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Molecular single photon double K-shell ionization
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Molecular single photon double K-shell ionization F. Penent a,b,∗ , M. Nakano c,d , M. Tashiro e , T.P. Grozdanov f , M. Zˇ itnik g , S. Carniato a,b , P. Selles a,b , L. Andric a,b , P. Lablanquie a,b , J. Palaudoux a,b , E. Shigemasa e , H. Iwayama e , Y. Hikosaka h , K. Soejima h , I.H. Suzuki c , N. Kouchi d , K. Ito c a
UPMC, Université Paris 06, LCPMR, 11 rue P. et M. Curie, 75231 Paris Cedex 05, France CNRS, LCPMR (UMR 7614), 11 rue P. et M. Curie, 75231 Paris Cedex 05, France c Photon Factory, Institute of Materials Structure Science, Oho, Tsukuba 305-0801, Japan d Department of Chemistry, Tokyo Institute of Technology, O-okayama, Tokyo 152-8551, Japan e Institute for Molecular Science, Okazaki 444-8585, Japan f Institute of Physics, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia g Joˇzef Stefan Institute, P.O. Box 3000, SI-1001 Ljubljana, Slovenia h Department of Environmental Science, Niigata University, Niigata 950-2181, Japan b
a r t i c l e
i n f o
Article history: Available online xxx Keywords: Double K-shell ionization Coincidence electron spectroscopy Auger spectroscopy Chemical shift
a b s t r a c t We have studied single photon double K-shell ionization of small molecules (N2 , CO, C2 H2n (n = 1–3), . . .) and the Auger decay of the resulting double core hole (DCH) molecular ions thanks to multi-electron coincidence spectroscopy using a magnetic bottle time-of-flight spectrometer. The relative cross-sections for single-site (K−2 ) and two-site (K−1 K−1 ) double K-shell ionization with respect to single K-shell (K−1 ) ionization have been measured that gives important information on the mechanisms of single photon double ionization. The spectroscopy of two-site (K−1 K−1 ) DCH states in the C2 H2n (n = 1–3) series shows important chemical shifts due to a strong dependence on the C C bond length. In addition, the complete cascade Auger decay following single site (K−2 ) ionization has been obtained. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Atomic ions with empty K-shell were identified in pioneering experiments [1] and further observed in multicharged ions [2] and electron [3] collisions before more precise measurements using synchrotron radiation led to substantial progress [4] in the understanding of their formation and decay mechanisms. The decay of hollow (K−2 ) ions proceeds through sequential X-ray or Auger electron emission and hypersatellite X-rays or hypersatellite Auger electrons are a clear signature of K−2 double core hole (DCH) atomic ions formation [5]. Much less was known on DCH molecular ions until Cederbaum et al. [6] pointed out, in 1986, that double Kshell ionization of two neighbor atoms (K−1 K−1 ) in C2 H2n (n = 1–3) series, could be a more sensitive probe of the chemical environment, particularly of the C C bond length, than conventional K−1 inner-shell electron spectroscopy. The interest of such a theoretical prediction for molecules was only highlighted recently by experimental results obtained simultaneously with X-ray free electron lasers (XFEL) that allow two-photon core double ionization [7–11]
∗ Corresponding author at: UPMC, Université Paris 06, LCPMR, 11 rue P. et M. Curie, 75231 Paris Cedex 05, France. Tel.: +33 144276431; fax: +33 642689199. E-mail address: [email protected] (F. Penent).
and with synchrotron radiation thanks to very efficient electron coincidence spectroscopy techniques [12–15] that can reveal minor single photon multi-ionization processes. In this paper we will present a brief summary of our recent experimental results [13–16] obtained at synchrotron centers for single photon core double ionization of small molecules and on their Auger decay. 2. Experimental The experiments were performed at the undulator beamlines BL-16A of the Photon Factory (PF) and TEMPO of SOLEIL. The PF and SOLEIL synchrotrons were operated in single-bunch mode (with respective period T of 624 ns and 1184 ns) with top-up (stored electron current of 50 mA and 11 mA respectively). Two similar set-ups were used, they consist of long (2–2.5 m) magnetic bottles electron time-of-flight spectrometers. The detectors are microchannelplates (MCP) assemblies (Z-stack) that allow the detection of successive electrons with a dead time down to 6 ns. The timeof-flight of each electron with respect to the incident light pulse is converted by a multi-hit time to digital converter. The data accumulation (in list mode) and the analysis were detailed elsewhere [17]. Electron time-of-flight to energy conversion was achieved by measuring photoelectrons of known kinetic energy- after precise calibration of the monochromator by well known atomic or
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Please cite this article in press as: F. Penent, et al., Molecular single photon double K-shell ionization, J. Electron Spectrosc. Relat. Phenom. (2013), http://dx.doi.org/10.1016/j.elspec.2013.10.002
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Fig. 1. (a) Energy correlation between two photoelectrons emitted in K−2 double core ionization of N2 at h = 1110 eV, associated with two Auger electrons in the kinetic energy ranges of [375–450 eV] and [300–375 eV]. (b) Histogram of the sum of the energies of the two photoelectrons giving the spectrum of K−2 states.
molecular absorption resonances -and by known Auger transitions. The electron absolute detection efficiency with kinetic energy was determined through coincidences between photoelectrons and Auger electrons and is about 70% from 0 to 200 eV, decreasing slowly to about 40% at 800 eV. The relative energy resolution E/E is typically from 1.5% to 2%. 3. Results and discussion 3.1. K−2 double core ionization The first experimental proof of K−2 double K-shell ionization was obtained at Photon Factory on N2 at a photon energy of 1110 eV [13] and at SOLEIL on oxygen compound O2 , CO and CO2 at a photon energy of 1300 eV for K−2 ionization of the oxygen K-shell. We will only recall here some results for N2 because the molecule is isoelectronic with CO and C2 H2 for which similar results were obtained later [14,15] and present interesting similarities. The excitation and decay processes are the following (1):
deduced from the hypersatellite Auger (or X-ray) lines intensities [4]. Only coincidence measurement between two photoelectrons and hypersatellite Auger electrons can show that the contribution of K−2 satellite states is about 25% of the main K−2 line [13]. The energy correlation between the two emitted Auger electrons can be obtained for the K−2 states and its satellites [13,16] and is accurately reproduced by theoretical calculations [16] that neglect the nuclear motion in the first Auger decay step. It appears however that the nuclear motion during the second Auger decay can no longer be neglected [16] due to the very strong repulsive potential curves of the N2 3+ intermediate ionic state. In Fig. 1a the continuous energy sharing between the two photoelectrons supports a direct double ionization process that can be scaled as Z*2 to the corresponding He double photoionization [13]. The K−2 double ionization process involves shakeoff and knockout contributions [14]. The ratio K−2 /K−1 of double to single photoionization is determined accurately from the experiment knowing the coincidence count rate and the detection efficiencies for all the electrons. This ratio is in good agreement with a simple
N2 + h → N2+ (K−2 ) + 2e− 2 1,2
(KEe1 , KEe2 ),
K−2 double core ionization
(K−2 ) → N3+ (K−1 V−2 ) + e− N2+ 2 2 A1
(KEeA1 [375 − 450 eV]),
1st step Augler decay
(K−1 V−2 ) N3+ 2
(KEeA2 [300 − 375 eV]),
2nd step Augler decay
→
N4+ (V−4 ) + e− 2 A2
In our experiment we select 4-electron coincidence events with one Auger electron in the [375–450 eV] energy range (Hypersatellite Auger electron) and a second Auger electron in the [300–375 eV] energy range (2nd step Auger electron) and two photoelectrons that share the available excess energy: KEe1 + KEe2 = h – (binding energy of K−2 states). The energy correlation between the two photoelectrons is plotted in Fig. 1a and the energy spectrum of K−2 states deduced from h − (KEe1 + KEe2 ) is shown in Fig. 1b. In addition to the first peak at 902.9 eV that corresponds to the K−2 ground state, satellite peaks appear at higher binding energy and result from additional excitation of a valence electron to unoccupied orbital. The contribution of K−2 satellite states is much higher than in K−1 single ionization due to the stronger perturbation of the valence electrons by the removal of two 1s electrons [13]. Since those satellite states decay also by emitting first a hypersatellite Auger electron, one should consider carefully the contribution of these states in the K−2 cross section
theoretical model that takes into account those two processes [15]. The dominant contribution at low excess energy is the knockout process resulting from the internal collision of a first 1s electron with the other 1s electron on the same atom. The typical ratios are of the order of 10−3 and are given in Refs. [13–15]. It is also interesting to compare the K−2 spectra between isoelectronic N2 , CO and C2 H2 molecules. In Fig. 2, we can see that the spectra present rather similar satellite structures and intensity ratios showing that the same molecular orbitals are involved in the K−2 satellites. The satellite spectra for N2 and CO show the same spacing. In the simplest approach, the electrons of the molecular orbitals involved in the K−2 satellite peaks feel the same Z = 14 charge of the two nucleus N + N or C + O while the spacing scales as Z2 for C2 H2 with Z = 12 (C + C). On the other hand, the K−2 satellites in the C2 H2n series [15] are very different because the molecular orbitals are very different.
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K
-2 −1
+1
K π π∗
890
900
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910
670
650
920
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930
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670
is only thanks to the very high detection efficiency (up to ∼70% for photoelectrons below 200 eV and ∼50% for Auger electrons leading to ∼25% for 3 electrons in coincidence and ∼12% for 4 electrons) of the magnetic bottle electron spectrometer that it was possible to observe this very weak signal.
C2H2 CO N2
-2
940
700
680
950
710
690
3
700
Binding energy (eV) Fig. 2. Spectra of the K−2 satellite states in the N2 , CO and C2 H2 isoelectronic molecules showing similar features. The photon energy was 1100 eV for N2 , 954 eV for CO and 770 eV for C2 H2 and accounts for the different energy resolution due to different kinetic energy of the electron pair. The respective K−2 thresholds are 902.5 eV (N2 ), 666.7 eV (CO) and 652.5 eV (C2 H2 ) have been shifted to allow an easy comparison between the spectra.
3.2. K−1 K−1 double core ionization Single photon double K-shell ionization on two neighbor atoms could not be observed in our first experiments on N2 [13] and was only confirmed later [15]. The first evidence of this process was observed on C2 H2 . The process is the following (2):
3.3. Auger decay from K−2 states in C2 H2n As mentioned in paragraph 1, the cascade Auger decay from K−2 states was clearly observed in N2 [13] by coincidences between the two Auger electrons that fall in different energy ranges. As mentioned in the previous paragraph, it is no longer possible to detect both Auger electrons in coincidence in K−1 K−1 two-site ionization (2) because only one of them can be detected due to the detector dead time. It was however possible to extract the Auger spectra corresponding to the sum of the two successive Auger decay in process (2) for C2 H2 [14]. Due to the very long acquisition time (∼12 h) necessary to extract the K−1 K−1 signal, the statistics on the K−2 signal that is accumulated simultaneously becomes excellent and it was possible to extract with very good statistics the coincidence between the two Auger electrons in the case of C2 H2 for K−2 ionization (Fig. 1 in Ref. [16]) and also for the first satellite states corresponding to (C1s−2 −1 *+1 ) (Fig. 7 in Ref. [16]). In the later case, the spectator Auger decay is dominant but the participator Auger decay is also observed that gives faster Auger electron (∼315 eV) than the hypersatellite Auger line from K−2 . Here we compare the Auger decay from K−2 double core hole states for C2 H2 , C2 H4 and C2 H6 . In Fig. 3a we show the 2D coincidence map between the two successive Auger electrons in the K−2 decay of C2 H2n (n = 1–3). In Fig. 3b the projection of the 2D map is displayed that shows clearly the
C2 H2 + h → C2 H2 2+ (K−1 K−1 ) + 2e−
(KEe1 , KEe2 ),
K−1 K−1 double core ionization
C2 H2 2+ (K−1 K−1 ) → C2 H2 3+ (K−1 V−2 ) + e− A1
(KEeA1 [220 − 250 eV]),
1st step Augler decay
C2 H2 3+ (K−1 V−2 ) → C2 H2 4+ (V−4 ) + e− A2
(KEeA2 [220 − 250 eV]),
2nd step Augler decay
The experimental difficulty to extract the corresponding K−1 K−1 signal is due to the fact that the signal is much weaker than for K−2 (see below) and that the two Auger electrons are in the same energy range. It is hence almost impossible to detect both Auger electrons due to the detector dead time of 6 ns at least. Four-fold electron coincidences that were very efficient to extract the K−2 signal with very few random coincidences are no longer efficient to extract the K−1 K−1 signal. It was however possible to isolate a characteristic peak in three-fold coincidences between the two photoelectrons and one of the two subsequent Auger electrons. After long accumulation time (∼12 h) a peak could be extracted that was assigned unambiguously to K−1 K−1 two-site DCH formation [14]. The peak appears at a binding energy of 595.6 ± 0.5 eV very close to the calculated value of 595.86 eV [14]. In a subsequent experiment, it was also possible to observe the K−1 K−1 peaks in C2 H4 , C2 H6 , N2 and CO [15]. The corresponding binding energies are 593.3 ± 0.5 eV (C2 H4 ), 590.2 ± 0.5 eV(C2 H6 ), 835.9 ± 1 eV (N2 ) and 855.4 ± 1 eV (CO) [15]. These experimental values are in good agreement with theoretical calculations [15,18] and confirm Cederbaum et al.’ prediction [6] on the chemical shift dependence on the bond length in the C2 H2n (n = 1–3) series that involves both Coulomb interaction between the two 1s holes and relaxation energies. The cross-section for K−1 K−1 formation decreases as 1/R2 with the C C bond length [15]. That is fully compatible with a knockout process where a 1s electron ionized on the first carbon atom collides and ionizes another 1s electron on the neighbor atom. The ratio of the K−1 K−1 /K−2 ionization cross sections is typically of the order of 1–2% and the ratio of K−1 K−1 /K−1 ionization cross sections is hence about 10−5 [14,15]. It
energy difference between the hypersatellite Auger electron and the second Auger electron. A detailed theoretical analysis for C2 H2 was given in Ref. [16] and compared with the experimental results but similar calculations remain to be done for C2 H4 and C2 H6 to compare with the present experimental results. The hypersatellite Auger lines are clearly different for the different molecules, while two peaks (at 300 and 285 eV) are clearly resolved (with the limited energy resolution of about 5 eV of the magnetic bottle spectrometer) for C2 H2 , the hypersatellite Auger spectra of C2 H4 and C2 H6 do not show well isolated peaks. A striking similarity between the K−2 hypersatellite Auger spectra and the normal Auger spectra following single K-shell ionization (K−1 ) for the different molecules can be observed in Fig. 3b. The Auger matrix element which results from Coulomb interaction between molecular valence electrons is basically the same. The difference comes from the different contraction (relaxation) of molecular orbitals following K−1 or K−2 ionization. The difference in Auger energies results also from the fact that one electron falls on a deeper potential when two electrons have been removed from the K-shell than when only one has been removed, hence the energy of Auger electron is higher. Using a simple Slater shielding constants [19] one could account for the energy difference between hypersatellite Auger electrons (K−2 ionization) and ‘normal’ Auger electrons (K−1 ionization). Conversely the second Auger electron that corresponds to the filling of the remaining Kshell vacancy does not show any strong similarity with the K−1 Auger spectrum although it is found in a close energy range. The intermediate C2 H2n 3+ (K−1 V−2 ) and final C2 H2n 4+ (V−4 ) molecular curves must be calculated to understand the Auger spectrum for
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a)
270
Slow Auger electron energy (eV)
4
260
C2H2
250 240 230
-1
K Auger electron energy (eV)
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140
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210
b)
200 260 280 300 320 Fast Auger electron energy (eV)
Slow Auger electron energy (eV)
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C2H4 C2H2
250 240 230 220 210 200
C2H4
260 280 300 320 Fast Auger electron energy (eV)
Slow Auger electron energy (eV)
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C2H6
250 240 230
C2H6
220 210
180
200
200
260 280 300 320 Fast Auger electron energy (eV)
220
240
260
280
300
320
-2
K Auger electron energy (eV)
Fig. 3. (a) 2D coincidence maps between the two Auger electrons following K−2 double K-shell ionization for C2 H2n (n = 1–3) molecules. (b) Solid lines: projections of the 2D maps showing the Auger spectra of the K−2 double core-hole state (energy bottom axis). Dashed lines: Auger spectra after single K-shell ionization plotted on the same figure and shifted (energy top axis) to compare with the hypersatellite Auger line corresponding to the first Auger decay from K−2 state.
the second electron. The differences for the second Auger decay between the different molecules appear to be smeared out, which may be the effect of nuclear motion [16]. 3.4. Below K−2 threshold We have pursued the analysis of our experimental data to look at the states converging to K−2 double ionization threshold [20]. Those states were observed in ref [12] for CH4 and NH3 .These states correspond to ionization of a 1s electron into the continuum and simultaneous excitation of the other 1s electron to a vacant orbital V and correspond to a singly charged ion with two K-shell vacancies: C2 H2n + (K−2 V). In this case, only one photoelectron is emitted but two Auger electrons are ejected due to the presence of two K-shell vacancies. The spectra can be reconstructed by plotting the energy of one photoelectron detected in coincidence with two Auger electrons that are in the same energy range as those following K−2 double ionization. The structures that appear below K−2 threshold have been assigned [20]: they correspond to direct and conjugate shake-up lines resulting from dipolar ionization (1s → p continuum) followed by monopole excitation (1s → V) (shake-up) or to dipolar excitation (1s → V) followed by monopole ionization (1s → continuum) (shake-off). These two processes are visible with
comparable intensities. Due to the different selection rules (dipole and monopole transitions) and to the different molecular orbitals in the C2 H2n series, the spectra are very different: in particular for ethylene and acetylene the LUMO * orbital is excited in a dipolar transition from 1s while the corresponding peak is missing for methane. 4. Conclusions With highly efficient multicoincidence electron spectroscopy, we have been able to reveal double core ionization processes resulting from electron correlation and to propose simple mechanisms that account for the relative cross-sections between different processes. The energy spectra of DCH states have been measured experimentally with high accuracy and the Auger decay of such states has been obtained. We have been able to confirm clearly the seminal proposal of Cederbaum et al. [6] on the two-site double K-shell (K−1 K−1 ) ionization for chemical analysis. Although innershell multiple ionization mechanisms are different when obtained with synchrotron or with XFEL (there, a few photons from the same light pulse can be absorbed by a molecule in a time comparable to the Auger decay time) light sources, these experiments provide complementary information on multiple inner-shell ionization and
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decay. With XFEL, count rates are too high to allow coincidence measurements, however the recent development of partial covariance mapping analysis of the XFEL data [21] offers a promising approach for further studies. Acknowledgements We are grateful to the PF and SOLEIL staff for the stable operation of the synchrotron in top-up mode that allows long time accumulation of the data. These works have been performed with the approval of PF and SOLEIL advisory committee from 2009 to 2011. References [1] G. Charpak, C.R. Hebd, Seances Acad. Sci. 237 (1953) 243. [2] P. Richard, W. Hodge, C. Fred Moore, Phys. Rev. Lett. 29 (1972) 393. [3] Briand F P., A. Touati, M. Frilley, P. Chevallier, A. Johnson, J.P. Rozet, M. Tavernier, S. Shafroth, M.O. Krause, J. Phys. B 9 (1976) 1055.
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[4] J. Hoszowska, et al., Phys. Rev. Lett. 102 (2009) 073006. [5] J.P. Briand, P. Chevallier, M. Tavernier, J.P. Rozet, Phys. Rev. Lett. 27 (1971) 777. [6] L.S. Cederbaum, F. Tarantelli, A. Sgamellotti, J. Schirmer, J. Chem. Phys. 85 (1986) 6513. [7] L. Fang, et al., Phys. Rev. Lett. 105 (2010) 083005. [8] N. Berrah, et al., Proc. Natl. Acad. Sci. U.S.A. 108 (2011) 16912. [9] P. Salén, et al., Phys. Rev. Lett. 108 (2012) 153003. [10] S. Santra, N.V. Kryzhevoi, L.S. Cederbaum, Phys. Rev. Lett. 103 (2009) 013002. [11] M. Tashiro, M. Ehara, K. Ueda, Chem. Phys. Lett. 496 (2010) 217. [12] J.H.D. Eland, M. Tashiro, P. Linusson, M. Ehara, K. Ueda, R. Feifel, Phys. Rev. Lett. 105 (2010) 213005. [13] P. Lablanquie, et al., Phys. Rev. Lett. 106 (2011) 063003. [14] P. Lablanquie, et al., Phys. Rev. Lett. 107 (2011) 193004. [15] M. Nakano, et al., Phys. Rev. Lett. 110 (2013) 163001. [16] M. Tashiro, et al., J. Chem. Phys. 137 (2012) 224306. [17] K. Ito, F. Penent, Y. Hikosaka, E. Shigemasa, I.H. Suzuki, J.H.D. Eland, P. Lablanquie, Rev. Sci. Instrum. 80 (2009) 123101. [18] M. Tashiro, M. Ehara, H. Fukuzawa, K. Ueda, C. Buth, N.V. Kryzhevoi, L.S. Cederbaum, J. Chem. Phys. 132 (2010) 184302. [19] J.C. Slater, Phys. Rev. 36 (1930) 57. [20] M. Nakano, et al., Phys. Rev. Lett. 111 (2013) 123001. [21] L.J. Frasinski, et al., Phys. Rev. Lett. 111 (2013) 073002.
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Molecular single photon double K-shell ionization F. Penent a,b,∗ , M. Nakano c,d , M. Tashiro e , T.P. Grozdanov f , M. Zˇ itnik g , S. Carniato a,b , P. Selles a,b , L. Andric a,b , P. Lablanquie a,b , J. Palaudoux a,b , E. Shigemasa e , H. Iwayama e , Y. Hikosaka h , K. Soejima h , I.H. Suzuki c , N. Kouchi d , K. Ito c a
UPMC, Université Paris 06, LCPMR, 11 rue P. et M. Curie, 75231 Paris Cedex 05, France CNRS, LCPMR (UMR 7614), 11 rue P. et M. Curie, 75231 Paris Cedex 05, France c Photon Factory, Institute of Materials Structure Science, Oho, Tsukuba 305-0801, Japan d Department of Chemistry, Tokyo Institute of Technology, O-okayama, Tokyo 152-8551, Japan e Institute for Molecular Science, Okazaki 444-8585, Japan f Institute of Physics, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia g Joˇzef Stefan Institute, P.O. Box 3000, SI-1001 Ljubljana, Slovenia h Department of Environmental Science, Niigata University, Niigata 950-2181, Japan b
a r t i c l e
i n f o
Article history: Available online xxx Keywords: Double K-shell ionization Coincidence electron spectroscopy Auger spectroscopy Chemical shift
a b s t r a c t We have studied single photon double K-shell ionization of small molecules (N2 , CO, C2 H2n (n = 1–3), . . .) and the Auger decay of the resulting double core hole (DCH) molecular ions thanks to multi-electron coincidence spectroscopy using a magnetic bottle time-of-flight spectrometer. The relative cross-sections for single-site (K−2 ) and two-site (K−1 K−1 ) double K-shell ionization with respect to single K-shell (K−1 ) ionization have been measured that gives important information on the mechanisms of single photon double ionization. The spectroscopy of two-site (K−1 K−1 ) DCH states in the C2 H2n (n = 1–3) series shows important chemical shifts due to a strong dependence on the C C bond length. In addition, the complete cascade Auger decay following single site (K−2 ) ionization has been obtained. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Atomic ions with empty K-shell were identified in pioneering experiments [1] and further observed in multicharged ions [2] and electron [3] collisions before more precise measurements using synchrotron radiation led to substantial progress [4] in the understanding of their formation and decay mechanisms. The decay of hollow (K−2 ) ions proceeds through sequential X-ray or Auger electron emission and hypersatellite X-rays or hypersatellite Auger electrons are a clear signature of K−2 double core hole (DCH) atomic ions formation [5]. Much less was known on DCH molecular ions until Cederbaum et al. [6] pointed out, in 1986, that double Kshell ionization of two neighbor atoms (K−1 K−1 ) in C2 H2n (n = 1–3) series, could be a more sensitive probe of the chemical environment, particularly of the C C bond length, than conventional K−1 inner-shell electron spectroscopy. The interest of such a theoretical prediction for molecules was only highlighted recently by experimental results obtained simultaneously with X-ray free electron lasers (XFEL) that allow two-photon core double ionization [7–11]
∗ Corresponding author at: UPMC, Université Paris 06, LCPMR, 11 rue P. et M. Curie, 75231 Paris Cedex 05, France. Tel.: +33 144276431; fax: +33 642689199. E-mail address: [email protected] (F. Penent).
and with synchrotron radiation thanks to very efficient electron coincidence spectroscopy techniques [12–15] that can reveal minor single photon multi-ionization processes. In this paper we will present a brief summary of our recent experimental results [13–16] obtained at synchrotron centers for single photon core double ionization of small molecules and on their Auger decay. 2. Experimental The experiments were performed at the undulator beamlines BL-16A of the Photon Factory (PF) and TEMPO of SOLEIL. The PF and SOLEIL synchrotrons were operated in single-bunch mode (with respective period T of 624 ns and 1184 ns) with top-up (stored electron current of 50 mA and 11 mA respectively). Two similar set-ups were used, they consist of long (2–2.5 m) magnetic bottles electron time-of-flight spectrometers. The detectors are microchannelplates (MCP) assemblies (Z-stack) that allow the detection of successive electrons with a dead time down to 6 ns. The timeof-flight of each electron with respect to the incident light pulse is converted by a multi-hit time to digital converter. The data accumulation (in list mode) and the analysis were detailed elsewhere [17]. Electron time-of-flight to energy conversion was achieved by measuring photoelectrons of known kinetic energy- after precise calibration of the monochromator by well known atomic or
0368-2048/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elspec.2013.10.002
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2
Fig. 1. (a) Energy correlation between two photoelectrons emitted in K−2 double core ionization of N2 at h = 1110 eV, associated with two Auger electrons in the kinetic energy ranges of [375–450 eV] and [300–375 eV]. (b) Histogram of the sum of the energies of the two photoelectrons giving the spectrum of K−2 states.
molecular absorption resonances -and by known Auger transitions. The electron absolute detection efficiency with kinetic energy was determined through coincidences between photoelectrons and Auger electrons and is about 70% from 0 to 200 eV, decreasing slowly to about 40% at 800 eV. The relative energy resolution E/E is typically from 1.5% to 2%. 3. Results and discussion 3.1. K−2 double core ionization The first experimental proof of K−2 double K-shell ionization was obtained at Photon Factory on N2 at a photon energy of 1110 eV [13] and at SOLEIL on oxygen compound O2 , CO and CO2 at a photon energy of 1300 eV for K−2 ionization of the oxygen K-shell. We will only recall here some results for N2 because the molecule is isoelectronic with CO and C2 H2 for which similar results were obtained later [14,15] and present interesting similarities. The excitation and decay processes are the following (1):
deduced from the hypersatellite Auger (or X-ray) lines intensities [4]. Only coincidence measurement between two photoelectrons and hypersatellite Auger electrons can show that the contribution of K−2 satellite states is about 25% of the main K−2 line [13]. The energy correlation between the two emitted Auger electrons can be obtained for the K−2 states and its satellites [13,16] and is accurately reproduced by theoretical calculations [16] that neglect the nuclear motion in the first Auger decay step. It appears however that the nuclear motion during the second Auger decay can no longer be neglected [16] due to the very strong repulsive potential curves of the N2 3+ intermediate ionic state. In Fig. 1a the continuous energy sharing between the two photoelectrons supports a direct double ionization process that can be scaled as Z*2 to the corresponding He double photoionization [13]. The K−2 double ionization process involves shakeoff and knockout contributions [14]. The ratio K−2 /K−1 of double to single photoionization is determined accurately from the experiment knowing the coincidence count rate and the detection efficiencies for all the electrons. This ratio is in good agreement with a simple
N2 + h → N2+ (K−2 ) + 2e− 2 1,2
(KEe1 , KEe2 ),
K−2 double core ionization
(K−2 ) → N3+ (K−1 V−2 ) + e− N2+ 2 2 A1
(KEeA1 [375 − 450 eV]),
1st step Augler decay
(K−1 V−2 ) N3+ 2
(KEeA2 [300 − 375 eV]),
2nd step Augler decay
→
N4+ (V−4 ) + e− 2 A2
In our experiment we select 4-electron coincidence events with one Auger electron in the [375–450 eV] energy range (Hypersatellite Auger electron) and a second Auger electron in the [300–375 eV] energy range (2nd step Auger electron) and two photoelectrons that share the available excess energy: KEe1 + KEe2 = h – (binding energy of K−2 states). The energy correlation between the two photoelectrons is plotted in Fig. 1a and the energy spectrum of K−2 states deduced from h − (KEe1 + KEe2 ) is shown in Fig. 1b. In addition to the first peak at 902.9 eV that corresponds to the K−2 ground state, satellite peaks appear at higher binding energy and result from additional excitation of a valence electron to unoccupied orbital. The contribution of K−2 satellite states is much higher than in K−1 single ionization due to the stronger perturbation of the valence electrons by the removal of two 1s electrons [13]. Since those satellite states decay also by emitting first a hypersatellite Auger electron, one should consider carefully the contribution of these states in the K−2 cross section
theoretical model that takes into account those two processes [15]. The dominant contribution at low excess energy is the knockout process resulting from the internal collision of a first 1s electron with the other 1s electron on the same atom. The typical ratios are of the order of 10−3 and are given in Refs. [13–15]. It is also interesting to compare the K−2 spectra between isoelectronic N2 , CO and C2 H2 molecules. In Fig. 2, we can see that the spectra present rather similar satellite structures and intensity ratios showing that the same molecular orbitals are involved in the K−2 satellites. The satellite spectra for N2 and CO show the same spacing. In the simplest approach, the electrons of the molecular orbitals involved in the K−2 satellite peaks feel the same Z = 14 charge of the two nucleus N + N or C + O while the spacing scales as Z2 for C2 H2 with Z = 12 (C + C). On the other hand, the K−2 satellites in the C2 H2n series [15] are very different because the molecular orbitals are very different.
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K
-2 −1
+1
K π π∗
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900
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is only thanks to the very high detection efficiency (up to ∼70% for photoelectrons below 200 eV and ∼50% for Auger electrons leading to ∼25% for 3 electrons in coincidence and ∼12% for 4 electrons) of the magnetic bottle electron spectrometer that it was possible to observe this very weak signal.
C2H2 CO N2
-2
940
700
680
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710
690
3
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Binding energy (eV) Fig. 2. Spectra of the K−2 satellite states in the N2 , CO and C2 H2 isoelectronic molecules showing similar features. The photon energy was 1100 eV for N2 , 954 eV for CO and 770 eV for C2 H2 and accounts for the different energy resolution due to different kinetic energy of the electron pair. The respective K−2 thresholds are 902.5 eV (N2 ), 666.7 eV (CO) and 652.5 eV (C2 H2 ) have been shifted to allow an easy comparison between the spectra.
3.2. K−1 K−1 double core ionization Single photon double K-shell ionization on two neighbor atoms could not be observed in our first experiments on N2 [13] and was only confirmed later [15]. The first evidence of this process was observed on C2 H2 . The process is the following (2):
3.3. Auger decay from K−2 states in C2 H2n As mentioned in paragraph 1, the cascade Auger decay from K−2 states was clearly observed in N2 [13] by coincidences between the two Auger electrons that fall in different energy ranges. As mentioned in the previous paragraph, it is no longer possible to detect both Auger electrons in coincidence in K−1 K−1 two-site ionization (2) because only one of them can be detected due to the detector dead time. It was however possible to extract the Auger spectra corresponding to the sum of the two successive Auger decay in process (2) for C2 H2 [14]. Due to the very long acquisition time (∼12 h) necessary to extract the K−1 K−1 signal, the statistics on the K−2 signal that is accumulated simultaneously becomes excellent and it was possible to extract with very good statistics the coincidence between the two Auger electrons in the case of C2 H2 for K−2 ionization (Fig. 1 in Ref. [16]) and also for the first satellite states corresponding to (C1s−2 −1 *+1 ) (Fig. 7 in Ref. [16]). In the later case, the spectator Auger decay is dominant but the participator Auger decay is also observed that gives faster Auger electron (∼315 eV) than the hypersatellite Auger line from K−2 . Here we compare the Auger decay from K−2 double core hole states for C2 H2 , C2 H4 and C2 H6 . In Fig. 3a we show the 2D coincidence map between the two successive Auger electrons in the K−2 decay of C2 H2n (n = 1–3). In Fig. 3b the projection of the 2D map is displayed that shows clearly the
C2 H2 + h → C2 H2 2+ (K−1 K−1 ) + 2e−
(KEe1 , KEe2 ),
K−1 K−1 double core ionization
C2 H2 2+ (K−1 K−1 ) → C2 H2 3+ (K−1 V−2 ) + e− A1
(KEeA1 [220 − 250 eV]),
1st step Augler decay
C2 H2 3+ (K−1 V−2 ) → C2 H2 4+ (V−4 ) + e− A2
(KEeA2 [220 − 250 eV]),
2nd step Augler decay
The experimental difficulty to extract the corresponding K−1 K−1 signal is due to the fact that the signal is much weaker than for K−2 (see below) and that the two Auger electrons are in the same energy range. It is hence almost impossible to detect both Auger electrons due to the detector dead time of 6 ns at least. Four-fold electron coincidences that were very efficient to extract the K−2 signal with very few random coincidences are no longer efficient to extract the K−1 K−1 signal. It was however possible to isolate a characteristic peak in three-fold coincidences between the two photoelectrons and one of the two subsequent Auger electrons. After long accumulation time (∼12 h) a peak could be extracted that was assigned unambiguously to K−1 K−1 two-site DCH formation [14]. The peak appears at a binding energy of 595.6 ± 0.5 eV very close to the calculated value of 595.86 eV [14]. In a subsequent experiment, it was also possible to observe the K−1 K−1 peaks in C2 H4 , C2 H6 , N2 and CO [15]. The corresponding binding energies are 593.3 ± 0.5 eV (C2 H4 ), 590.2 ± 0.5 eV(C2 H6 ), 835.9 ± 1 eV (N2 ) and 855.4 ± 1 eV (CO) [15]. These experimental values are in good agreement with theoretical calculations [15,18] and confirm Cederbaum et al.’ prediction [6] on the chemical shift dependence on the bond length in the C2 H2n (n = 1–3) series that involves both Coulomb interaction between the two 1s holes and relaxation energies. The cross-section for K−1 K−1 formation decreases as 1/R2 with the C C bond length [15]. That is fully compatible with a knockout process where a 1s electron ionized on the first carbon atom collides and ionizes another 1s electron on the neighbor atom. The ratio of the K−1 K−1 /K−2 ionization cross sections is typically of the order of 1–2% and the ratio of K−1 K−1 /K−1 ionization cross sections is hence about 10−5 [14,15]. It
energy difference between the hypersatellite Auger electron and the second Auger electron. A detailed theoretical analysis for C2 H2 was given in Ref. [16] and compared with the experimental results but similar calculations remain to be done for C2 H4 and C2 H6 to compare with the present experimental results. The hypersatellite Auger lines are clearly different for the different molecules, while two peaks (at 300 and 285 eV) are clearly resolved (with the limited energy resolution of about 5 eV of the magnetic bottle spectrometer) for C2 H2 , the hypersatellite Auger spectra of C2 H4 and C2 H6 do not show well isolated peaks. A striking similarity between the K−2 hypersatellite Auger spectra and the normal Auger spectra following single K-shell ionization (K−1 ) for the different molecules can be observed in Fig. 3b. The Auger matrix element which results from Coulomb interaction between molecular valence electrons is basically the same. The difference comes from the different contraction (relaxation) of molecular orbitals following K−1 or K−2 ionization. The difference in Auger energies results also from the fact that one electron falls on a deeper potential when two electrons have been removed from the K-shell than when only one has been removed, hence the energy of Auger electron is higher. Using a simple Slater shielding constants [19] one could account for the energy difference between hypersatellite Auger electrons (K−2 ionization) and ‘normal’ Auger electrons (K−1 ionization). Conversely the second Auger electron that corresponds to the filling of the remaining Kshell vacancy does not show any strong similarity with the K−1 Auger spectrum although it is found in a close energy range. The intermediate C2 H2n 3+ (K−1 V−2 ) and final C2 H2n 4+ (V−4 ) molecular curves must be calculated to understand the Auger spectrum for
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a)
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Slow Auger electron energy (eV)
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C2H2
250 240 230
-1
K Auger electron energy (eV)
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C2H4 C2H2
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C2H4
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Slow Auger electron energy (eV)
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C2H6
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C2H6
220 210
180
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260 280 300 320 Fast Auger electron energy (eV)
220
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-2
K Auger electron energy (eV)
Fig. 3. (a) 2D coincidence maps between the two Auger electrons following K−2 double K-shell ionization for C2 H2n (n = 1–3) molecules. (b) Solid lines: projections of the 2D maps showing the Auger spectra of the K−2 double core-hole state (energy bottom axis). Dashed lines: Auger spectra after single K-shell ionization plotted on the same figure and shifted (energy top axis) to compare with the hypersatellite Auger line corresponding to the first Auger decay from K−2 state.
the second electron. The differences for the second Auger decay between the different molecules appear to be smeared out, which may be the effect of nuclear motion [16]. 3.4. Below K−2 threshold We have pursued the analysis of our experimental data to look at the states converging to K−2 double ionization threshold [20]. Those states were observed in ref [12] for CH4 and NH3 .These states correspond to ionization of a 1s electron into the continuum and simultaneous excitation of the other 1s electron to a vacant orbital V and correspond to a singly charged ion with two K-shell vacancies: C2 H2n + (K−2 V). In this case, only one photoelectron is emitted but two Auger electrons are ejected due to the presence of two K-shell vacancies. The spectra can be reconstructed by plotting the energy of one photoelectron detected in coincidence with two Auger electrons that are in the same energy range as those following K−2 double ionization. The structures that appear below K−2 threshold have been assigned [20]: they correspond to direct and conjugate shake-up lines resulting from dipolar ionization (1s → p continuum) followed by monopole excitation (1s → V) (shake-up) or to dipolar excitation (1s → V) followed by monopole ionization (1s → continuum) (shake-off). These two processes are visible with
comparable intensities. Due to the different selection rules (dipole and monopole transitions) and to the different molecular orbitals in the C2 H2n series, the spectra are very different: in particular for ethylene and acetylene the LUMO * orbital is excited in a dipolar transition from 1s while the corresponding peak is missing for methane. 4. Conclusions With highly efficient multicoincidence electron spectroscopy, we have been able to reveal double core ionization processes resulting from electron correlation and to propose simple mechanisms that account for the relative cross-sections between different processes. The energy spectra of DCH states have been measured experimentally with high accuracy and the Auger decay of such states has been obtained. We have been able to confirm clearly the seminal proposal of Cederbaum et al. [6] on the two-site double K-shell (K−1 K−1 ) ionization for chemical analysis. Although innershell multiple ionization mechanisms are different when obtained with synchrotron or with XFEL (there, a few photons from the same light pulse can be absorbed by a molecule in a time comparable to the Auger decay time) light sources, these experiments provide complementary information on multiple inner-shell ionization and
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