- Email: [email protected]
Inner-valence Auger decay in chloroform after Cl 2p ionization
Nuclear Inst. and Methods in Physics Research B 461 (2019) 133–136
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Inner-valence Auger decay in chloroform after Cl 2p ionization a
b
c
a
c
D.N. Vasconcelos , M.A. MacDonald , B.N.C. Tenório , M.M. Sant'Anna , A.B. Rocha , ⁎ V. Morcelled, V.S. Bonfima, N. Appathuraib, L. Zuinb, A.C.F. Santosa,
T
a
Instituto de Física, Universidade Federal do Rio de Janeiro, 21941-972 Rio de Janeiro, RJ, Brazil Canadian Light Source Inc., Saskatoon, SK S7N 2V3, Canada Instituto de Química, Universidade Federal do Rio de Janeiro, 21941-909 Rio de Janeiro, RJ, Brazil d Departamento de Física, Universidade Federal Rural do Rio de Janeiro, RJ CEP 23890-000, Brazil b c
ARTICLE INFO
ABSTRACT
Keywords: Auger decay Chloroform Density functional theory
The inner-valence Auger electron spectra after the chlorine 2p ionization in the gas phase CHCl3 molecule were investigated both theoretically and experimentally. The total ion yield spectrum exhibits a broad structure at 217 eV, above the Cl 2p ionization threshold, which can be attributed to either a shape resonance or a shake-up. In addition, we have performed a series of high-level ab initio quantum mechanical calculations at the density functional theory level to compute the Auger lines.
1. Introduction
2. Experiment
Chloroform (CHCl3), a significant chloro-substituted methane largely employed in industry as organic solvent, is one of the chemicals responsible for the depletion of the stratospheric ozone (O3) layer [1]. Nonetheless, because of its short lifetime and its mostly natural origin, CHCl3 is not listed in the Montreal Protocol, an agreement that controls the manufacture of ozone-depleting substances. It has been shown that global chloroform emissions have increased in the last decade [2]. In addition, it has been suggested [2] that if chloroform emissions do not decrease, the restoration of the stratospheric O3 layer could be delayed. The inner-shell photoionization of an isolated molecule can provide several pieces of information on the electronic structure and relaxation processes. Various investigations have been carried out on Auger electron spectroscopy of free molecules [3–6]. The CHCl3 ionization and/or fragmentation have been investigated largely using EUV and Xray photons and electron energy loss [7–9]. In this paper, we present Auger electron spectra after the chlorine 2p ionization in the gas phase CHCl3 molecule. This paper is organized as follows: In the next section, we describe the experimental apparatus. In the third section we delineate the quantum mechanical calculations, performed at the density functional theory level, used to calculate the Cl 2p−1 VV Auger lines. In the section devoted to present the results, the calculations are compared with the experimental spectra. Finally, some conclusions are drawn.
The measurements presented in this paper were performed at the VLS-PGM beamline at the Canadian Light Source, Saskatoon, Canada [10–13]. This beamline is optimized for an energy range of 5–250 eV. An end-station dedicated to measure photoelectrons consists of a toroidal electron energy analyzer [12] working at the pass energy of 10 eV. In the spectrometer, the incident photon beam crosses the vapor jet, producing Auger electrons. The electrons ejected in the perpendicular plane with respect to the photon beam are focused at the toroid entrance slit, which can collect electrons with a 100° azimuthal angular range. Energy selected electrons pass through the toroidal analyzer and reach the exit slits. Those slits then accelerate and refocus the electrons to their respective microchannel plate detectors. In order to estimate the spectrometer resolution, a photoelectron spectrum of argon, corresponding to the Ar(KL3s23p6) → Ar+ (KL3s23p5) ionization, for 195.75 eV photons was acquired (See Fig. 1). The measured analyzer resolution was 160 meV.
⁎
3. Calculations The assignments of the molecular bands were carried on by Density functional theory calculations performed using the hybrid version of the Perdew-Burke-Ernzerhof functional (PBE0) and the aug-cc-pVTZ
Corresponding author. E-mail address: [email protected] (A.C.F. Santos).
https://doi.org/10.1016/j.nimb.2019.09.039 Received 3 June 2019; Received in revised form 18 September 2019; Accepted 24 September 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 461 (2019) 133–136
D.N. Vasconcelos, et al.
Fig. 1. Photoelectron spectrum of argon, Ar (KL 3s23p6) → Ar+ (KL 3s23p5), for 195.75 eV photons. The peaks located at 179.6 eV and 179.8 eV correspond, respectively, to the 2P3/2 and 2P1/2 states. The electron energy resolution is 160 meV. The small peak just below 179.4 eV is due to a residual contribution of N2 molecule.
Fig. 2. Total ion yield of CHCl3 molecule as a function of the photon energy around the Cl 2p edge taken with 0.01 eV steps. The two vertical bars indicate the ionization potentials (IP) for Cl 2p3/2,1/2, respectively. The red line represents a smooth in the experimental data and it is drawn to guide the eyes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
basis set. Relativistic effects have been neglected in the calculations. The kinetic energy of the leaving Auger electron, Ek, is determined by the conservation of energy formula:
hole, but just the electronic density. The TDDFT is known to be inefficient to recover the relaxation effects when dealing with vertical excitation energies, thus, a larger shift in the energy scale of 6.5 eV is applied to match theoretical and the first Cl 2p (5a1) → 10a1 experimental peak. It is interesting to observe in both the calculated cross section spectrum of Fig. 3 and the experimental TIY in Fig. 2, that there is a strong electronic transition around 218 eV which give rise to a shoulder in the region between 218 and 220 eV of the spectrum. The measurement performed with photons of 217.0 eV energy though give rise to processes mainly involving electronic processes related with this intense transition near 218 eV. Even more, one may suggest the existence of shape resonances or two electrons transition (shake-up) in this region of the spectra. The intense peak near 217 eV is mainly related to the Cl 2p (3e, 4a1) → 18e electronic transition according to our calculations. It is worth mentioning that the spectral region below 217.0 eV is mainly dominated by transitions resulting from the upper Cl 2p (5e, 5a1) shell while the region above 217.0 eV, on another hand, is dominated by contributions from the lower Cl 2p (3e, 4a1) shell. Fig. 4 shows the Auger electron spectra of CHCl3 molecule measured at 217.0 eV and 230.0 eV. Due to the large number of outer-shell electrons, giving rise to several allowed final double-hole states, the Auger spectra shown in Fig. 4 exhibit rich structures. Both spectra are virtually similar, but the Auger electron peak positions at 217.0 eV photon energy are shifted by +1 eV in relation to their corresponding peaks obtained at 230.0 eV photon energy. If we consider the normal Auger processes ionizing the lower Cl 2p (3e,4a1) shell with the same final states shown in Table 1, we would obtain the Ek values shifted by +0.4 eV, which is in tune with the experimental shifts of approximately +1 eV. As we have a shift of 4.2 eV in the Cl 2p ionization energy compared to the experimental value, our calculated Auger kinetic energy, Ek, must be accordingly shifted by the same factor of +4.2 eV. In Table 1 we present the calculated energies of the leaving Auger electron, Ek, obtained with the equation (1a) and the respective shifted values. Relating the values of the Table 1 with the molecular bands of Fig. 4 is easy to assign the first experimental band, centered at 177.5 eV, mostly to the Auger decay to the final state3 of the CHCl32+ cation with contributions of the state1 and state5 configurations. The next strong band centered at 176.0 eV, on the other hand, may be assigned to the configuration staste8 with the hole on the (8e)−2 valence orbital and the lowest bands at 174 and 175 eV may be assigned to the configuration state7 with contribution of the state2 with the holes on the molecular
(1)
Ek = Eh - E f
where Eh stands for the energy of the Cl 2p ionized state of the molecule and Ef for the final doubled charged cation energy. The CHCl3 Cl 2p photoionization cross section spectrum has also been calculated with an analytic continuation procedure [14–16]. The analytic continuation procedure is able to describe, on the same ground, both the discrete and continuum regions of the spectrum using square-integrable functions (also known as L2 basis set) once the full restrict-channel discrete pseudo-spectrum is obtained, what can be easily achieved for medium-to-large size molecules at the TDDFT level [15]. Since information on the photoionization process is also wanted, an additional L2 basis set with ten spherical functions of S, P, and D type obtained from the work of Kaufmann et al [17] with n ranging from 2 to 6 was added to the center of mass of the system to correctly describe the continuum region of the spectrum. 4. Results and discussions The CHCl3 molecule belongs to the C3v symmetry point with the ground state configuration as described in Ref. [7]. The Cl 2p shell orbitals may be grouped according to its energy as (4a1)1 (3e)2 (1a2)1 (4e)2 (5e)2 (5a1)1.The process of LVV Auger emission after Cl 2p ionization can be written as
h + CHCl3
CHCl3+ (2p 1 ) + eph
CHCl32 + (V1 1 V 2 1) + eLVV
(2)
Total ion yield (TIY) spectrum of CHCl3 is presented in Fig. 2 in the 206–230 eV photon energy range. This spectrum was measured by scanning the photon energy around the Cl 2p edge and measuring all positive fragments of CHCl3 at the toroid spectrometer. The ionization threshold for CHCl3 is located at 206.0 eV (Cl 2p3/2) [7,9]. The position of the Cl 2p1/2 edge was estimated from the expected spin-orbit split of 1.6 eV as being 207.6 eV (Cl2p1/2). The calculated CHCl3 Cl 2p photoionization cross section spectrum is presented in Fig. 3. The theoretical CHCl3 Cl 2p vertical ionization threshold was obtained with the same basis set at the TDDFT/PBE0 level and the result, 202.1 eV, is indicated by the green vertical dashed line in Fig. 3. The vertical ionization energy is calculated as the difference between the ground state energy and that one of the systems with a core-hole at the equilibrium geometry, i.e., without relaxing the geometry to the core134
Nuclear Inst. and Methods in Physics Research B 461 (2019) 133–136
D.N. Vasconcelos, et al.
Fig. 3. Photoionization cross section of the CHCl3 molecule obtained at the TDDFT level. The calculated spectrum was rigidly shifted by Δ = +6.5 eV to match the first Cl 2p (5a1) → 10a1 experimental peak at 200.6 eV. The green and red dashed lines represent the experimental and calculated Cl 2p ionization energies. The blue line represents a convolution of the cross sections. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Calculated energies of the leaving Auger electron, Ek, obtained with the Eq. (1) and the respective shifted values. CHCl32+ configuration state
CHCl32+ valence orbital holes (Ef)
Ek in eV
Ek + 4.2 eV
state1 state2 state3 state4 state5 state6 state7 state8
(9a1)−2 (8a1)−1(9a1)−1 (2a2)−2 (9a1)−1(2a2)−1 (9e)−2 (7a1)−1(8a1)−1 (8a1)−1(1a2)−1 (8e)−2
172.8 168.4 173.1 173.8 173.6 158.8 168.8 171.6
177.0 172.6 177.3 178.0 177.8 163.0 173.0 175.8
have shown that the theoretical Auger electron energies of CHCl3, calculated using series of high-level ab initio quantum mechanical calculations of density functional theory, explains qualitatively the experimental results. The total ion yield spectrum exhibits a broad structure at 217 eV, above the Cl 2p ionization threshold, which can be attributed to either a shape resonance or a shake-up. The Auger spectra taken at 217 eV and 230 eV mimic each other, but the peak positions obtained at 217 eV are shifted by +1 eV with respect to the spectrum taken at 230 eV. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 4. Auger electron spectra of CHCl3 after Cl 2p photoionization by photons, 217.0 eV and 230.0 eV, respectively. The green lines represent Gaussian fittings to the observed peaks. The red lines represent the sum of all fittings. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Acknowledgments This work is supported in part by CNPq and CAPES. One of the authors, A.C.F. Santos, acknowledges support from Programa Abdias Nascimento. Research described in this paper was performed at the Canadian Light Source, which is supported by the Canada Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research.
CHCl32+ cation on the (8a1)−1(1a2)−1 and (8a1)−1(9a1)−1 combinations. 5. Conclusions This paper describes the experimental and theoretical investigations on the Auger decay after Cl 2p−1 ionization of CHCl3 molecule. We 135
Nuclear Inst. and Methods in Physics Research B 461 (2019) 133–136
D.N. Vasconcelos, et al.
References
A.B. Rocha, V. Morcelle, N. Appathurai, L. Zuin, J. Chem. Phys. 149 (2018) 054303. [11] A.C.F. Santos, D.N. Vasconcelos, M.A. MacDonald, M.M. Sant’Anna, B.N.C. Tenório, A.B. Rocha, V. Morcelle, N. Appathurai, L. Zuin, Eur. Phys. J. D 73 (2019) 81. [12] T.J. Reddish, G. Richmond, G.W. Bagley, J.P. Wightman, S. Cvejanovic, Rev. Sci. Instrum. 68 (1997) 2685. [13] Y.F. Hu, L. Zuin, G. Wright, R. Igarashi, M. McKibben, T. Wilson, S.Y. Chen, T. Johnson, D. Maxwell, B.W. Yates, Rev. Sci. Instrum. 78 (2007) 083109. [14] B.N.C. Tenorio, M.A.C. Nascimento, S. Coriani, A.B. Rocha, J. Chem. Theory Comp. 12 (2016) 4440–4459. [15] B.N.C. Tenorio, M.A.C. Nascimento, A.B. Rocha, J. Chem. Phys. 148 (2018) 074104. [16] B.N.C. Tenorio, R.R. Oliveira, M.A.C. Nascimento, A.B. Rocha, J. Chem. Theory Comp. 14 (2018) 5324–5338. [17] K. Kaufmann, W. Baumeister, M. Jungen, J. Phys. B: At. Mol. Opt. Phys. 22 (1989) 2223–2240.
[1] S. Tegtmeier, Nat. Geosci. 12 (2019) 84–86. [2] X. Fang, et al., Nat. Geosci. 12 (2019) 89–93. [3] E. Kokkonen, K. Jänkälä, M. Patane, W. Cao, M. Hrast, K. Bučar, M. Žitnik, M. Huttula, J. Chem. Phys. 148 (2018) 174301. [4] O. Travnikova, et al., Phys. Rev. Lett. 118 (2017) 213001. [5] O. Travnikova, et al., Phys. Rev. Lett. 116 (2016) 213001. [6] H. Sann, et al., Phys. Rev. Lett. 117 (2017) 243002. [7] A.F. Lago, A.C.F. Santos, G.G.B. de Souza, J. Chem. Phys. 120 (2004) 9547–9555. [8] A.F. Lago, A.C.F. Santos, G.G.B. de Souza, Int. J. Mass. Spectr. 262 (2007) 187–194. [9] A.P. Hitchcock, C.E. Brion, J. Electron Spectrosc. Relat. Phenom. 14 (1978) 417–441. [10] A.C.F. Santos, D.N. Vasconcelos, M.A. MacDonald, M.M. Sant’Anna, B.N.C. Tenório,
136
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Inner-valence Auger decay in chloroform after Cl 2p ionization a
b
c
a
c
D.N. Vasconcelos , M.A. MacDonald , B.N.C. Tenório , M.M. Sant'Anna , A.B. Rocha , ⁎ V. Morcelled, V.S. Bonfima, N. Appathuraib, L. Zuinb, A.C.F. Santosa,
T
a
Instituto de Física, Universidade Federal do Rio de Janeiro, 21941-972 Rio de Janeiro, RJ, Brazil Canadian Light Source Inc., Saskatoon, SK S7N 2V3, Canada Instituto de Química, Universidade Federal do Rio de Janeiro, 21941-909 Rio de Janeiro, RJ, Brazil d Departamento de Física, Universidade Federal Rural do Rio de Janeiro, RJ CEP 23890-000, Brazil b c
ARTICLE INFO
ABSTRACT
Keywords: Auger decay Chloroform Density functional theory
The inner-valence Auger electron spectra after the chlorine 2p ionization in the gas phase CHCl3 molecule were investigated both theoretically and experimentally. The total ion yield spectrum exhibits a broad structure at 217 eV, above the Cl 2p ionization threshold, which can be attributed to either a shape resonance or a shake-up. In addition, we have performed a series of high-level ab initio quantum mechanical calculations at the density functional theory level to compute the Auger lines.
1. Introduction
2. Experiment
Chloroform (CHCl3), a significant chloro-substituted methane largely employed in industry as organic solvent, is one of the chemicals responsible for the depletion of the stratospheric ozone (O3) layer [1]. Nonetheless, because of its short lifetime and its mostly natural origin, CHCl3 is not listed in the Montreal Protocol, an agreement that controls the manufacture of ozone-depleting substances. It has been shown that global chloroform emissions have increased in the last decade [2]. In addition, it has been suggested [2] that if chloroform emissions do not decrease, the restoration of the stratospheric O3 layer could be delayed. The inner-shell photoionization of an isolated molecule can provide several pieces of information on the electronic structure and relaxation processes. Various investigations have been carried out on Auger electron spectroscopy of free molecules [3–6]. The CHCl3 ionization and/or fragmentation have been investigated largely using EUV and Xray photons and electron energy loss [7–9]. In this paper, we present Auger electron spectra after the chlorine 2p ionization in the gas phase CHCl3 molecule. This paper is organized as follows: In the next section, we describe the experimental apparatus. In the third section we delineate the quantum mechanical calculations, performed at the density functional theory level, used to calculate the Cl 2p−1 VV Auger lines. In the section devoted to present the results, the calculations are compared with the experimental spectra. Finally, some conclusions are drawn.
The measurements presented in this paper were performed at the VLS-PGM beamline at the Canadian Light Source, Saskatoon, Canada [10–13]. This beamline is optimized for an energy range of 5–250 eV. An end-station dedicated to measure photoelectrons consists of a toroidal electron energy analyzer [12] working at the pass energy of 10 eV. In the spectrometer, the incident photon beam crosses the vapor jet, producing Auger electrons. The electrons ejected in the perpendicular plane with respect to the photon beam are focused at the toroid entrance slit, which can collect electrons with a 100° azimuthal angular range. Energy selected electrons pass through the toroidal analyzer and reach the exit slits. Those slits then accelerate and refocus the electrons to their respective microchannel plate detectors. In order to estimate the spectrometer resolution, a photoelectron spectrum of argon, corresponding to the Ar(KL3s23p6) → Ar+ (KL3s23p5) ionization, for 195.75 eV photons was acquired (See Fig. 1). The measured analyzer resolution was 160 meV.
⁎
3. Calculations The assignments of the molecular bands were carried on by Density functional theory calculations performed using the hybrid version of the Perdew-Burke-Ernzerhof functional (PBE0) and the aug-cc-pVTZ
Corresponding author. E-mail address: [email protected] (A.C.F. Santos).
https://doi.org/10.1016/j.nimb.2019.09.039 Received 3 June 2019; Received in revised form 18 September 2019; Accepted 24 September 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Nuclear Inst. and Methods in Physics Research B 461 (2019) 133–136
D.N. Vasconcelos, et al.
Fig. 1. Photoelectron spectrum of argon, Ar (KL 3s23p6) → Ar+ (KL 3s23p5), for 195.75 eV photons. The peaks located at 179.6 eV and 179.8 eV correspond, respectively, to the 2P3/2 and 2P1/2 states. The electron energy resolution is 160 meV. The small peak just below 179.4 eV is due to a residual contribution of N2 molecule.
Fig. 2. Total ion yield of CHCl3 molecule as a function of the photon energy around the Cl 2p edge taken with 0.01 eV steps. The two vertical bars indicate the ionization potentials (IP) for Cl 2p3/2,1/2, respectively. The red line represents a smooth in the experimental data and it is drawn to guide the eyes. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
basis set. Relativistic effects have been neglected in the calculations. The kinetic energy of the leaving Auger electron, Ek, is determined by the conservation of energy formula:
hole, but just the electronic density. The TDDFT is known to be inefficient to recover the relaxation effects when dealing with vertical excitation energies, thus, a larger shift in the energy scale of 6.5 eV is applied to match theoretical and the first Cl 2p (5a1) → 10a1 experimental peak. It is interesting to observe in both the calculated cross section spectrum of Fig. 3 and the experimental TIY in Fig. 2, that there is a strong electronic transition around 218 eV which give rise to a shoulder in the region between 218 and 220 eV of the spectrum. The measurement performed with photons of 217.0 eV energy though give rise to processes mainly involving electronic processes related with this intense transition near 218 eV. Even more, one may suggest the existence of shape resonances or two electrons transition (shake-up) in this region of the spectra. The intense peak near 217 eV is mainly related to the Cl 2p (3e, 4a1) → 18e electronic transition according to our calculations. It is worth mentioning that the spectral region below 217.0 eV is mainly dominated by transitions resulting from the upper Cl 2p (5e, 5a1) shell while the region above 217.0 eV, on another hand, is dominated by contributions from the lower Cl 2p (3e, 4a1) shell. Fig. 4 shows the Auger electron spectra of CHCl3 molecule measured at 217.0 eV and 230.0 eV. Due to the large number of outer-shell electrons, giving rise to several allowed final double-hole states, the Auger spectra shown in Fig. 4 exhibit rich structures. Both spectra are virtually similar, but the Auger electron peak positions at 217.0 eV photon energy are shifted by +1 eV in relation to their corresponding peaks obtained at 230.0 eV photon energy. If we consider the normal Auger processes ionizing the lower Cl 2p (3e,4a1) shell with the same final states shown in Table 1, we would obtain the Ek values shifted by +0.4 eV, which is in tune with the experimental shifts of approximately +1 eV. As we have a shift of 4.2 eV in the Cl 2p ionization energy compared to the experimental value, our calculated Auger kinetic energy, Ek, must be accordingly shifted by the same factor of +4.2 eV. In Table 1 we present the calculated energies of the leaving Auger electron, Ek, obtained with the equation (1a) and the respective shifted values. Relating the values of the Table 1 with the molecular bands of Fig. 4 is easy to assign the first experimental band, centered at 177.5 eV, mostly to the Auger decay to the final state3 of the CHCl32+ cation with contributions of the state1 and state5 configurations. The next strong band centered at 176.0 eV, on the other hand, may be assigned to the configuration staste8 with the hole on the (8e)−2 valence orbital and the lowest bands at 174 and 175 eV may be assigned to the configuration state7 with contribution of the state2 with the holes on the molecular
(1)
Ek = Eh - E f
where Eh stands for the energy of the Cl 2p ionized state of the molecule and Ef for the final doubled charged cation energy. The CHCl3 Cl 2p photoionization cross section spectrum has also been calculated with an analytic continuation procedure [14–16]. The analytic continuation procedure is able to describe, on the same ground, both the discrete and continuum regions of the spectrum using square-integrable functions (also known as L2 basis set) once the full restrict-channel discrete pseudo-spectrum is obtained, what can be easily achieved for medium-to-large size molecules at the TDDFT level [15]. Since information on the photoionization process is also wanted, an additional L2 basis set with ten spherical functions of S, P, and D type obtained from the work of Kaufmann et al [17] with n ranging from 2 to 6 was added to the center of mass of the system to correctly describe the continuum region of the spectrum. 4. Results and discussions The CHCl3 molecule belongs to the C3v symmetry point with the ground state configuration as described in Ref. [7]. The Cl 2p shell orbitals may be grouped according to its energy as (4a1)1 (3e)2 (1a2)1 (4e)2 (5e)2 (5a1)1.The process of LVV Auger emission after Cl 2p ionization can be written as
h + CHCl3
CHCl3+ (2p 1 ) + eph
CHCl32 + (V1 1 V 2 1) + eLVV
(2)
Total ion yield (TIY) spectrum of CHCl3 is presented in Fig. 2 in the 206–230 eV photon energy range. This spectrum was measured by scanning the photon energy around the Cl 2p edge and measuring all positive fragments of CHCl3 at the toroid spectrometer. The ionization threshold for CHCl3 is located at 206.0 eV (Cl 2p3/2) [7,9]. The position of the Cl 2p1/2 edge was estimated from the expected spin-orbit split of 1.6 eV as being 207.6 eV (Cl2p1/2). The calculated CHCl3 Cl 2p photoionization cross section spectrum is presented in Fig. 3. The theoretical CHCl3 Cl 2p vertical ionization threshold was obtained with the same basis set at the TDDFT/PBE0 level and the result, 202.1 eV, is indicated by the green vertical dashed line in Fig. 3. The vertical ionization energy is calculated as the difference between the ground state energy and that one of the systems with a core-hole at the equilibrium geometry, i.e., without relaxing the geometry to the core134
Nuclear Inst. and Methods in Physics Research B 461 (2019) 133–136
D.N. Vasconcelos, et al.
Fig. 3. Photoionization cross section of the CHCl3 molecule obtained at the TDDFT level. The calculated spectrum was rigidly shifted by Δ = +6.5 eV to match the first Cl 2p (5a1) → 10a1 experimental peak at 200.6 eV. The green and red dashed lines represent the experimental and calculated Cl 2p ionization energies. The blue line represents a convolution of the cross sections. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Calculated energies of the leaving Auger electron, Ek, obtained with the Eq. (1) and the respective shifted values. CHCl32+ configuration state
CHCl32+ valence orbital holes (Ef)
Ek in eV
Ek + 4.2 eV
state1 state2 state3 state4 state5 state6 state7 state8
(9a1)−2 (8a1)−1(9a1)−1 (2a2)−2 (9a1)−1(2a2)−1 (9e)−2 (7a1)−1(8a1)−1 (8a1)−1(1a2)−1 (8e)−2
172.8 168.4 173.1 173.8 173.6 158.8 168.8 171.6
177.0 172.6 177.3 178.0 177.8 163.0 173.0 175.8
have shown that the theoretical Auger electron energies of CHCl3, calculated using series of high-level ab initio quantum mechanical calculations of density functional theory, explains qualitatively the experimental results. The total ion yield spectrum exhibits a broad structure at 217 eV, above the Cl 2p ionization threshold, which can be attributed to either a shape resonance or a shake-up. The Auger spectra taken at 217 eV and 230 eV mimic each other, but the peak positions obtained at 217 eV are shifted by +1 eV with respect to the spectrum taken at 230 eV. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Fig. 4. Auger electron spectra of CHCl3 after Cl 2p photoionization by photons, 217.0 eV and 230.0 eV, respectively. The green lines represent Gaussian fittings to the observed peaks. The red lines represent the sum of all fittings. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Acknowledgments This work is supported in part by CNPq and CAPES. One of the authors, A.C.F. Santos, acknowledges support from Programa Abdias Nascimento. Research described in this paper was performed at the Canadian Light Source, which is supported by the Canada Foundation for Innovation, Natural Sciences and Engineering Research Council of Canada, the University of Saskatchewan, the Government of Saskatchewan, Western Economic Diversification Canada, the National Research Council Canada, and the Canadian Institutes of Health Research.
CHCl32+ cation on the (8a1)−1(1a2)−1 and (8a1)−1(9a1)−1 combinations. 5. Conclusions This paper describes the experimental and theoretical investigations on the Auger decay after Cl 2p−1 ionization of CHCl3 molecule. We 135
Nuclear Inst. and Methods in Physics Research B 461 (2019) 133–136
D.N. Vasconcelos, et al.
References
A.B. Rocha, V. Morcelle, N. Appathurai, L. Zuin, J. Chem. Phys. 149 (2018) 054303. [11] A.C.F. Santos, D.N. Vasconcelos, M.A. MacDonald, M.M. Sant’Anna, B.N.C. Tenório, A.B. Rocha, V. Morcelle, N. Appathurai, L. Zuin, Eur. Phys. J. D 73 (2019) 81. [12] T.J. Reddish, G. Richmond, G.W. Bagley, J.P. Wightman, S. Cvejanovic, Rev. Sci. Instrum. 68 (1997) 2685. [13] Y.F. Hu, L. Zuin, G. Wright, R. Igarashi, M. McKibben, T. Wilson, S.Y. Chen, T. Johnson, D. Maxwell, B.W. Yates, Rev. Sci. Instrum. 78 (2007) 083109. [14] B.N.C. Tenorio, M.A.C. Nascimento, S. Coriani, A.B. Rocha, J. Chem. Theory Comp. 12 (2016) 4440–4459. [15] B.N.C. Tenorio, M.A.C. Nascimento, A.B. Rocha, J. Chem. Phys. 148 (2018) 074104. [16] B.N.C. Tenorio, R.R. Oliveira, M.A.C. Nascimento, A.B. Rocha, J. Chem. Theory Comp. 14 (2018) 5324–5338. [17] K. Kaufmann, W. Baumeister, M. Jungen, J. Phys. B: At. Mol. Opt. Phys. 22 (1989) 2223–2240.
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