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Electronic and vibrational excitation in gas phase thymine and 5-bromouracil by electron impact
Chemical Physics Letters 381 (2003) 486–494 www.elsevier.com/locate/cplett
Electronic and vibrational excitation in gas phase thymine and 5-bromouracil by electron impact R. Abouaf *, J. Pommier, H. Dunet Laboratoire des Collisions Atomiques et Mol eculaires (UMR 8625), Universit e Paris Sud (Bat. 351), 91405 Orsay Cedex, France Received 1 July 2003 Published online: 31 October 2003
Abstract In thymine (T) and 5-bromouracil (BrU) the excitation of the lowest electronic states and vibrational excitation using EEL spectroscopy (0–100 eV) with angular analysis is reported. In the two molecules the singlet electronic states have been found blue-shifted by about 0.3 eV compared to the UV–Vis absorption results. Evidence has been found of triplet states at 3.6 eV for T and 3.35 eV for BrU (±0.08 eV). For vibrational excitation both molecules show two resonance regions, around 1–2 and 4–5 eV. The modes excited around 1–2 eV reveal an excitation of the carbonyl, CC double bond and NH stretch modes. At 5 eV, the NH stretch modes are still present but CO and CC double bond stretch modes are much less excited. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Interaction of ionizing radiations with biological material generates after several cascades a large number of rather low energy electrons. In the energy range 0–20 eV, the importance of the role of these electrons in producing substantial damages to this material has recently been demonstrated [1,2]. In this energy range, the secondary electrons are well known, via the dissociative electron attachment process to break efficiently the molecular bonds even at very low energy. Several studies have recently underlined the efficient production of negative ions in DNA bases and derivatives [3–6]. *
Corresponding author. E-mail address: [email protected] (R. Abouaf).
The secondary electrons colliding with molecules can also be responsible of vibrational and electronic excitations. If a lot of information on the lowest singlet excited states of these molecules have already been obtained, both experimentally by various techniques and theoretically (see for example Broo and Holmen [7] and references therein), only a few studies concern the electronic excitation by rather low energy electrons interaction [8] and to our knowledge, no studies have been published for vibrational excitation of these molecules by electron impact. 5-Bromouracil (BrU) is very similar to thymine (T), a DNA base, as BrU is obtained by replacement of the methyl group of T by a Br atom. It is known that BrU can act as a radiosensitizer after replacement of T in DNA, enhancing the damage
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.09.121
R. Abouaf et al. / Chemical Physics Letters 381 (2003) 486–494
to the biological material submitted to ionizing radiations [9,10]. After the studies in T and BrU [3–5], showing strong differences in the cross-sections for negative ions production, the present work has been undertaken to extend the comparison of these two molecules under electron impact (0–100 eV) to their excitation to the lowest electronic states, singlet and triplet states, and at lower energy (0–5 eV) to pure vibrational excitation.
2. Experimental The device used for the present experiment is an electrostatic electron spectrometer using two hemispherical energy analysers in tandem both in the electron gun and the analyser sections. The optics and magnetic shielding have been carefully designed to allow both the electron gun and the electron analyser to work down to zero energy, preserving close to zero energy, an electron resolution of about 0.050 eV FWHM. Above 1 eV, the e resolution becomes better and reaches 0.025 eV FWHM. To check the neutral beam production, mass analysis of both positive and negative ions is achieved with a time of flight system based on a Mac-Laren geometry, the ions being collected onto microchannel plates. Effusive beams of thymine and bromouracil are produced by vaporizing commercial products in a double stage tantalum oven, using a needle on top to allow a better definition of the collision center. After one day of outgassing, the beams are obtained at about 150 °C, that is well below their decomposition temperature of about 300 °C [3]. The whole electron spectrometer and the time of flight system are heated at the same temperature to reduce insulating deposits which forbid any experiment after 1 or 2 h. In these conditions runs of a few days could be performed before a full cleaning of the device.
487
loss spectroscopy (EELS) method. The EEL spectra have been recorded in the electron constant residual energy mode, i.e. the residual energy Er of the scattered electron is fixed whereas the incident electron energy is varied. This mode allows a better comparison between the different processes, as the energy above the threshold is always the same for all processes observed on the spectrum. When residual energy is low (below 2 eV for example) this avoids also discrimination effects in the analysis section of the spectrometer. Varying the residual energy Er and observation angle h helps to distinguish between dipole allowed transitions to singlet states and transitions to triplet states. Detailed conditions of h and Er can be found in Hall and Read [11]. Briefly, at an incident energy definitely higher than the excitation energy (Einc Eexc ), the collision responsible of the excitation occurs at large impact parameter, the incident electron looses only a small amount of its energy and is only slightly deviated from its incident trajectory. In these conditions exchange electron process with the target is not likely. Therefore at Einc Eexc and at low observation angles (a few degrees), the EEL spectrum will strongly favour dipole allowed transitions to the singlet states and will then resemble the absorption spectrum. On the opposite, when the incident energy is reduced and becomes comparable to the excitation energy (Einc Eexc ), the collision has to occur at rather small impact parameters. The incident electron penetrates the molecular electron cloud and the electron exchange process becomes likely; the scattered electron can be then appreciably deviated and it will be observed at various scattering angles. In these conditions, transitions to singlet states are less favoured, and the generally weaker contribution of transitions to triplet states can appear on the spectrum. 3.2. Thymine
3. Results and discussion 3.1. Electronic excitation We have investigated the excitation of the lowest electronic states using the electron energy
EEL spectra at 5° observation angle and for residual energies Er ranging from 90 to 0.5 eV are presented in Fig. 1a and b. From Er ¼ 90 eV down to Er ¼ 5:6 eV, three bands are observed at 4.95, 6.2 and 7.4 eV (all values ± 0.08 eV). These bands
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Fig. 1. (a and b) Electronic excitation of thymine. EEL spectra recorded at scattering angle h ¼ 5°, and at residual energies Er ranging from 90 to 0.5 eV. A new band at 3.6 eV appears at low residual energy is attributed to a triplet state. (c and d) Angular variation of the EEL spectra at residual energies 80 and 2 eV. Transitions observed at Er ¼ 80 eV are strongly forward peaked whereas transitions at low residual energy extend to much larger angles.
are strongly forward peaked, their intensity decreasing by a factor of about 20 between h ¼ 4° and 12° (Fig. 1c). This angular behaviour and the observation of these processes at high residual energies are indicating dipole allowed transitions to singlet states. Even if the general shape of the spectrum remains the same between 90 and 10 eV, it can be observed that the intensity of the transitions at 6.2 and 7.4 eV seems to decrease slightly more quickly than the transition at 4.95 eV. This could be simply an indication that the condition
(Einc Eexc becomes less valid at smaller Einc for the larger Eexc . Energy location of the transitions found in the present work in the gas phase are in reasonable agreement with previous experimental observations done on thin films, either under 25 keV electron impact [12] or with UV–Vis absorption spectrum [13], both techniques revealing dipole allowed transitions (see Table 1). We note that in these films experiments, the first two transitions appear red shifted by about 0.3 eV compared to our gas phase results. The three bands are
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489
Table 1 Lowest electronic excitation energies (eV) for thymine and 5-bromouracil This work
Abs UV films [13]
e impact films [12]
INDO6
CASPT [7]
VEPPP [14]
Thymine 3.6 ± 0.08 (T) 4.95 ± 0.08 6.2 ± 0.08
– 4.63 5.86
– 4.66 ± 0.18 5.94 ± 0.18
7.4 ± 0.08
7.04
7.08 ± 0.18
– 4.98 5.81 6.54 7.16
– 4.88 5.88 6.1 7.13
1.88 4.82 5.5 6.27 7.10
This work
Abs UV s. aq [17]
e
5-Bromouracil 3.35 ± 0.08 (T) 4.66 ± 0.08 6.12 ± 0.08
– 4.47 5.87
– 4.70 ± 0.18 5.93 ± 0.18
7.20 ± 0.08 7.88 ± 0.08 8.60 ± 0.08
– – –
6.93 ± 0.18 7.85 8.45 ± 0.18
impact films (U) [12]
INDO (U)6
VEPPP (U) [14]
– 5.09 5.99 6.42 6.95 – –
1.78 5.09 5.82 6.40 7.11 7.98 –
(T) indicates a triplet state. For 5-bromouracil several values are not available. (U) indicates values measured or calculated for uracil, given here only for comparison.
attributed to p p transitions. Different types of semiempirical or ab initio calculations [7,14 and references therein] reproduced the main features of the experimental results within about 0.3 eV (Table 1). However, depending upon the type of calculations (INDO/S or CASPT2 in [7]), the ordering of the second and third transitions is different. It is worth noting also that these calculations predict two bands of similar oscillator strengths in the 6 eV region, separated by about 0.7 eV whereas most experiments observe only one band in this energy region. We should mention however that some shoulder could possibly be observed around 5.9 eV in our EEL spectrum at Er ¼ 90 eV (Fig. 1). On this last spectrum one can also note several tiny shoulders on the high energy side of the first band, revealing a vibrational structure (spacings about 0.1 eV), possibly due to CO wag modes, similar to the observations reported by Becker et al. (740 cm1 ) in a low temperature absorption experiment [15]. At lower residual energies (Er 6 2 eV), a fourth band is clearly observable peaking at an energy loss of 3.6 ± 0.08 eV. Its relative intensity in the EEL spectrum increases with decreasing residual energy (Fig. 1b) and it becomes a dominant band at Er ¼ 0:5 eV. Variation of the intensity of this
process with the observation angle is presented in Fig. 1d. This angular behaviour extending to larger angles is very different from the dipole allowed transitions (strongly peaked at very small angles). The appearance at low Er and the angular behaviour clearly indicates a transition to a triplet state. The energy position at 3.6 eV is somewhat different from the calculated value of Srivastava et al. [14]. 3.3. 5-Bromouracil The EEL spectra recorded at Er ranging from 90 to 1 eV are displayed in Fig. 2a. At high residual energies the p p transitions to singlet states appear at 4.66, 6.12, 7.20, 7.88, 8.60 eV (all values ± 0.08 eV), and around 11 and 13 eV. These last two bands present some vibrational structure showing spacings about 0.4 eV which could be attributed either to N1 H or N3 H stretch modes [16]. Like in the case of thymine, all these processes are strongly peaked at very small observation angles, as expected for dipole allowed transitions. At Er lower than 60 eV the intensity of the transitions above 7.2 eV decrease very quickly, being hardly observable at Er ¼ 10 or 5 eV. As already mentioned for T, at these residual energies and for excitation energies of the order of 10 eV the condition
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Fig. 2. (a) Electronic excitation of 5-bromouracil. EEL spectra at h ¼ 5° scattering angle, and at residual energies ranging from 90 to 1 eV. (b) Angular variation of the EEL spectrum at Er ¼ 1 eV from scattering angles h ¼ 10°–90°. The band at 3.35 V is attributed to a triplet state.
Einc Eexc , favouring the appearance of singlet states, is not anymore valid. Comparison of our values with UV–Vis results in water solution of Lohmann [17] for the first two bands shows a good agreement, the gas phase results appearing slightly blue shifted (Table 1). The agreement is also good (Table 1) with the values of 25 keV electron impact on uracil thin films reported by Isaacson [12] at lower energy resolution. We can even confirm the existence of a clear peak at 7.88 eV, which was suspected by Isaacson at 7.85 eV. At Er ¼ 1 eV a new band is observed at an energy loss of 3.35 ± 0.08 eV. Its observation at low Er and its angular behaviour extending to large angles, up to 90° where this transition becomes dominant in the spectrum (Fig. 2b), is in favour of a transition to a triplet state. The other peaks observed at Er ¼ 1 eV and at large angles could also be evidence of triplet states associated to the singlet states and lying almost at the same energies. 3.4. Vibrational excitation It is well known [18] that the low energy interaction of electrons with molecules is dominated by the occurrence of very short lived metastable anion states (resonances) due to the temporary capture of the incident electron by the target. The very
short lifetime of these states (1010 –1015 s) does not generally allow their direct observation. They are observed through their decay which can occur either by rejection of the captured electron, or by dissociation. If the lifetime of the temporary anion state is not too short, in most cases the rejection of the electron occurs at a distance different from the ground state equilibrium distance, leading to elastic scattering or to resonant vibrational excitation of the neutral molecule. This process is always in competition with the dissociation of the resonant state, leading to a negative ion via the dissociative electron attachment (DEA) process, which is very efficient to dissociate molecules even at very low electron energies. 3.5. Thymine Evidence of resonances in the electron scattering on thymine has been demonstrated in electron transmission spectroscopy experiments by Aflatooni et al. [19] and by DEA [3]. Aflatooni et al. found three resonances at 0.29, 1.71 and 4.05 eV. They are understood as due to electron capture in the first p empty orbitals, in good agreement with calculations [19,20]. Due to the limited resolution of our device (0.050 eV FWHM) when the incident energy is very close to zero, we have investigated only the resonance regions around 1.7 and 4 eV.
R. Abouaf et al. / Chemical Physics Letters 381 (2003) 486–494
Energy loss spectra at 1.7 eV incident energy, recorded with an energy electron resolution of 0.025 eV, are presented at different scattering angles in Fig. 3. They reveal the excitation of three groups of vibrational modes in the regions around 0.1, 0.2 and 0.4 eV. At 90°, the main energy losses are found at 0.090, 0.180 and 0.430 eV (all values ± 0.010 eV). At 60°, besides the mode at 0.090 eV, two modes are clearly separated in the 0.2 and
Fig. 3. Vibrational excitation of thymine in the second resonance region. Energy loss spectra recorded at incident energy Ei ¼ 1:7 eV at scattering angles h ¼ 90° (a), 60° (b), and 30° (c). Three regions of vibrational excitation are observed around 0.1, 0.2 and 0.4 eV (see text).
491
0.4 eV regions showing modes at 0.175, 0.220, 0.375 and 0.430 eV (all values ± 0.010 eV). At 30°, the evolution with scattering angle is spectacular for the mode at 0.220 eV which was only a weak shoulder at 90° and becomes dominant at 30°. Using the numbering of the atoms and the values of Colarusso et al. [21], the modes excited around 0.430 eV have been assigned to N1 H and/ or N3 H stretch modes (respectively 0.430 and 0.426 eV). C6 H stretch (0.381 eV) or CH stretch from the methyl group (0.365 eV), are also considered responsible for the peak at 0.375 eV, whereas the 0.220 eV peak is assigned to C2 O and/or C4 O stretch modes (respectively 0.220 and 0.214 eV) and also to C5 C6 stretch mode (0.209 eV). Interpretation of the peak around 0.175 eV is more difficult as many vibrational modes have been identified in the energy region from 0.180 to 0.173 eV, including N1 H bending, C2 N3 stretch, N1 H, N3 H or methyl group bending modes. Around 0.090 eV the modes involved are assigned to C2 O, C4 O wag modes and C4 C5 stretch mode. Concerning the second resonance region, the EEL spectra have been recorded at Einc ¼ 5 eV (Fig. 4) as the vibrational excitation appears slightly larger than at 4 eV. The group of vibrational modes excited is similar to those excited at 1.7 eV but present however some differences. The N1 H and/or N3 H stretch modes around 0.430 eV and the C6 H (0.380 eV) or CH stretch from the methyl group (0.365 eV) are more strongly excited and a small excitation of the first harmonic is even observed around 0.75 and 0.85 eV. On the opposite, the C2 O or C4 O stretch modes (respectively 0.220 and 0.214 eV) seem less excited. The observations at 1.7 eV are an indication that the p orbital occupied by the captured electron has an antibonding character on the carbonyl groups and the C5 C6 double bond, as expected for a p orbital [22], but also some r character to excite the NH stretch bonds. Around 5 eV, the nature of the reached orbital clearly affects even more the NH or CH stretch bonds than the CO or CC double bond stretch modes. The carbonyl wag modes are however still excited.
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Fig. 4. Vibrational excitation of thymine in the third resonance region. Energy loss spectra recorded at incident energy Ei ¼ 5 eV at scattering angles h ¼ 90° (a) and 30° (b). Like in Fig. 3, three groups of vibrational modes are found excited (see text).
3.6. 5-Bromouracil (BrU) Contrary to thymine, there is no transmission spectroscopy results for BrU precising the energy position of the resonances. However the similarity between thymine and uracil for the energy position of their lowest excited electronic states [13], and the location of their low energy resonances [19], and between thymine and BrU (Figs. 1 and 2), let expect two resonances regions, around 1–2 and 3–5 eV, besides the lowest resonance very close to zero. EEL spectra recorded at 5 and 1.2 eV are displayed in Fig. 5. At 5 eV they show excitation of vibrational modes at 0.085, 0.150 (shoulder), 0.180, 0.215, 0.385, 0.435 and 0.830 eV (all values ± 0.010
eV) At 1.2 eV the excitations around 0.150–0.180 eV are slightly reduced and the excitation at 0.385 eV have also almost disappeared. According to IR measurements [16], these excitations corresponds basically to the same modes as for thymine. Modes close to 0.090 eV indicate the excitation of C2 O and C4 O wag modes and C4 C5 stretch mode, whereas C2 O, C4 O and C5 C6 stretch modes are concerned around 0.200 eV, N1 H and/or N3 H stretch modes being responsible of the peaks in the 0.4 eV region. These observations are indicating that the p orbital occupied by the extra electron in the resonance around 1.2 eV allows some excitation of the carbonyl modes (stretch and wag), and the C5 C6 double bond stretch, together with a clear excitation of the N1 H and N3 H stretch modes. Around 4
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493
Fig. 5. Vibrational excitation of 5-bromouracil at scattering angle h ¼ 90° and incident energy Ei ¼ 1.2 eV (b) (second resonance region), and Ei ¼ 5 eV (a) (third resonance region). See text.
eV, the NH stretch modes are still strongly excited, a weak harmonic band being even observed around 0.770–0.840 eV. The character of the vacant orbital reached in this energy region is then certainly somewhat Ôless p Õ than at lower energy.
4. Conclusions We have obtained in the isolated molecules, precise location of the lowest singlet electronic states. Our results appear in fair agreement with previous experimental and theoretical results. Compared to observations reported in thin films by high energy e impact or UV–Vis absorption, they appear blue-shifted by about 0.3 eV. New transi-
tions, not observable in these last experiments, have been found at 3.6 eV for T and 3.35 eV for BrU. They are assigned to transitions to triplet states. For both T and BrU resonant vibrational excitation have been observed in the regions 1–2 and 4–5 eV. The vibrational modes excited in the low resonance region reveal excitation of three main groups of modes. CO carbonyl stretch modes and CC double bond stretch mode are excited as well as CO wag and C4 C5 stretch modes. Excitation of NH groups and CH (stretch and bending modes) is also clearly observed. In the energy domain of the present experiment, the electronic and vibrational excitations of the T and BrU appear very similar. Although occurring in the same energy range, the similarity of these
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processes are in contrast with the important differences observed for negative ion production via dissociative electron attachment processes in T and BrU [3–5]. As far as the gas phase processes can account for the complex problems of the electron interaction with T and BrU in the biological medium, electronic and vibrational excitations do not seem to be involved in explaining the different behaviour of these molecules under ionizing radiations. References [1] B. Bouda€ıffa, P. Cloutier, D. Hunting, M.A. Huels, L. Sanche, Science 287 (2000) 1658. [2] H. Abdoul-Carime, P.C. Dugal, L. Sanche, Radiat. Res. 153 (2000) 23. [3] M.A. Huels, I. Hahndorf, E. Illenberger, L. Sanche, J. Chem. Phys. 108 (1998) 1309. [4] H. Abdoul-Carime, M.A. Huels, F. Bruning, E. Illenberger, L. Sanche, J. Chem. Phys. 113 (2000) 2517. [5] R. Abouaf, J. Pommier, H. Dunet, Int. J. Mass Spectrometry 226 (2003) 397. [6] G. Hanel, B. Gstir, F. Denifl, P. Scheier, M. Probst, B. Farizon, M. Farizon, E. Illenberger, T.D. M€ark, Phys. Rev. Lett. 90 (2003) 188104.
[7] A. Broo, A. Holmen, J. Phys. Chem. A 101 (1997) 3589. [8] M.A. Dillon, H. Tanaka, D. Spence, Radiat. Res. 117 (1989) 1. [9] S. Zamenhof, R. De Giovanni, S. Greer, Nature 181 (1958) 827. [10] L.L. Ling, J.F. Ward, Radiat. Res. 121 (1990) 76. [11] R.I. Hall, F.H. Read, in: I. Shimamura, K. Takayanagi (Eds.), Electron Molecule Collisions, Plenum Press, New York, 1984. [12] M. Isaacson, J. Chem. Phys. 56 (1972) 1803. [13] Y. Yamada, H. Fukutome, Biopolymers 6 (1968) 43. [14] S.K. Srivastava, P.C. Mishra, Int. J. Quantum Chem. XVIII (1980) 827. [15] R.S. Becker, G. Kogan, Photochem. Photobiol. 31 (1980) 5. [16] Z. Zwierzchowska, K. Dobrosz-Teperek, W. Lewandowski, R. Kolos, K. Bajdor, J.Cz. Dobrowolski, A.P. Mazurek, J. Mol. Struct. 410–411 (1997) 415. [17] W. Lohmann, Z. Naturforsch. 29c (1974) 493. [18] G.J. Schulz, Rev. Mod. Phys. 45 (1973) 423. [19] K. Aflatooni, G.A. Gallup, P.D. Burrow, J. Phys. Chem. A 102 (1998) 6205. [20] M.D. Sevilla, B. Besler, A.O. Colson, J. Phys. Chem. 99 (1995) 1060. [21] P. Colarusso, K. Zang, B. Guo, P.F. Bernath, Chem. Phys. Lett. 269 (1997) 39. [22] L.C. Snyder, R.C. Shulman, D.B. Neumann, J. Chem. Phys. 53 (1970) 256.
Electronic and vibrational excitation in gas phase thymine and 5-bromouracil by electron impact R. Abouaf *, J. Pommier, H. Dunet Laboratoire des Collisions Atomiques et Mol eculaires (UMR 8625), Universit e Paris Sud (Bat. 351), 91405 Orsay Cedex, France Received 1 July 2003 Published online: 31 October 2003
Abstract In thymine (T) and 5-bromouracil (BrU) the excitation of the lowest electronic states and vibrational excitation using EEL spectroscopy (0–100 eV) with angular analysis is reported. In the two molecules the singlet electronic states have been found blue-shifted by about 0.3 eV compared to the UV–Vis absorption results. Evidence has been found of triplet states at 3.6 eV for T and 3.35 eV for BrU (±0.08 eV). For vibrational excitation both molecules show two resonance regions, around 1–2 and 4–5 eV. The modes excited around 1–2 eV reveal an excitation of the carbonyl, CC double bond and NH stretch modes. At 5 eV, the NH stretch modes are still present but CO and CC double bond stretch modes are much less excited. Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction Interaction of ionizing radiations with biological material generates after several cascades a large number of rather low energy electrons. In the energy range 0–20 eV, the importance of the role of these electrons in producing substantial damages to this material has recently been demonstrated [1,2]. In this energy range, the secondary electrons are well known, via the dissociative electron attachment process to break efficiently the molecular bonds even at very low energy. Several studies have recently underlined the efficient production of negative ions in DNA bases and derivatives [3–6]. *
Corresponding author. E-mail address: [email protected] (R. Abouaf).
The secondary electrons colliding with molecules can also be responsible of vibrational and electronic excitations. If a lot of information on the lowest singlet excited states of these molecules have already been obtained, both experimentally by various techniques and theoretically (see for example Broo and Holmen [7] and references therein), only a few studies concern the electronic excitation by rather low energy electrons interaction [8] and to our knowledge, no studies have been published for vibrational excitation of these molecules by electron impact. 5-Bromouracil (BrU) is very similar to thymine (T), a DNA base, as BrU is obtained by replacement of the methyl group of T by a Br atom. It is known that BrU can act as a radiosensitizer after replacement of T in DNA, enhancing the damage
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2003.09.121
R. Abouaf et al. / Chemical Physics Letters 381 (2003) 486–494
to the biological material submitted to ionizing radiations [9,10]. After the studies in T and BrU [3–5], showing strong differences in the cross-sections for negative ions production, the present work has been undertaken to extend the comparison of these two molecules under electron impact (0–100 eV) to their excitation to the lowest electronic states, singlet and triplet states, and at lower energy (0–5 eV) to pure vibrational excitation.
2. Experimental The device used for the present experiment is an electrostatic electron spectrometer using two hemispherical energy analysers in tandem both in the electron gun and the analyser sections. The optics and magnetic shielding have been carefully designed to allow both the electron gun and the electron analyser to work down to zero energy, preserving close to zero energy, an electron resolution of about 0.050 eV FWHM. Above 1 eV, the e resolution becomes better and reaches 0.025 eV FWHM. To check the neutral beam production, mass analysis of both positive and negative ions is achieved with a time of flight system based on a Mac-Laren geometry, the ions being collected onto microchannel plates. Effusive beams of thymine and bromouracil are produced by vaporizing commercial products in a double stage tantalum oven, using a needle on top to allow a better definition of the collision center. After one day of outgassing, the beams are obtained at about 150 °C, that is well below their decomposition temperature of about 300 °C [3]. The whole electron spectrometer and the time of flight system are heated at the same temperature to reduce insulating deposits which forbid any experiment after 1 or 2 h. In these conditions runs of a few days could be performed before a full cleaning of the device.
487
loss spectroscopy (EELS) method. The EEL spectra have been recorded in the electron constant residual energy mode, i.e. the residual energy Er of the scattered electron is fixed whereas the incident electron energy is varied. This mode allows a better comparison between the different processes, as the energy above the threshold is always the same for all processes observed on the spectrum. When residual energy is low (below 2 eV for example) this avoids also discrimination effects in the analysis section of the spectrometer. Varying the residual energy Er and observation angle h helps to distinguish between dipole allowed transitions to singlet states and transitions to triplet states. Detailed conditions of h and Er can be found in Hall and Read [11]. Briefly, at an incident energy definitely higher than the excitation energy (Einc Eexc ), the collision responsible of the excitation occurs at large impact parameter, the incident electron looses only a small amount of its energy and is only slightly deviated from its incident trajectory. In these conditions exchange electron process with the target is not likely. Therefore at Einc Eexc and at low observation angles (a few degrees), the EEL spectrum will strongly favour dipole allowed transitions to the singlet states and will then resemble the absorption spectrum. On the opposite, when the incident energy is reduced and becomes comparable to the excitation energy (Einc Eexc ), the collision has to occur at rather small impact parameters. The incident electron penetrates the molecular electron cloud and the electron exchange process becomes likely; the scattered electron can be then appreciably deviated and it will be observed at various scattering angles. In these conditions, transitions to singlet states are less favoured, and the generally weaker contribution of transitions to triplet states can appear on the spectrum. 3.2. Thymine
3. Results and discussion 3.1. Electronic excitation We have investigated the excitation of the lowest electronic states using the electron energy
EEL spectra at 5° observation angle and for residual energies Er ranging from 90 to 0.5 eV are presented in Fig. 1a and b. From Er ¼ 90 eV down to Er ¼ 5:6 eV, three bands are observed at 4.95, 6.2 and 7.4 eV (all values ± 0.08 eV). These bands
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Fig. 1. (a and b) Electronic excitation of thymine. EEL spectra recorded at scattering angle h ¼ 5°, and at residual energies Er ranging from 90 to 0.5 eV. A new band at 3.6 eV appears at low residual energy is attributed to a triplet state. (c and d) Angular variation of the EEL spectra at residual energies 80 and 2 eV. Transitions observed at Er ¼ 80 eV are strongly forward peaked whereas transitions at low residual energy extend to much larger angles.
are strongly forward peaked, their intensity decreasing by a factor of about 20 between h ¼ 4° and 12° (Fig. 1c). This angular behaviour and the observation of these processes at high residual energies are indicating dipole allowed transitions to singlet states. Even if the general shape of the spectrum remains the same between 90 and 10 eV, it can be observed that the intensity of the transitions at 6.2 and 7.4 eV seems to decrease slightly more quickly than the transition at 4.95 eV. This could be simply an indication that the condition
(Einc Eexc becomes less valid at smaller Einc for the larger Eexc . Energy location of the transitions found in the present work in the gas phase are in reasonable agreement with previous experimental observations done on thin films, either under 25 keV electron impact [12] or with UV–Vis absorption spectrum [13], both techniques revealing dipole allowed transitions (see Table 1). We note that in these films experiments, the first two transitions appear red shifted by about 0.3 eV compared to our gas phase results. The three bands are
R. Abouaf et al. / Chemical Physics Letters 381 (2003) 486–494
489
Table 1 Lowest electronic excitation energies (eV) for thymine and 5-bromouracil This work
Abs UV films [13]
e impact films [12]
INDO6
CASPT [7]
VEPPP [14]
Thymine 3.6 ± 0.08 (T) 4.95 ± 0.08 6.2 ± 0.08
– 4.63 5.86
– 4.66 ± 0.18 5.94 ± 0.18
7.4 ± 0.08
7.04
7.08 ± 0.18
– 4.98 5.81 6.54 7.16
– 4.88 5.88 6.1 7.13
1.88 4.82 5.5 6.27 7.10
This work
Abs UV s. aq [17]
e
5-Bromouracil 3.35 ± 0.08 (T) 4.66 ± 0.08 6.12 ± 0.08
– 4.47 5.87
– 4.70 ± 0.18 5.93 ± 0.18
7.20 ± 0.08 7.88 ± 0.08 8.60 ± 0.08
– – –
6.93 ± 0.18 7.85 8.45 ± 0.18
impact films (U) [12]
INDO (U)6
VEPPP (U) [14]
– 5.09 5.99 6.42 6.95 – –
1.78 5.09 5.82 6.40 7.11 7.98 –
(T) indicates a triplet state. For 5-bromouracil several values are not available. (U) indicates values measured or calculated for uracil, given here only for comparison.
attributed to p p transitions. Different types of semiempirical or ab initio calculations [7,14 and references therein] reproduced the main features of the experimental results within about 0.3 eV (Table 1). However, depending upon the type of calculations (INDO/S or CASPT2 in [7]), the ordering of the second and third transitions is different. It is worth noting also that these calculations predict two bands of similar oscillator strengths in the 6 eV region, separated by about 0.7 eV whereas most experiments observe only one band in this energy region. We should mention however that some shoulder could possibly be observed around 5.9 eV in our EEL spectrum at Er ¼ 90 eV (Fig. 1). On this last spectrum one can also note several tiny shoulders on the high energy side of the first band, revealing a vibrational structure (spacings about 0.1 eV), possibly due to CO wag modes, similar to the observations reported by Becker et al. (740 cm1 ) in a low temperature absorption experiment [15]. At lower residual energies (Er 6 2 eV), a fourth band is clearly observable peaking at an energy loss of 3.6 ± 0.08 eV. Its relative intensity in the EEL spectrum increases with decreasing residual energy (Fig. 1b) and it becomes a dominant band at Er ¼ 0:5 eV. Variation of the intensity of this
process with the observation angle is presented in Fig. 1d. This angular behaviour extending to larger angles is very different from the dipole allowed transitions (strongly peaked at very small angles). The appearance at low Er and the angular behaviour clearly indicates a transition to a triplet state. The energy position at 3.6 eV is somewhat different from the calculated value of Srivastava et al. [14]. 3.3. 5-Bromouracil The EEL spectra recorded at Er ranging from 90 to 1 eV are displayed in Fig. 2a. At high residual energies the p p transitions to singlet states appear at 4.66, 6.12, 7.20, 7.88, 8.60 eV (all values ± 0.08 eV), and around 11 and 13 eV. These last two bands present some vibrational structure showing spacings about 0.4 eV which could be attributed either to N1 H or N3 H stretch modes [16]. Like in the case of thymine, all these processes are strongly peaked at very small observation angles, as expected for dipole allowed transitions. At Er lower than 60 eV the intensity of the transitions above 7.2 eV decrease very quickly, being hardly observable at Er ¼ 10 or 5 eV. As already mentioned for T, at these residual energies and for excitation energies of the order of 10 eV the condition
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Fig. 2. (a) Electronic excitation of 5-bromouracil. EEL spectra at h ¼ 5° scattering angle, and at residual energies ranging from 90 to 1 eV. (b) Angular variation of the EEL spectrum at Er ¼ 1 eV from scattering angles h ¼ 10°–90°. The band at 3.35 V is attributed to a triplet state.
Einc Eexc , favouring the appearance of singlet states, is not anymore valid. Comparison of our values with UV–Vis results in water solution of Lohmann [17] for the first two bands shows a good agreement, the gas phase results appearing slightly blue shifted (Table 1). The agreement is also good (Table 1) with the values of 25 keV electron impact on uracil thin films reported by Isaacson [12] at lower energy resolution. We can even confirm the existence of a clear peak at 7.88 eV, which was suspected by Isaacson at 7.85 eV. At Er ¼ 1 eV a new band is observed at an energy loss of 3.35 ± 0.08 eV. Its observation at low Er and its angular behaviour extending to large angles, up to 90° where this transition becomes dominant in the spectrum (Fig. 2b), is in favour of a transition to a triplet state. The other peaks observed at Er ¼ 1 eV and at large angles could also be evidence of triplet states associated to the singlet states and lying almost at the same energies. 3.4. Vibrational excitation It is well known [18] that the low energy interaction of electrons with molecules is dominated by the occurrence of very short lived metastable anion states (resonances) due to the temporary capture of the incident electron by the target. The very
short lifetime of these states (1010 –1015 s) does not generally allow their direct observation. They are observed through their decay which can occur either by rejection of the captured electron, or by dissociation. If the lifetime of the temporary anion state is not too short, in most cases the rejection of the electron occurs at a distance different from the ground state equilibrium distance, leading to elastic scattering or to resonant vibrational excitation of the neutral molecule. This process is always in competition with the dissociation of the resonant state, leading to a negative ion via the dissociative electron attachment (DEA) process, which is very efficient to dissociate molecules even at very low electron energies. 3.5. Thymine Evidence of resonances in the electron scattering on thymine has been demonstrated in electron transmission spectroscopy experiments by Aflatooni et al. [19] and by DEA [3]. Aflatooni et al. found three resonances at 0.29, 1.71 and 4.05 eV. They are understood as due to electron capture in the first p empty orbitals, in good agreement with calculations [19,20]. Due to the limited resolution of our device (0.050 eV FWHM) when the incident energy is very close to zero, we have investigated only the resonance regions around 1.7 and 4 eV.
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Energy loss spectra at 1.7 eV incident energy, recorded with an energy electron resolution of 0.025 eV, are presented at different scattering angles in Fig. 3. They reveal the excitation of three groups of vibrational modes in the regions around 0.1, 0.2 and 0.4 eV. At 90°, the main energy losses are found at 0.090, 0.180 and 0.430 eV (all values ± 0.010 eV). At 60°, besides the mode at 0.090 eV, two modes are clearly separated in the 0.2 and
Fig. 3. Vibrational excitation of thymine in the second resonance region. Energy loss spectra recorded at incident energy Ei ¼ 1:7 eV at scattering angles h ¼ 90° (a), 60° (b), and 30° (c). Three regions of vibrational excitation are observed around 0.1, 0.2 and 0.4 eV (see text).
491
0.4 eV regions showing modes at 0.175, 0.220, 0.375 and 0.430 eV (all values ± 0.010 eV). At 30°, the evolution with scattering angle is spectacular for the mode at 0.220 eV which was only a weak shoulder at 90° and becomes dominant at 30°. Using the numbering of the atoms and the values of Colarusso et al. [21], the modes excited around 0.430 eV have been assigned to N1 H and/ or N3 H stretch modes (respectively 0.430 and 0.426 eV). C6 H stretch (0.381 eV) or CH stretch from the methyl group (0.365 eV), are also considered responsible for the peak at 0.375 eV, whereas the 0.220 eV peak is assigned to C2 O and/or C4 O stretch modes (respectively 0.220 and 0.214 eV) and also to C5 C6 stretch mode (0.209 eV). Interpretation of the peak around 0.175 eV is more difficult as many vibrational modes have been identified in the energy region from 0.180 to 0.173 eV, including N1 H bending, C2 N3 stretch, N1 H, N3 H or methyl group bending modes. Around 0.090 eV the modes involved are assigned to C2 O, C4 O wag modes and C4 C5 stretch mode. Concerning the second resonance region, the EEL spectra have been recorded at Einc ¼ 5 eV (Fig. 4) as the vibrational excitation appears slightly larger than at 4 eV. The group of vibrational modes excited is similar to those excited at 1.7 eV but present however some differences. The N1 H and/or N3 H stretch modes around 0.430 eV and the C6 H (0.380 eV) or CH stretch from the methyl group (0.365 eV) are more strongly excited and a small excitation of the first harmonic is even observed around 0.75 and 0.85 eV. On the opposite, the C2 O or C4 O stretch modes (respectively 0.220 and 0.214 eV) seem less excited. The observations at 1.7 eV are an indication that the p orbital occupied by the captured electron has an antibonding character on the carbonyl groups and the C5 C6 double bond, as expected for a p orbital [22], but also some r character to excite the NH stretch bonds. Around 5 eV, the nature of the reached orbital clearly affects even more the NH or CH stretch bonds than the CO or CC double bond stretch modes. The carbonyl wag modes are however still excited.
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Fig. 4. Vibrational excitation of thymine in the third resonance region. Energy loss spectra recorded at incident energy Ei ¼ 5 eV at scattering angles h ¼ 90° (a) and 30° (b). Like in Fig. 3, three groups of vibrational modes are found excited (see text).
3.6. 5-Bromouracil (BrU) Contrary to thymine, there is no transmission spectroscopy results for BrU precising the energy position of the resonances. However the similarity between thymine and uracil for the energy position of their lowest excited electronic states [13], and the location of their low energy resonances [19], and between thymine and BrU (Figs. 1 and 2), let expect two resonances regions, around 1–2 and 3–5 eV, besides the lowest resonance very close to zero. EEL spectra recorded at 5 and 1.2 eV are displayed in Fig. 5. At 5 eV they show excitation of vibrational modes at 0.085, 0.150 (shoulder), 0.180, 0.215, 0.385, 0.435 and 0.830 eV (all values ± 0.010
eV) At 1.2 eV the excitations around 0.150–0.180 eV are slightly reduced and the excitation at 0.385 eV have also almost disappeared. According to IR measurements [16], these excitations corresponds basically to the same modes as for thymine. Modes close to 0.090 eV indicate the excitation of C2 O and C4 O wag modes and C4 C5 stretch mode, whereas C2 O, C4 O and C5 C6 stretch modes are concerned around 0.200 eV, N1 H and/or N3 H stretch modes being responsible of the peaks in the 0.4 eV region. These observations are indicating that the p orbital occupied by the extra electron in the resonance around 1.2 eV allows some excitation of the carbonyl modes (stretch and wag), and the C5 C6 double bond stretch, together with a clear excitation of the N1 H and N3 H stretch modes. Around 4
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493
Fig. 5. Vibrational excitation of 5-bromouracil at scattering angle h ¼ 90° and incident energy Ei ¼ 1.2 eV (b) (second resonance region), and Ei ¼ 5 eV (a) (third resonance region). See text.
eV, the NH stretch modes are still strongly excited, a weak harmonic band being even observed around 0.770–0.840 eV. The character of the vacant orbital reached in this energy region is then certainly somewhat Ôless p Õ than at lower energy.
4. Conclusions We have obtained in the isolated molecules, precise location of the lowest singlet electronic states. Our results appear in fair agreement with previous experimental and theoretical results. Compared to observations reported in thin films by high energy e impact or UV–Vis absorption, they appear blue-shifted by about 0.3 eV. New transi-
tions, not observable in these last experiments, have been found at 3.6 eV for T and 3.35 eV for BrU. They are assigned to transitions to triplet states. For both T and BrU resonant vibrational excitation have been observed in the regions 1–2 and 4–5 eV. The vibrational modes excited in the low resonance region reveal excitation of three main groups of modes. CO carbonyl stretch modes and CC double bond stretch mode are excited as well as CO wag and C4 C5 stretch modes. Excitation of NH groups and CH (stretch and bending modes) is also clearly observed. In the energy domain of the present experiment, the electronic and vibrational excitations of the T and BrU appear very similar. Although occurring in the same energy range, the similarity of these
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processes are in contrast with the important differences observed for negative ion production via dissociative electron attachment processes in T and BrU [3–5]. As far as the gas phase processes can account for the complex problems of the electron interaction with T and BrU in the biological medium, electronic and vibrational excitations do not seem to be involved in explaining the different behaviour of these molecules under ionizing radiations. References [1] B. Bouda€ıffa, P. Cloutier, D. Hunting, M.A. Huels, L. Sanche, Science 287 (2000) 1658. [2] H. Abdoul-Carime, P.C. Dugal, L. Sanche, Radiat. Res. 153 (2000) 23. [3] M.A. Huels, I. Hahndorf, E. Illenberger, L. Sanche, J. Chem. Phys. 108 (1998) 1309. [4] H. Abdoul-Carime, M.A. Huels, F. Bruning, E. Illenberger, L. Sanche, J. Chem. Phys. 113 (2000) 2517. [5] R. Abouaf, J. Pommier, H. Dunet, Int. J. Mass Spectrometry 226 (2003) 397. [6] G. Hanel, B. Gstir, F. Denifl, P. Scheier, M. Probst, B. Farizon, M. Farizon, E. Illenberger, T.D. M€ark, Phys. Rev. Lett. 90 (2003) 188104.
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