mostly a result of single electron capture, rather than of single capture with ionization and double capture. Single electron

capture occurring in collision of uracil with C2+ leads to the high fragmentation probability of the uracil ion produced. The analysis has allowed us to reveal that the main mechanism of single electron capture by C2+ and O2+ ions likely involves the rearrangement of the projectile ion core electron configuration. Moreover, the analysis done indicates the important role of metastable doubly charged ions within initial beam. The authors acknowledge the financial support from the Russian Foundation for Basic Researches, Grant No. 14-03-00367. OSV is grateful to the Russian Foundation for Basic Researches for financial support, project No. 13-02-00589.

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