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Vibrational excitation of tetrahydrofuran by electron impact in the low energy range
Chemical Physics Letters 443 (2007) 17–21 www.elsevier.com/locate/cplett
Vibrational excitation of tetrahydrofuran by electron impact in the low energy range Marcin Dampc a, Ireneusz Linert a, Aleksandar R. Milosavljevic´ b, Mariusz Zubek a
a,*
Department of Physics of Electronic Phenomena, Gdan´sk University of Technology, 80-952 Gdan´sk, Poland b Institute of Physics, Pregrevica 118, 11080 Belgrade, Serbia Received 25 April 2007; in final form 5 June 2007 Available online 15 June 2007
Abstract Vibrational excitation of tetrahydrofuran has been studied over the incident electron energy range from 5 to 14 eV and over a wide range of scattering angles (20–180). Measurement of vibrational excitation function of the CH2 stretch modes (C–H stretching) reveals three structures at 6.0, 7.9 and 10.3 eV which can be attributed to the formation of negative-ion resonances. Differential and integral cross sections for vibrational excitation of the CH2 stretch modes have also been determined. 2007 Elsevier B.V. All rights reserved.
1. Introduction The tetrahydrofuran (THF) molecule, C4H8O, has been the subject of several investigations since it represents a structural unit of a number of carbohydrate and biological molecules. It also serves as a model to study conformation and ring puckering vibrations in small ring molecules [1,2]. In recent years, there has been renewed interest in THF stimulated by the investigations of radiation damage in biological systems and the role of secondary low energy electrons (E 6 20 eV) in the chemical and structural changes of cellular DNA [3]. The secondary electrons, although having energies below the ionization threshold, can induce single and double strand breaks in DNA. Such damage is partially attributable to the process of dissociative electron attachment to building blocks of DNA which proceed through formation of negative-ion resonances [4,5]. Since THF, a five-membered hydrocarbon ring containing an oxygen atom (Fig. 1), is incorporated in the deoxyribose sugar, it is considered to be a simple analogue for the study of the effects of secondary electrons on the
*
Corresponding author. Fax: +48 58 347 2821. E-mail address: [email protected] (M. Zubek).
0009-2614/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.06.048
deoxyribose ring and further on the sugar chain of the backbone of DNA [6]. In the present Letter, we have studied vibrational excitation of THF in the 5–14 eV incident electron energy range. The measurements have been carried out over a wide range of scattering angles from 20 to 180 applying the magnetic angle-changing technique for scattering from 90 to 180 [7]. To determine energies of negative-ion resonances occurring in vibrational excitation we have measured the excitation function of the observed CH2 stretch vibrational modes (C–H stretching) at fixed scattering angles. Identification of resonances allows us to understand the processes of electron energy transfer to the furanose ring leading to relaxation of equilibrium geometry of the ring. From the measured energy loss spectra we have obtained, to our knowledge, the first absolute differential and integral cross sections for excitation of the CH2 stretch vibrational modes. These absolute vibrational cross sections may serve as input parameters in Monte Carlo simulations of deposition of ionizing radiation energy in human tissue [8,9]. The first and up to now the only detailed investigations of electron impact vibrational excitations of THF in the gaseous and condensed multilayer phases have been performed by Lepage et al. [10] in the 1–30 eV energy range. However, these authors focused mainly on the resonance enhanced
18
M. Dampc et al. / Chemical Physics Letters 443 (2007) 17–21
Fig. 1. Diagram of the tetrahydrofuran molecule, C4H8O in the C2 symmetry point group. Oxygen atom is red (dark), carbon atom gray (gray), and hydrogen atom green (dark gray). (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
excitation of solid THF which revealed existence of several overlapping negative-ion resonances in the 4–12 eV range. In recent experimental works in the gaseous THF which include measurements of the total [11] and differential elastic [12,13] cross sections and studies of dissociative electron attachment [14,15] several resonance processes below 12 eV have been seen in agreement with the first observation of resonances at about 8.5 eV in [10]. Very recently, vibrational excitation of THF in the solid films have been monitored in the studies of the electron-stimulated reactions in the films [16]. The above experimental results are accompanied by a number of very recent ab initio calculations of elastic electron scattering in THF [17–19] which predict presence of overlapping shape resonances in the region 6–15 eV and calculations of integral elastic and electronically inelastic cross sections [20] which found a few core-excited resonances for energies up to 10 eV.
arates the electrons scattered in the backward directions from the incident electron beam and allows us to detect electron scattering in the angular range up to 180. The scattering angle scale has been determined with an uncertainty of ±2 against the position of a minimum at 117.5 in the elastic electron scattering in argon at 10 eV. The angular resolution of the spectrometer is estimated from the electron optics computational simulations to be less than 5. In the present Letter, electron energy loss spectra have been measured in THF at fixed incident electron energies in the scattering angle range from 20 to 180 in 10 steps. The background in these spectra has been measured by introducing the target gas directly to the vacuum chamber through a side valve. These background contributions have been removed from the measured spectra. It has been found in the previous measurements [7] that the variation of the transmission of electron analyzer in the energy loss range up to 1.0 eV is small, less than 5%, and no attempts have been made to correct measured energy loss spectra for such transmission. The energy resolution of the spectrometer in the energy loss mode of the present operation was 70–80 meV. The incident electron energy was calibrated to within ±30 meV with respect to the position of the 2 P3/2 resonance in argon at 11.098 eV. The excitation function measured in this work has been corrected for variation of spectrometer transmission with the incident electron energy. That correction has been determined with the help of helium elastic scattering. The anhydrous THF with a declared purity of better than 99.9% was purchased from Aldrich and was used after a few outgasing cycles under vacuum. 3. Results and discussion 3.1. Energy loss spectra
2. Experimental details Measurements were performed using a hemispherical electron spectrometer, which has been described in detail by Linert and Zubek [7]. Briefly, it consists of an electron beam source, an analyzer for scattered electrons and a magnetic angle changer, which surrounds the electron scattering center. The electron beam source and scattered electron analyzer contain two hemispherical energy selectors arranged in series and are equipped with systems of cylindrical lenses and deflectors. The incident electron beam is crossed at right angle with a target gas beam produced by a stainless steel tube with 0.6 mm inner diameter. The electron analyzer can be rotated around the target gas beam in the angular range from about 90 to 90. The range of accessible scattering angles is further extended up to 180 by using magnetic angle changer [21]. The angle changer provides a static, localized magnetic field which is perpendicular to the scattering plane. The incident electron beam and the scattered electrons are deflected by the magnetic field. The magnetic field sep-
We have measured electron energy loss spectra in THF at incident energies of 7 and 10 eV. These values of incident electron energy correspond to the energy range of observed negative-ion resonances (see Section 3.2). Fig. 2a and b show the spectra obtained at 7 eV and 90 and at 10 eV and 110, respectively. In both spectra two prominent energy loss peaks have been observed at energies of 140 meV and 350 meV. The peaks are 20–30 meV wider than the elastic scattering peak indicating excitation of groups of vibrational modes with similar energies. THF has 33 normal vibrational modes but the energies of the fundamental modes of vibration of the same type are similar and in general are grouped in certain energy ranges [10]. Eight modes of the a- and b-CH2 symmetric and asymmetric stretch vibrations have energies between 350 meV and 370 meV, the other CH2 group vibrations lie in the 110–190 meV range, while the C–C stretch modes appear from 110 meV to 130 meV. Energies of the fundamental vibrational modes matching the positions of the observed energy loss peaks are indicated in Fig. 2a and b.
M. Dampc et al. / Chemical Physics Letters 443 (2007) 17–21 3
CH 2 vibrations
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C-C stretch
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19
with higher energy resolution for cyclopentane (C5H10) which is a homogenous five-member saturated hydrocarbon. In that work a group of three energy loss peaks assigned to the C–C stretch and CH2 scissoring and rocking modes but probably comprising more than three vibrations has been found in the 100–200 meV energy loss range and a prominent single peak has been seen at 365 meV consisting of several vibrations of the CH2 stretch modes. Such similarities in the vibrational energy loss spectra have also been noticed in [22] for three-member ring molecules, cyclopropane (C3H6) and oxygen containing ethylene oxide (C2H4O). Longer progressions of vibrational peaks recorded in the present energy loss spectra for THF are indicative of excitation through mechanism of negative-ion resonances. This is in contrast to direct excitation observed by Milosavljevic´ et al. [23] in an energy loss spectrum measured at 30 eV and 10 which shows peaks corresponding essentially to excitation of the fundamental CH2 vibrational modes. The intensities of the peaks of the fundamental vibrational modes observed by Milosavljevic´ et al. (e.g. for the CH2 stretch) were small (1/3000 of the elastic intensity) compared to about 1/15 in the present energy loss spectra.
x5
3.2. Excitation function
6 x50
4
0.4
0.6
Fig. 2. Energy loss spectra measured at incident electron energy and scattering angle of, (a) 7 eV and 90, (b) 10 eV and 110, respectively. The energies of the fundamental vibrational modes corresponding to the positions of observed energy loss peaks are shown by bars.
Here, the modes of the totally symmetric vibrations (belonging to A representation of the C2 symmetry point group of THF, Fig. 1) are shown as the more likely to be excited through the resonance processes. The energy loss peaks at 140 meV correspond well to combined excitation of the C–C stretch (m13) and the CH2 rocking (m11), twisting (m9), scissoring (m5, m6) and wagging (m7, m8) modes. The 350 meV peaks indicate excitation of the a- and b-CH2 stretch (m1–m4) modes (C–H stretching). At an energy loss of 510 meV excitation of combination modes involving the CH2 stretch modes and the 140 meV bands are also clearly visible in the spectra. At higher energy losses of 690 meV (Fig. 2a) and 950 meV overtones of the CH2 stretch modes and combination modes have been also detected, respectively. Our two spectra (Fig. 2) compare well with the THF gas phase energy loss spectrum obtained by Lepage et al. [10] at scattering angle of 30 and electron energy of 8.5 eV. The above features of the vibrational excitation in THF are reminiscent of those obtained by Allan and Andric [22]
10 -1
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Electron energy loss (eV)
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0.0
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-22
0
The excitation function of the CH2 stretch vibrational modes of the 350 meV energy loss peak measured at a scattering angle of 80 is shown in Fig. 3. It is normalized to the average differential cross section obtained at 7 eV and 10 eV (see Section 3.3). The excitation function displays a wide peak with two resonance maxima at 7.9 eV and 10.3 eV. There is also a clear resonance shoulder at about 6.0 eV on the rising slope of the wide peak. These results can be compared with the excitation function of the m2aCH2 asymmetric stretch mode obtained by Lepage et al.
Differential cross section (10 m sr )
2
6
4
THF o
80 ΔE = 0.350 eV
2
0 4
6
8
10
12
14
Electron energy (eV) Fig. 3. Excitation function of the CH2 stretch modes of the 350 meV energy loss peak in tetrahydrofuran measured at the scattering angle of 80.
20
M. Dampc et al. / Chemical Physics Letters 443 (2007) 17–21
energy loss peak measured at 7 eV and 10 eV in the angular range 20–180. These cross sections have been derived from the ratios of vibrational to elastic intensities. The ratios have been deduced from energy loss spectra using a gaussian peak fitting procedure, taking into account the increased width of the inelastic peaks. These ratios have been normalized against absolute elastic cross sections of THF measured recently [13] using a relative flow technique. The uncertainties in the obtained differential cross sections are 30%. The angular dependencies of the cross sections for both energies are similar in shape and show maxima near 100–110 which may be reminiscent of d-wave contributions in the scattering. The obtained differential cross sections have been extended down to 0, as is shown in Fig. 4 and then integrated to estimate the integral vibrational cross sections. Here, first the intensities ratios have been extrapolated to 0 and then the extrapolated values multiplied by the elastic cross sections taken from the theoretical calculations [17], but adjusted for 0–30 region as it has been done previously in the determination of the integral elastic cross section [13]. The resulting integral cross sections are equal to 1.66 · 10 20 m2 and 1.14 · 10 20 m2 at 7 eV and 10 eV, respectively. In addition, using the above procedure we have estimated the integral cross section for excitation of the vibrational modes of the 140 meV peak, the second most intense energy loss peak. At 7 eV, we have estimated a value of 3.76 · 10 20 m2. This locates the total integral vibrational cross section for 7 eV at about 5.5 · 10 20 m2 which coincides well with the difference of recently measured total [11] and integral elastic [13] cross sections. The uncertainty in the present integral vibrational cross sections is 35%.
3.3. Differential cross sections
4. Conclusions
Fig. 4 presents the differential cross sections for excitation of the CH2 stretch vibrational modes of the 350 meV
We have studied electron impact vibrational excitation of tetrahydrofuran in the 5–14 eV incident energy range. The excitation function measured for the CH2 stretch modes reveals structures caused by overlapping negativeion resonances at 6.0, 7.9 and 10.3 eV. Some of the resonances, particularly those at higher energy may correspond to the short-lived r* shape resonances found by Allan and Andric [22] in the linear alkanes and related saturated organic molecules. We have also measured cross sections for vibrational excitation of the CH2 stretch modes of tetrahydrofuran which give the first estimate of the integral vibrational cross sections.
30
THF
-22
2
-1
Differential cross section (10 m sr )
[10] at 30 which reveals a resonance peak at 8.5 eV. The present energies of the resonances can also be compared with the positions of structures observed at about 6 eV and 8.5 eV in the total cross section measurements by Mo_zejko et al. [11], who in their discussion ascribed them to short-lived overlapping resonances. Further, excitation functions of various fundamental vibrational modes of THF measured for condensed multilayers [10] showed at least three resonances at 4, 7.5 and 10 eV. The shape of our excitation function, resembles that measured for CH2 stretch modes in cyclopentane [22], which exhibits a broad peak with a maximum at about 6.5 eV and a shoulder at about 9.5 eV, both being interpreted as resulting from several overlapping resonances. It is also interesting to note, that the positions of resonances obtained in the present measurements correlate with the maxima in the dissociative electron attachment found in the total negative ion yield by Aflatooni et al. [15] at 6.2 eV and 8.0 eV and in the C2HO ion yield by Sulzer et al. [14] at 7.6 eV. These authors suggest that in the observed dissociative attachment processes core-excited resonances are involved. Overlapping shape negative-ion resonances have been predicted in the theoretical calculations of elastic electron scattering in THF [17–19]. Winstead and McKoy [17] in their Schwinger multichannel calculations of the integral cross section found a resonance peak at about 8.3 eV and a resonance related shoulder at 13–14 eV. Trevisan et al. [18] reported a broad resonance in the momentum transfer cross section at about 8.6 eV resulting from two overlapping shape resonances of A1 and B2 symmetries. The good agreement of our experimental position of the resonance (7.9 eV) with the theoretically predicted value (8.5 eV) may support its assignment as a shape resonance.
ΔE = 0.350 eV
25
7 eV 10 eV
20 15 10
Acknowledgements
5 0 0
20
40
60
80
100
120
140
160
180
Scattering angle (deg) Fig. 4. Differential cross sections for excitation of the CH2 stretch modes of the 350 meV energy loss peak in tetrahydrofuran measured at the incident electron energies of 7 eV and 10 eV.
This work has been carried out within the European Science Foundation programme ‘Electron Induced Processing at the Molecular Level’ (EIPAM). One of us (ARM) acknowledges receiving fellowships for two research visits from European Science Foundation through the EIPAM programme. This work is also partly supported by the Pol-
M. Dampc et al. / Chemical Physics Letters 443 (2007) 17–21
ish State Committee for Scientific Research and by MSEP of Serbia under the project 141011. References [1] B. Cadioli, E. Gallinella, C. Coulombeau, H. Jobic, G. Berthier, J. Phys. Chem. 97 (1993) 7844. [2] V.M. Rayo´n, J.A. Sordo, J. Chem. Phys. 122 (2005) 204303, and references therein. [3] B. Boudaiffa, P. Cloutier, D. Hunting, M.A. Huels, L. Sanche, Science 287 (2000) 1658. [4] G. Hanel et al., Phys. Rev. Lett. 90 (2003) 188104. [5] H. Abdoul-Carime, S. Gohlke, E. Illenberger, Phys. Rev. Lett. 92 (2004) 168103. [6] L. Sanche, Eur. Phys. J. D 35 (2005) 367. [7] I. Linert, M. Zubek, J. Phys. B 39 (2006) 4087. [8] A. Rolda´n, J.M. Pe´rez, A. Williart, F. Blanco, G. Garcia, J. Appl. Phys. 95 (2004) 5865. [9] J.C. Oller, A. Munoz, J.M. Pere´z, F. Blanco, P. Limao-Viera, G. Garcia, Chem. Phys. Lett. 421 (2006) 439. [10] M. Lepage, S. Letarte, M. Michaud, F. Motte-Tollet, M.-J. HubinFranskin, D. Roy, L. Sanche, J. Chem. Phys. 109 (1998) 5980.
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[11] P. Mo_zejko, E. Ptasin´ska-Denga, A. Domaracka, C. Szmytkowski, Phys. Rev. A 74 (2006) 012708. [12] C.J. Colyer, V. Vizcaino, J.P. Sullivan, M.J. Brunger, S.J. Buckman, New J. Phys. 9 (2007) 41. [13] M. Dampc, A.R. Milosavljevic´, I. Linert, B.P. Marinkovic´, M. Zubek, Phys. Rev. A 75 (2007) 042710. [14] P. Sulzer et al., J. Chem. Phys. 125 (2006) 044304. [15] K. Aflatooni, A.M. Scheer, P.D. Burrow, J. Chem. Phys. 125 (2006) 054301. [16] C. Ja¨ggle, P. Swiderek, S.-P. Breton, M. Michaud, L. Sanche, J. Phys. Chem. B 110 (2006) 12512. [17] C. Winstead, V. McKoy, J. Chem. Phys. 125 (2006) 074302. [18] C.S. Trevisan, A.E. Orel, T.N. Rescigno, J. Phys. B 39 (2006) L255. [19] S. Tonzani, C.H. Greene, J. Chem. Phys. 125 (2006) 094504. [20] D. Bouchiha, J.D. Gorfinkiel, L.G. Caron, L. Sanche, J. Phys. B 39 (2006) 975. [21] I. Linert, G.C. King, M. Zubek, J. Electron Spectrosc. 134 (2004) 1. [22] M. Allan, L. Andric, J. Chem. Phys. 105 (1996) 3559. [23] A.R. Milosavljevic´, A. Giuliani, D. Sˇevic´, M.-J. Hubin-Franskin, B.P. Marinkovic´, Eur. Phys. J. D 35 (2005) 411.
Vibrational excitation of tetrahydrofuran by electron impact in the low energy range Marcin Dampc a, Ireneusz Linert a, Aleksandar R. Milosavljevic´ b, Mariusz Zubek a
a,*
Department of Physics of Electronic Phenomena, Gdan´sk University of Technology, 80-952 Gdan´sk, Poland b Institute of Physics, Pregrevica 118, 11080 Belgrade, Serbia Received 25 April 2007; in final form 5 June 2007 Available online 15 June 2007
Abstract Vibrational excitation of tetrahydrofuran has been studied over the incident electron energy range from 5 to 14 eV and over a wide range of scattering angles (20–180). Measurement of vibrational excitation function of the CH2 stretch modes (C–H stretching) reveals three structures at 6.0, 7.9 and 10.3 eV which can be attributed to the formation of negative-ion resonances. Differential and integral cross sections for vibrational excitation of the CH2 stretch modes have also been determined. 2007 Elsevier B.V. All rights reserved.
1. Introduction The tetrahydrofuran (THF) molecule, C4H8O, has been the subject of several investigations since it represents a structural unit of a number of carbohydrate and biological molecules. It also serves as a model to study conformation and ring puckering vibrations in small ring molecules [1,2]. In recent years, there has been renewed interest in THF stimulated by the investigations of radiation damage in biological systems and the role of secondary low energy electrons (E 6 20 eV) in the chemical and structural changes of cellular DNA [3]. The secondary electrons, although having energies below the ionization threshold, can induce single and double strand breaks in DNA. Such damage is partially attributable to the process of dissociative electron attachment to building blocks of DNA which proceed through formation of negative-ion resonances [4,5]. Since THF, a five-membered hydrocarbon ring containing an oxygen atom (Fig. 1), is incorporated in the deoxyribose sugar, it is considered to be a simple analogue for the study of the effects of secondary electrons on the
*
Corresponding author. Fax: +48 58 347 2821. E-mail address: [email protected] (M. Zubek).
0009-2614/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.06.048
deoxyribose ring and further on the sugar chain of the backbone of DNA [6]. In the present Letter, we have studied vibrational excitation of THF in the 5–14 eV incident electron energy range. The measurements have been carried out over a wide range of scattering angles from 20 to 180 applying the magnetic angle-changing technique for scattering from 90 to 180 [7]. To determine energies of negative-ion resonances occurring in vibrational excitation we have measured the excitation function of the observed CH2 stretch vibrational modes (C–H stretching) at fixed scattering angles. Identification of resonances allows us to understand the processes of electron energy transfer to the furanose ring leading to relaxation of equilibrium geometry of the ring. From the measured energy loss spectra we have obtained, to our knowledge, the first absolute differential and integral cross sections for excitation of the CH2 stretch vibrational modes. These absolute vibrational cross sections may serve as input parameters in Monte Carlo simulations of deposition of ionizing radiation energy in human tissue [8,9]. The first and up to now the only detailed investigations of electron impact vibrational excitations of THF in the gaseous and condensed multilayer phases have been performed by Lepage et al. [10] in the 1–30 eV energy range. However, these authors focused mainly on the resonance enhanced
18
M. Dampc et al. / Chemical Physics Letters 443 (2007) 17–21
Fig. 1. Diagram of the tetrahydrofuran molecule, C4H8O in the C2 symmetry point group. Oxygen atom is red (dark), carbon atom gray (gray), and hydrogen atom green (dark gray). (For interpretation of the references in colour in this figure legend, the reader is referred to the web version of this article.)
excitation of solid THF which revealed existence of several overlapping negative-ion resonances in the 4–12 eV range. In recent experimental works in the gaseous THF which include measurements of the total [11] and differential elastic [12,13] cross sections and studies of dissociative electron attachment [14,15] several resonance processes below 12 eV have been seen in agreement with the first observation of resonances at about 8.5 eV in [10]. Very recently, vibrational excitation of THF in the solid films have been monitored in the studies of the electron-stimulated reactions in the films [16]. The above experimental results are accompanied by a number of very recent ab initio calculations of elastic electron scattering in THF [17–19] which predict presence of overlapping shape resonances in the region 6–15 eV and calculations of integral elastic and electronically inelastic cross sections [20] which found a few core-excited resonances for energies up to 10 eV.
arates the electrons scattered in the backward directions from the incident electron beam and allows us to detect electron scattering in the angular range up to 180. The scattering angle scale has been determined with an uncertainty of ±2 against the position of a minimum at 117.5 in the elastic electron scattering in argon at 10 eV. The angular resolution of the spectrometer is estimated from the electron optics computational simulations to be less than 5. In the present Letter, electron energy loss spectra have been measured in THF at fixed incident electron energies in the scattering angle range from 20 to 180 in 10 steps. The background in these spectra has been measured by introducing the target gas directly to the vacuum chamber through a side valve. These background contributions have been removed from the measured spectra. It has been found in the previous measurements [7] that the variation of the transmission of electron analyzer in the energy loss range up to 1.0 eV is small, less than 5%, and no attempts have been made to correct measured energy loss spectra for such transmission. The energy resolution of the spectrometer in the energy loss mode of the present operation was 70–80 meV. The incident electron energy was calibrated to within ±30 meV with respect to the position of the 2 P3/2 resonance in argon at 11.098 eV. The excitation function measured in this work has been corrected for variation of spectrometer transmission with the incident electron energy. That correction has been determined with the help of helium elastic scattering. The anhydrous THF with a declared purity of better than 99.9% was purchased from Aldrich and was used after a few outgasing cycles under vacuum. 3. Results and discussion 3.1. Energy loss spectra
2. Experimental details Measurements were performed using a hemispherical electron spectrometer, which has been described in detail by Linert and Zubek [7]. Briefly, it consists of an electron beam source, an analyzer for scattered electrons and a magnetic angle changer, which surrounds the electron scattering center. The electron beam source and scattered electron analyzer contain two hemispherical energy selectors arranged in series and are equipped with systems of cylindrical lenses and deflectors. The incident electron beam is crossed at right angle with a target gas beam produced by a stainless steel tube with 0.6 mm inner diameter. The electron analyzer can be rotated around the target gas beam in the angular range from about 90 to 90. The range of accessible scattering angles is further extended up to 180 by using magnetic angle changer [21]. The angle changer provides a static, localized magnetic field which is perpendicular to the scattering plane. The incident electron beam and the scattered electrons are deflected by the magnetic field. The magnetic field sep-
We have measured electron energy loss spectra in THF at incident energies of 7 and 10 eV. These values of incident electron energy correspond to the energy range of observed negative-ion resonances (see Section 3.2). Fig. 2a and b show the spectra obtained at 7 eV and 90 and at 10 eV and 110, respectively. In both spectra two prominent energy loss peaks have been observed at energies of 140 meV and 350 meV. The peaks are 20–30 meV wider than the elastic scattering peak indicating excitation of groups of vibrational modes with similar energies. THF has 33 normal vibrational modes but the energies of the fundamental modes of vibration of the same type are similar and in general are grouped in certain energy ranges [10]. Eight modes of the a- and b-CH2 symmetric and asymmetric stretch vibrations have energies between 350 meV and 370 meV, the other CH2 group vibrations lie in the 110–190 meV range, while the C–C stretch modes appear from 110 meV to 130 meV. Energies of the fundamental vibrational modes matching the positions of the observed energy loss peaks are indicated in Fig. 2a and b.
M. Dampc et al. / Chemical Physics Letters 443 (2007) 17–21 3
CH 2 vibrations
40
THF
C-C stretch
7 eV 90
ο
Intensity (counts)
}
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with higher energy resolution for cyclopentane (C5H10) which is a homogenous five-member saturated hydrocarbon. In that work a group of three energy loss peaks assigned to the C–C stretch and CH2 scissoring and rocking modes but probably comprising more than three vibrations has been found in the 100–200 meV energy loss range and a prominent single peak has been seen at 365 meV consisting of several vibrations of the CH2 stretch modes. Such similarities in the vibrational energy loss spectra have also been noticed in [22] for three-member ring molecules, cyclopropane (C3H6) and oxygen containing ethylene oxide (C2H4O). Longer progressions of vibrational peaks recorded in the present energy loss spectra for THF are indicative of excitation through mechanism of negative-ion resonances. This is in contrast to direct excitation observed by Milosavljevic´ et al. [23] in an energy loss spectrum measured at 30 eV and 10 which shows peaks corresponding essentially to excitation of the fundamental CH2 vibrational modes. The intensities of the peaks of the fundamental vibrational modes observed by Milosavljevic´ et al. (e.g. for the CH2 stretch) were small (1/3000 of the elastic intensity) compared to about 1/15 in the present energy loss spectra.
x5
3.2. Excitation function
6 x50
4
0.4
0.6
Fig. 2. Energy loss spectra measured at incident electron energy and scattering angle of, (a) 7 eV and 90, (b) 10 eV and 110, respectively. The energies of the fundamental vibrational modes corresponding to the positions of observed energy loss peaks are shown by bars.
Here, the modes of the totally symmetric vibrations (belonging to A representation of the C2 symmetry point group of THF, Fig. 1) are shown as the more likely to be excited through the resonance processes. The energy loss peaks at 140 meV correspond well to combined excitation of the C–C stretch (m13) and the CH2 rocking (m11), twisting (m9), scissoring (m5, m6) and wagging (m7, m8) modes. The 350 meV peaks indicate excitation of the a- and b-CH2 stretch (m1–m4) modes (C–H stretching). At an energy loss of 510 meV excitation of combination modes involving the CH2 stretch modes and the 140 meV bands are also clearly visible in the spectra. At higher energy losses of 690 meV (Fig. 2a) and 950 meV overtones of the CH2 stretch modes and combination modes have been also detected, respectively. Our two spectra (Fig. 2) compare well with the THF gas phase energy loss spectrum obtained by Lepage et al. [10] at scattering angle of 30 and electron energy of 8.5 eV. The above features of the vibrational excitation in THF are reminiscent of those obtained by Allan and Andric [22]
10 -1
0.2
Electron energy loss (eV)
2
0.0
8
-22
0
The excitation function of the CH2 stretch vibrational modes of the 350 meV energy loss peak measured at a scattering angle of 80 is shown in Fig. 3. It is normalized to the average differential cross section obtained at 7 eV and 10 eV (see Section 3.3). The excitation function displays a wide peak with two resonance maxima at 7.9 eV and 10.3 eV. There is also a clear resonance shoulder at about 6.0 eV on the rising slope of the wide peak. These results can be compared with the excitation function of the m2aCH2 asymmetric stretch mode obtained by Lepage et al.
Differential cross section (10 m sr )
2
6
4
THF o
80 ΔE = 0.350 eV
2
0 4
6
8
10
12
14
Electron energy (eV) Fig. 3. Excitation function of the CH2 stretch modes of the 350 meV energy loss peak in tetrahydrofuran measured at the scattering angle of 80.
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energy loss peak measured at 7 eV and 10 eV in the angular range 20–180. These cross sections have been derived from the ratios of vibrational to elastic intensities. The ratios have been deduced from energy loss spectra using a gaussian peak fitting procedure, taking into account the increased width of the inelastic peaks. These ratios have been normalized against absolute elastic cross sections of THF measured recently [13] using a relative flow technique. The uncertainties in the obtained differential cross sections are 30%. The angular dependencies of the cross sections for both energies are similar in shape and show maxima near 100–110 which may be reminiscent of d-wave contributions in the scattering. The obtained differential cross sections have been extended down to 0, as is shown in Fig. 4 and then integrated to estimate the integral vibrational cross sections. Here, first the intensities ratios have been extrapolated to 0 and then the extrapolated values multiplied by the elastic cross sections taken from the theoretical calculations [17], but adjusted for 0–30 region as it has been done previously in the determination of the integral elastic cross section [13]. The resulting integral cross sections are equal to 1.66 · 10 20 m2 and 1.14 · 10 20 m2 at 7 eV and 10 eV, respectively. In addition, using the above procedure we have estimated the integral cross section for excitation of the vibrational modes of the 140 meV peak, the second most intense energy loss peak. At 7 eV, we have estimated a value of 3.76 · 10 20 m2. This locates the total integral vibrational cross section for 7 eV at about 5.5 · 10 20 m2 which coincides well with the difference of recently measured total [11] and integral elastic [13] cross sections. The uncertainty in the present integral vibrational cross sections is 35%.
3.3. Differential cross sections
4. Conclusions
Fig. 4 presents the differential cross sections for excitation of the CH2 stretch vibrational modes of the 350 meV
We have studied electron impact vibrational excitation of tetrahydrofuran in the 5–14 eV incident energy range. The excitation function measured for the CH2 stretch modes reveals structures caused by overlapping negativeion resonances at 6.0, 7.9 and 10.3 eV. Some of the resonances, particularly those at higher energy may correspond to the short-lived r* shape resonances found by Allan and Andric [22] in the linear alkanes and related saturated organic molecules. We have also measured cross sections for vibrational excitation of the CH2 stretch modes of tetrahydrofuran which give the first estimate of the integral vibrational cross sections.
30
THF
-22
2
-1
Differential cross section (10 m sr )
[10] at 30 which reveals a resonance peak at 8.5 eV. The present energies of the resonances can also be compared with the positions of structures observed at about 6 eV and 8.5 eV in the total cross section measurements by Mo_zejko et al. [11], who in their discussion ascribed them to short-lived overlapping resonances. Further, excitation functions of various fundamental vibrational modes of THF measured for condensed multilayers [10] showed at least three resonances at 4, 7.5 and 10 eV. The shape of our excitation function, resembles that measured for CH2 stretch modes in cyclopentane [22], which exhibits a broad peak with a maximum at about 6.5 eV and a shoulder at about 9.5 eV, both being interpreted as resulting from several overlapping resonances. It is also interesting to note, that the positions of resonances obtained in the present measurements correlate with the maxima in the dissociative electron attachment found in the total negative ion yield by Aflatooni et al. [15] at 6.2 eV and 8.0 eV and in the C2HO ion yield by Sulzer et al. [14] at 7.6 eV. These authors suggest that in the observed dissociative attachment processes core-excited resonances are involved. Overlapping shape negative-ion resonances have been predicted in the theoretical calculations of elastic electron scattering in THF [17–19]. Winstead and McKoy [17] in their Schwinger multichannel calculations of the integral cross section found a resonance peak at about 8.3 eV and a resonance related shoulder at 13–14 eV. Trevisan et al. [18] reported a broad resonance in the momentum transfer cross section at about 8.6 eV resulting from two overlapping shape resonances of A1 and B2 symmetries. The good agreement of our experimental position of the resonance (7.9 eV) with the theoretically predicted value (8.5 eV) may support its assignment as a shape resonance.
ΔE = 0.350 eV
25
7 eV 10 eV
20 15 10
Acknowledgements
5 0 0
20
40
60
80
100
120
140
160
180
Scattering angle (deg) Fig. 4. Differential cross sections for excitation of the CH2 stretch modes of the 350 meV energy loss peak in tetrahydrofuran measured at the incident electron energies of 7 eV and 10 eV.
This work has been carried out within the European Science Foundation programme ‘Electron Induced Processing at the Molecular Level’ (EIPAM). One of us (ARM) acknowledges receiving fellowships for two research visits from European Science Foundation through the EIPAM programme. This work is also partly supported by the Pol-
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