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Ionic fragmentation of deep core-level (Cl1s) excited chloroform molecule
Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 161–163
Ionic fragmentation of deep core-level (Cl1s) excited chloroform molecule A.F. Lago a, 1 , A.C.F. Santos b , W.C. Stolte c , A.S. Schlachter c , G.G.B. de Souza a, ∗ b
a Instituto de Qu´ımica, Universidade Federal do Rio de Janeiro, 21949-900 Rio de Janeiro, RJ, Brazil Instituto de F´ısica, Universidade Federal do Rio de Janeiro, Caixa Postal 68528, Rio de Janeiro 21941-972, RJ, Brazil c Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
Available online 24 February 2005
Abstract The ionic fragmentation of the chloroform (CHCl3 ) molecule has been studied along the Cl1s edge, using synchrotron radiation and a magnetic mass spectrometer. The following cations were observed: Cl+ , Cl2+ , CCl+ , CCl2 + , CHCl+ , Cl3+ , H+ and C+ . While all ions are seen to be formed at the intense resonance which dominates the photoabsorption spectrum below the chlorine 1s edge (taken to occur at 2828.6 eV), the probability of observation of the CHCl+ , C+ and Cl3+ cations is seen to significantly decrease above the ionization edge. © 2005 Elsevier B.V. All rights reserved. Keywords: Chloroform; Ionic fragmentation; Inner shell; Synchrotron radiation
1. Introduction The photofragmentation of molecules following deep core-level ionization has been the subject of considerable recent interest [1–4]. In contrast to the ionization of shallowcore level electrons in molecules, which is usually followed by Auger processes involving the depletion of valence-shell electrons, the ionization of deep core-level electrons is normally followed by a much more complicated array of processes involving cascading Auger mechanisms and giving rise to highly charged species. As such, the study of deep corelevel photoionization may also bring important information to the investigation of multi-charged molecular fragments using electron impact or advanced theoretical techniques [5–8]. Within our knowledge, the study of deep-core level ionization using high energy photons (synchrotron radiation) has so far concentrated on small (diatomic or linear) molecules. In the present work the ionic fragmentation of the chloroform molecule, CHCl3 , has been studied using a magnetic mass ∗
Corresponding author. Fax: +55 21 290 4746. E-mail address: [email protected] (G.G.B. de Souza). 1 Present address: Department of Chemistry, The University of North Carolina, Chapel Hill, NC 27599-3290, USA. 0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.039
spectrometer and synchrotron radiation, in the 2815–2845 eV energy range.
2. Experiment The experiment was performed using X-ray synchrotron radiation from beamline 9.3.1 at the Advanced Light Source (ALS) in Berkeley, CA. 9.3.1 is a bending magnet beamline covering the 2–6 keV photon-energy range. This beamline provides a flux of 1011 photons s−1 in a bandpass ≤0.5 eV. The photon energy was determined with an accuracy of 0.2 eV. The apparatus consists of a 180◦ magnetic mass spectrometer [9], an electrostatic lens to focus the ions created in the interaction region onto the entrance slit of the spectrometer, and an effusive-jet gas cell containing push and extraction plates to move the ions from the interaction region into the lens. Ions are detected with a channel electron multiplier (CEM) at the exit slit of the spectrometer. The polarity of the lens, the magnetic field, and the CEM may be switched to allow measurement of either cations or anions produced in the interaction region. This spectrometer has a mass-to-charge resolution of approximately 1 part in 50. During the experiment, the pressure was maintained
162
A.F. Lago et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 161–163
around 10−5 Torr (background was 10−7 Torr), and the gas needle was kept at ground potential. The emergent beam was recorded by a light-sensitive diode. Energy calibration was checked against a previously measured total ion yield spectrum for the methyl chloride molecule, which has a known resonance at 2823.4 eV. The samples were commercially obtained from Sigma–Aldrich with purity better than 99.5%. No further purification was used except for degassing the liquid samples by multiple freeze–pump–thaw cycles before admitting the vapor into the chamber.
3. Results and discussion Chloroform, CHCl3 , is an important chloro-substituted methane which is widely employed in chemistry as organic solvent and other applications. The CHCl3 molecule is highly symmetrical in the ground state (C3v ) with most of the electron charge clouds being concentrated on the three peripheral chlorine atoms because of their higher electronegativity as compared to the carbon and hydrogen atoms. Very recently, the dissociative photoionization of the chloroform and chloroform-d molecules in the valence region and around the chlorine 2p edge was studied by Lago et al. using timeof-flight mass spectrometry in the coincidence mode and a He I lamp and tunable synchrotron radiation as light sources [10]. In the present work, we use synchrotron radiation in the X-rays range to investigate the deep core-level dissociative photoionization of the chloroform molecule. The total ion yield of the CHCl3 molecule, which mimics the photoabsorption spectrum, was measured as a function of the photon energy, around the chlorine K edge and is shown in Fig. 1. The difference in shape observed between the Cl1s and the corresponding Cl2p photoabsorption spectrum is of course associated with the different symmetries of the orbitals to which the Cl2p and Cl1s are associated in the molecule. In the absence of theoretical calculations, we speculate that the intense peak observed at 2823.0 eV is probably associated with transitions to an unoccupied, anti-
Fig. 1. Total ion yield of the CHCl3 molecule as a function of the photon energy near the Cl K edge.
bonding molecular orbital, while the other resonances may be associated in principle to transitions with mixed Rydberg/valence character, as observed for HCl and Cl2 [1–3]. Considering the lack of a known experimental value for the chloroform molecule, we have arbitrarily taken the chlorine 1s binding energy of chloroform as occurring approximately at 2828.6 eV, which is the known chlorine 1s binding energy for a related molecule, carbon tetrachloride, CCl4 as given by Perera et al. [11]. It is perhaps interesting mentioning that other chlorine-containing molecules with available experimental data, such as HCl (2823.9 eV) and Cl2 (2821.3 eV) also present intense around 2820 eV. These molecules also present ionization energy values very close to 2830 eV [12]. Analogously, it may be assumed that the resonances observed above the ionization edge in the chloroform molecule contain contributions from doubly excited states [12]. The individual contributions of the observed fragments as a function of the photon energy are presented in Fig. 2. The largest contribution to the ion yield originates from the Cl+ ion, followed by Cl2+ . These ions are seen to be formed with significant intensity both below and above the ionization edge. The CCl+ cation comes next in relative intensity, but is formed with much less intensity outside the main resonance. It is interesting to observe that all ions are formed at the intense resonance at 2823 eV an effective demonstration of its
Fig. 2. Individual cation contributions as a function of the photon energy.
A.F. Lago et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 161–163
strong dissociative character. The probability of observation of the CHCl+ , C+ and Cl3+ fragments, on the other hand, is seen to significantly decrease outside the low energy resonance. An opposite behavior is observed for the H+ cation, as its intensity is seen to increase as it moves from the 2823 eV resonance to higher photon energies. Finally, the extremely low probability of observation of C+ , which may be taken as the signature of the complete atomization of the molecule, should be pointed out. Negative ions have also been observed as a result of the excitation of the chloroform molecule around the chlorine 1s edge. However, a detailed discussion of these results will be presented in a forthcoming publication [13]. 4. Conclusions The ionic fragmentation of the chloroform (CHCl3 ) molecule has been studied along the Cl 1s edge, using synchrotron radiation and a magnetic mass spectrometer. The following cations were observed: Cl+ , Cl2+ , CCl+ , CCl2 + , CHCl+ , Cl3+ , H+ and C+ , in order of decreasing probability. While all ions were seen to be formed at the intense resonance at 2823 eV, the probability of observation of the CHCl+ , C+ and Cl3+ cations is seen to significantly decrease above the ionization edge. For a more detailed description of the fragmentation mechanism, further theoretical calculations and also electron-ion coincidence measurements would be highly desirable. Acknowledgments The authors thank the ALS staff for their assistance during the experiments and CNPq and FAPERJ (Brazil) for
163
financial support. ACFS is particularly indebted to Prof. Robert DuBois for the support to perform this experiment and CNPq (Brazil) for a post-doctoral fellowship at the University of Missouri-Rolla.
References [1] D.L. Hansen, M.E. Arrasate, J. Cotter, G.R. Fischer, K.T. Leung, J.C. Levin, R. Martin, P. Neill, R.C.C. Perera, I.A. Sellin, M. Simon, Y. Uehara, B. Vanderford, S.B. Whitfield, D.W. Lindle, Phys. Rev. A 57 (1998) 2608. [2] D.L. Hansen, M.E. Arrasate, J. Cotter, G.R. Fischer, K.T. Leung, J.C. Levin, R. Martin, P. Neill, R.C.C. Perera, I.A. Sellin, M. Simon, Y. Uehara, B. Vanderford, S.B. Whitfield, D.W. Lindle, Phys. Rev. A 57 (1998) R4090. [3] D.L. Hansen, M.E. Arrasate, J. Cotter, G.R. Fischer, K.T. Leung, J.C. Levin, R. Martin, P. Neill, R.C.C. Perera, I.A. Sellin, M. Simon, Y. Uehara, B. Vanderford, S.B. Whitfield, D.W. Lindle, Phys. Rev. A 58 (1998) 3757. [4] J.J. Neville, T. Tyliszczak, A.P. Hitchcock, J. Electron Spectr. Relat. Phenom. 101–103 (1999) 119. [5] L. Morvay, I. Cornides, Int. J. Mass Spectr. I. Proc. 62 (1984) 263. [6] G. Handke, F. Tarantelli, A. Sgamellotti, L.S. Cederbaum, J. Chem. Phys. 104 (1996) 9531. [7] C. Tian, C.R. Vidal, Phys. Rev. A 58 (1998) 3783. [8] F. Scheuerman, E. Salzborn, F. Hagelberg, P. Scheier, J. Chem. Phys. 114 (2001) 9875. [9] W.C. Stolte, Y. Yu, J.A.R. Samson, O. Hemmers, D.L. Jansen, S.B. Whitfield, H. Wang, P. Glans, D.W. Lindle, J. Phys. B 30 (1997) 4489. [10] A.F. Lago, A.C.F. Santos, G.G.B. de Souza, J. Chem. Phys. 120 (2004) 9547. [11] R.C.C. Perera, R.E. LaVilla, G.V. Gibbs, J. Chem. Phys. 86 (1987) 4824. [12] S. Bodeur, J.L. Marechal, C. Reynaud, D. Bazin, I. Nenner, Z. Phys. D 17 (1990) 291. [13] A.F. Lago, A.C. F. Santos, A.S. Schlachter, W.C. Stolte, G.G.B. de Souza, in preparation.
Ionic fragmentation of deep core-level (Cl1s) excited chloroform molecule A.F. Lago a, 1 , A.C.F. Santos b , W.C. Stolte c , A.S. Schlachter c , G.G.B. de Souza a, ∗ b
a Instituto de Qu´ımica, Universidade Federal do Rio de Janeiro, 21949-900 Rio de Janeiro, RJ, Brazil Instituto de F´ısica, Universidade Federal do Rio de Janeiro, Caixa Postal 68528, Rio de Janeiro 21941-972, RJ, Brazil c Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
Available online 24 February 2005
Abstract The ionic fragmentation of the chloroform (CHCl3 ) molecule has been studied along the Cl1s edge, using synchrotron radiation and a magnetic mass spectrometer. The following cations were observed: Cl+ , Cl2+ , CCl+ , CCl2 + , CHCl+ , Cl3+ , H+ and C+ . While all ions are seen to be formed at the intense resonance which dominates the photoabsorption spectrum below the chlorine 1s edge (taken to occur at 2828.6 eV), the probability of observation of the CHCl+ , C+ and Cl3+ cations is seen to significantly decrease above the ionization edge. © 2005 Elsevier B.V. All rights reserved. Keywords: Chloroform; Ionic fragmentation; Inner shell; Synchrotron radiation
1. Introduction The photofragmentation of molecules following deep core-level ionization has been the subject of considerable recent interest [1–4]. In contrast to the ionization of shallowcore level electrons in molecules, which is usually followed by Auger processes involving the depletion of valence-shell electrons, the ionization of deep core-level electrons is normally followed by a much more complicated array of processes involving cascading Auger mechanisms and giving rise to highly charged species. As such, the study of deep corelevel photoionization may also bring important information to the investigation of multi-charged molecular fragments using electron impact or advanced theoretical techniques [5–8]. Within our knowledge, the study of deep-core level ionization using high energy photons (synchrotron radiation) has so far concentrated on small (diatomic or linear) molecules. In the present work the ionic fragmentation of the chloroform molecule, CHCl3 , has been studied using a magnetic mass ∗
Corresponding author. Fax: +55 21 290 4746. E-mail address: [email protected] (G.G.B. de Souza). 1 Present address: Department of Chemistry, The University of North Carolina, Chapel Hill, NC 27599-3290, USA. 0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.039
spectrometer and synchrotron radiation, in the 2815–2845 eV energy range.
2. Experiment The experiment was performed using X-ray synchrotron radiation from beamline 9.3.1 at the Advanced Light Source (ALS) in Berkeley, CA. 9.3.1 is a bending magnet beamline covering the 2–6 keV photon-energy range. This beamline provides a flux of 1011 photons s−1 in a bandpass ≤0.5 eV. The photon energy was determined with an accuracy of 0.2 eV. The apparatus consists of a 180◦ magnetic mass spectrometer [9], an electrostatic lens to focus the ions created in the interaction region onto the entrance slit of the spectrometer, and an effusive-jet gas cell containing push and extraction plates to move the ions from the interaction region into the lens. Ions are detected with a channel electron multiplier (CEM) at the exit slit of the spectrometer. The polarity of the lens, the magnetic field, and the CEM may be switched to allow measurement of either cations or anions produced in the interaction region. This spectrometer has a mass-to-charge resolution of approximately 1 part in 50. During the experiment, the pressure was maintained
162
A.F. Lago et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 161–163
around 10−5 Torr (background was 10−7 Torr), and the gas needle was kept at ground potential. The emergent beam was recorded by a light-sensitive diode. Energy calibration was checked against a previously measured total ion yield spectrum for the methyl chloride molecule, which has a known resonance at 2823.4 eV. The samples were commercially obtained from Sigma–Aldrich with purity better than 99.5%. No further purification was used except for degassing the liquid samples by multiple freeze–pump–thaw cycles before admitting the vapor into the chamber.
3. Results and discussion Chloroform, CHCl3 , is an important chloro-substituted methane which is widely employed in chemistry as organic solvent and other applications. The CHCl3 molecule is highly symmetrical in the ground state (C3v ) with most of the electron charge clouds being concentrated on the three peripheral chlorine atoms because of their higher electronegativity as compared to the carbon and hydrogen atoms. Very recently, the dissociative photoionization of the chloroform and chloroform-d molecules in the valence region and around the chlorine 2p edge was studied by Lago et al. using timeof-flight mass spectrometry in the coincidence mode and a He I lamp and tunable synchrotron radiation as light sources [10]. In the present work, we use synchrotron radiation in the X-rays range to investigate the deep core-level dissociative photoionization of the chloroform molecule. The total ion yield of the CHCl3 molecule, which mimics the photoabsorption spectrum, was measured as a function of the photon energy, around the chlorine K edge and is shown in Fig. 1. The difference in shape observed between the Cl1s and the corresponding Cl2p photoabsorption spectrum is of course associated with the different symmetries of the orbitals to which the Cl2p and Cl1s are associated in the molecule. In the absence of theoretical calculations, we speculate that the intense peak observed at 2823.0 eV is probably associated with transitions to an unoccupied, anti-
Fig. 1. Total ion yield of the CHCl3 molecule as a function of the photon energy near the Cl K edge.
bonding molecular orbital, while the other resonances may be associated in principle to transitions with mixed Rydberg/valence character, as observed for HCl and Cl2 [1–3]. Considering the lack of a known experimental value for the chloroform molecule, we have arbitrarily taken the chlorine 1s binding energy of chloroform as occurring approximately at 2828.6 eV, which is the known chlorine 1s binding energy for a related molecule, carbon tetrachloride, CCl4 as given by Perera et al. [11]. It is perhaps interesting mentioning that other chlorine-containing molecules with available experimental data, such as HCl (2823.9 eV) and Cl2 (2821.3 eV) also present intense around 2820 eV. These molecules also present ionization energy values very close to 2830 eV [12]. Analogously, it may be assumed that the resonances observed above the ionization edge in the chloroform molecule contain contributions from doubly excited states [12]. The individual contributions of the observed fragments as a function of the photon energy are presented in Fig. 2. The largest contribution to the ion yield originates from the Cl+ ion, followed by Cl2+ . These ions are seen to be formed with significant intensity both below and above the ionization edge. The CCl+ cation comes next in relative intensity, but is formed with much less intensity outside the main resonance. It is interesting to observe that all ions are formed at the intense resonance at 2823 eV an effective demonstration of its
Fig. 2. Individual cation contributions as a function of the photon energy.
A.F. Lago et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 161–163
strong dissociative character. The probability of observation of the CHCl+ , C+ and Cl3+ fragments, on the other hand, is seen to significantly decrease outside the low energy resonance. An opposite behavior is observed for the H+ cation, as its intensity is seen to increase as it moves from the 2823 eV resonance to higher photon energies. Finally, the extremely low probability of observation of C+ , which may be taken as the signature of the complete atomization of the molecule, should be pointed out. Negative ions have also been observed as a result of the excitation of the chloroform molecule around the chlorine 1s edge. However, a detailed discussion of these results will be presented in a forthcoming publication [13]. 4. Conclusions The ionic fragmentation of the chloroform (CHCl3 ) molecule has been studied along the Cl 1s edge, using synchrotron radiation and a magnetic mass spectrometer. The following cations were observed: Cl+ , Cl2+ , CCl+ , CCl2 + , CHCl+ , Cl3+ , H+ and C+ , in order of decreasing probability. While all ions were seen to be formed at the intense resonance at 2823 eV, the probability of observation of the CHCl+ , C+ and Cl3+ cations is seen to significantly decrease above the ionization edge. For a more detailed description of the fragmentation mechanism, further theoretical calculations and also electron-ion coincidence measurements would be highly desirable. Acknowledgments The authors thank the ALS staff for their assistance during the experiments and CNPq and FAPERJ (Brazil) for
163
financial support. ACFS is particularly indebted to Prof. Robert DuBois for the support to perform this experiment and CNPq (Brazil) for a post-doctoral fellowship at the University of Missouri-Rolla.
References [1] D.L. Hansen, M.E. Arrasate, J. Cotter, G.R. Fischer, K.T. Leung, J.C. Levin, R. Martin, P. Neill, R.C.C. Perera, I.A. Sellin, M. Simon, Y. Uehara, B. Vanderford, S.B. Whitfield, D.W. Lindle, Phys. Rev. A 57 (1998) 2608. [2] D.L. Hansen, M.E. Arrasate, J. Cotter, G.R. Fischer, K.T. Leung, J.C. Levin, R. Martin, P. Neill, R.C.C. Perera, I.A. Sellin, M. Simon, Y. Uehara, B. Vanderford, S.B. Whitfield, D.W. Lindle, Phys. Rev. A 57 (1998) R4090. [3] D.L. Hansen, M.E. Arrasate, J. Cotter, G.R. Fischer, K.T. Leung, J.C. Levin, R. Martin, P. Neill, R.C.C. Perera, I.A. Sellin, M. Simon, Y. Uehara, B. Vanderford, S.B. Whitfield, D.W. Lindle, Phys. Rev. A 58 (1998) 3757. [4] J.J. Neville, T. Tyliszczak, A.P. Hitchcock, J. Electron Spectr. Relat. Phenom. 101–103 (1999) 119. [5] L. Morvay, I. Cornides, Int. J. Mass Spectr. I. Proc. 62 (1984) 263. [6] G. Handke, F. Tarantelli, A. Sgamellotti, L.S. Cederbaum, J. Chem. Phys. 104 (1996) 9531. [7] C. Tian, C.R. Vidal, Phys. Rev. A 58 (1998) 3783. [8] F. Scheuerman, E. Salzborn, F. Hagelberg, P. Scheier, J. Chem. Phys. 114 (2001) 9875. [9] W.C. Stolte, Y. Yu, J.A.R. Samson, O. Hemmers, D.L. Jansen, S.B. Whitfield, H. Wang, P. Glans, D.W. Lindle, J. Phys. B 30 (1997) 4489. [10] A.F. Lago, A.C.F. Santos, G.G.B. de Souza, J. Chem. Phys. 120 (2004) 9547. [11] R.C.C. Perera, R.E. LaVilla, G.V. Gibbs, J. Chem. Phys. 86 (1987) 4824. [12] S. Bodeur, J.L. Marechal, C. Reynaud, D. Bazin, I. Nenner, Z. Phys. D 17 (1990) 291. [13] A.F. Lago, A.C. F. Santos, A.S. Schlachter, W.C. Stolte, G.G.B. de Souza, in preparation.