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Evidence of radiative charge transfer in argon dimers
Chemical Physics Letters 441 (2007) 16–19 www.elsevier.com/locate/cplett
Evidence of radiative charge transfer in argon dimers N. Saito a, Y. Morishita a, I.H. Suzuki a,b, S.D. Stoychev c, A.I. Kuleff c, L.S. Cederbaum c, X.-J. Liu d, H. Fukuzawa d, G. Pru¨mper d, K. Ueda d,* a
National Institute of Advanced Industrial Science and Technology (AIST), NMIJ, Tsukuba 305-8568, Japan b Photon Factory, Institute of Materials Structure Science, KEK, Oho 1-1, Tsukuba, 305-0801, Japan c Theoretische Chemie, Physikalisch-Chemisches Institut, Universita¨t Heidelberg, D-69120 Heidelberg, Germany d Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Received 13 February 2007; in final form 23 April 2007 Available online 27 April 2007
Abstract Auger electron spectra recorded in coincidence with two Ar+ ions produced from Ar2þ 2 suggest that the bound one-site two-hole state, Ar (3p2)–Ar, decays further only via radiative charge transfer to the dissociative two-site two-hole states Ar+(3p1)–Ar+(3p1). The measured kinetic energy release of Ar2þ 2 agrees well with the theoretical estimate based on this process. Ó 2007 Elsevier B.V. All rights reserved. 2+
1. Introduction Inner-shell ionization of molecules leads to ion formation with an energy well above the double ionization thresholds. Then the inner-shell ionized states can decay by electron emission. This process is known as the Auger process [1]. The Auger final states are dicationic ‘two-hole’ states. In case of diatomic molecules such as N2, these twohole states dissociate into N+ and N+ due to Coulomb explosion independently of whether the two holes are created on one atom (i.e., N2+–N), or on two atoms (i.e., N+–N+). The situation is significantly different for van der Waals rare gas dimers such as Ar2, where the internuclear distance is significantly larger [2], say, three times larger than in N2. The Auger spectra are generally considered as ‘self imaging’, i.e., fingerprinting images of the atoms where the inner-shell hole is created (see for example [3] and references therein). If we apply this self-imaging picture to Ar2 after 2p ionization, the ‘atomic’ Ar LMM Auger takes place, creating two holes in one Ar site. In Ar2, two holes
*
Corresponding author. Fax: +81 22 217 5380. E-mail address: [email protected] (K. Ueda).
0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.04.077
can be located on a single Ar site and such dicationic states are not necessarily dissociative [4]. Previously, De Fanis et al. [5] and Morishita et al. [6] observed the Ar+–Ar+ formation using electron-ion-ion coincidence, to a certain branching ratio, after 2p ionization of Ar2 ½Ar–Ar þ hm ! ½Arþ ð2p1 Þ–Ar þ e photo ! Arþ þ Arþ þ e þ e photo : Ueda et al. [7] suggested three possible processes leading to the Ar+–Ar+ dissociation: 8 2þ 1 1 > > > ½Ar ð3s 3p Þ–Ar þ e > þ þ > 2 > ! Ar ð3p n‘Þ þ Ar þ e ðaÞ > > > < ½Ar2þ ð3p2 Þ–Ar þ e ½Arþ ð2p1 Þ–Ar ! > ! Arþ ð3p1 Þ þ Arþ ð3p1 Þ þ e þ hm ðbÞ > > > > > ½Arþ ð3p1 Þ–Arþ ð3p1 Þ þ e > > > : ! Arþ ð3p1 Þ þ Arþ ð3p1 Þ þ e ðcÞ In pathway (a) the one-site states Ar2+(3s13p1), in which 3s and 3p holes are located on one Ar site after L23M1M23 Auger decay, couple with the two-site satellite states Ar+(3p2n‘)-Ar+(3p1) with n‘ such as 3d, 4s, 4p, 4d, 5s,
N. Saito et al. / Chemical Physics Letters 441 (2007) 16–19
The experiment was the same as the one previously reported [6]. Thus only a brief account is repeated here. The experiment has been carried out on the c branch of the soft X-ray photochemistry beamline 27SU [9,10] at SPring-8. The storage ring was operated in the 35 singlebunches + 6/42 filling mode, which provides a single-bunch separation of 114.0 ns. Argon dimers were produced by expanding argon gas at a stagnation pressure of 2 bar cooled to a temperature of 170 K through a pinhole of 30 lm diameter and 0.25 mm thickness. The coincidence measurements described below were performed at a photon energy of 257.05 eV, i.e. 8.4 and 6.3 eV above the atomic Ar 2p1 2P3/2 and 2p1 2P1/2 ionization thresholds [11]. Our electron–ion–ion coincidence momentum imaging is based on recording the electron and ion times-of-flight (TOFs) with multi-hit two-dimensional position sensitive detectors [12–14]. Knowledge of position and arrival time on the particle detectors, (x, y, t), allows us to extract information about the linear momentum (px, py, pz) for each particle. The two TOF spectrometers are placed face to face. In the present setting of the electric and magnetic field conditions, all the electrons up to 12 eV in kinetic energy and all the Ar+ ions up to 10 eV kinetic energy, both ejected in 4p sr, were accelerated onto the MCP detectors, whereas the energetic Auger electrons with 200 eV only ejected towards the detector in 4p/20 sr could be detected.
3.1. Auger electron spectra We recorded coincidence events between one electron and two ions, as well as those between one electron and one ion, in a list mode. In Fig. 1, the Auger spectrum of the Ar2 recorded in coincidence with the Ar+–Ar+ ion pairs is compared with the Auger spectrum of the Ar atoms recorded in coincidence with the Ar2+ ions. The energy resolution is only 20 eV. Counts of the Ar dimer Auger spectrum is much lower than those of the monomer Auger spectrum. Within the statistical uncertainties, however, these two Auger spectra coincide. In the figure, the energies of the atomic Auger transitions to 3s13p1 (L23M1M23 at 190 eV) and to 3p2 (L23M23M23 at 205 eV) are indicated by vertical dashed and solid lines, respectively [15,16]. It is clear that Ar+–Ar+ ion pair can be produced not only from the one-site Ar2+(3s13p1)–Ar states but also from the one-site Ar2+(3p2)–Ar states and that the latter is predominant. 3.2. Kinetic energy release The KERs in the fragmentation into Ar+–Ar+ are shown in Fig. 2. We plot the KERs coincident with photoelectrons (open circles) and with Auger electrons (closed symbols) selected with the three regions in Fig. 1. The profiles of the KERs do not depend on the Auger electron energies. The KER distribution is peaked at 5.3 eV and exhibits a longer tail in the low energy side. The corresponding internuclear distance estimated from the two-hole ˚ , is significantly shorter than Coulomb repulsion, 2.7 A Auger3 Auger2 Auger1
L23M1M23
L23M23M23
3000
40
Ar + - Ar +
Ar 2+
2000
20 1000
0 160
200
Counts for Ar 2+
2. Experiment
3. Results and discussion
Counts for Ar + - Ar +
5p and thus may dissociate to Ar+ and Ar+. Pathway (b) represents a radiative charge transfer [8], i.e., the radiative decay from the one-site Ar2+(3p2)–Ar states, populated by the atomic L23M23M23 Auger decay, to two-site Ar+(3p1)–Ar+(3p1) ion-pair states. Besides the selfimaging ‘atomic’ Auger decay, ‘interatomic’ Auger decay, i.e., the core-hole decay in which an electron in the neighboring atom participates, can also take place and then the two holes are created directly at the two different Ar sites – the pathway (c). With the limited knowledge experimentally obtained, Ueda et al. speculated that the potential coupling mechanism (a) may be most likely [7]. The decisive conclusion on the prominent channel to the Ar+–Ar+ ion-pair formation can, however, be extracted from the Auger-electron–ion–ion coincidence hidden in the previous data set [6]. We thus have reanalyzed the data. The Auger electron energy spectra recorded in coincidence with two Ar+ ions indicates that the single-site two-hole state Ar2+(3p2)–Ar that is supposed to be bound can produce the Ar+–Ar+ ion-pair, suggesting that these bound states further decay via radiative charge transfer to the dissociative two-site two-hole states Ar+(3p1)–Ar+(3p1), i.e., process (b). Measured kinetic energy release (KER) gives reasonable agreement with the predictions based on ab initio adiabatic potential energy curves of one-site and two-site states in Ar2 and the radiative charge transfer (b) between them.
17
0 240
Electron Energy (eV) Fig. 1. (color online) The Auger spectra of Ar2 dimers recorded in coincidence with the Ar+-Ar+ ion pairs and of the Ar atoms recorded in coincidence with the Ar2+ ions. Vertical solid and dashed lines are the energies of the L23M23M23 and L23M1M23 atomic Auger transitions, respectively. The kinetic energy release is analyzed in coincidence with the electrons with the kinetic energies in the region of the labels of Auger1, Auger2 and Auger3 above the figure. The scales for the counts were adjusted so that the areas of the counts are the same in the figure.
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N. Saito et al. / Chemical Physics Letters 441 (2007) 16–19
50
45
E (eV)
1000
Counts
1
All Auger1(x21) Auger2(x25) AUger3(x42)
500
3
4 5 Kinetic Energy Release (eV)
6
Fig. 2. (color online) The total kinetic energy release (KER) in the Ar2þ 2 fragmentation into the two Ar+ ions. Open circles show the KER coincident with photoelectrons; closed squares, circles, and triangles denote the KERs coincident with Auger electrons selected in the regions of Auger1, Auger2, and Auger3, respectively. The scales for the counts were adjusted so that the areas of the counts are the same in the figure.
the equilibrium bond length in the neutral ground state ˚ ). It implies that nuclear motion (bond shortening) (3.8 A takes place in the final state of the Auger decay, Ar2+(3p2)–Ar, before a radiative transition to the two-site states Ar+(3p1)–Ar+(3p1) occurs. The Auger states are thus only intermediate states in this pathway to the Ar+ + Ar+ ion production. Let us briefly discuss the relation between the coincident count ratio of Ar+–Ar2+ to Ar+–Ar+ and the Auger transition branching ratios. The coincident count ratio of Ar+– Ar2+ to Ar+–Ar+ is 0.12 after the detection efficiency correction. On the other hand, Auger transition branching ratios are 0.092 for the Auger electron energies of 177– 180 eV (3p33d), 0.139 for 187–193 eV (3s13p1), and 0.727 for 201–208 eV (3p2) [16]. Our previous Letter [6] suggests that Ar2 in the 3p33d state decays into Ar+– Ar2+. The present Auger-electron–ion coincident results suggest that Ar2 in the 3s13p1 and 3p2 states decays into Ar+–Ar+. Then the Auger transition ratio of 3p33d to 3s13p1 and 3p2 is 0.11, which is very close to the coincident count ratio of Ar+–Ar2+ to Ar+–Ar+. 3.3. Radiative charge transfer We have performed ab initio calculations to obtain the potential energies of Ar2. The calculations were done by use of the formalism of Green’s functions in the so-called algebraic diagrammatic construction (ADC) scheme, in particular the ADC(2) for the particle–particle propagator [17] as implemented by Tarantelli [18]. The resulting double ionization potentials are added to the potential energy curve (PEC) of the ground state of Ar2 calculated via the CCSD(T) method. A full account for the calculations will be given elsewhere [19]. It should be noted that all computed ionic state PECs displayed in Fig. 3 have been shifted such that they match the experimental ionic energies at large R.
Ar2+(3p-2 1D)-Ar
Σ-u
Ar2+(3p-2 3P)-Ar
3 + Σu
Πu Σ-u 3
0
Ar2+(3p-2 1S)-Ar 1
3
35
1 + Σg
1 + Σg
3
40
Σ-u
Πu
1 + Σg
Ar+(3p-1 2P)-Ar+(3p-1 2P) 3
4
5
6
R (Å) Fig. 3. (color online) Calculated potential energy curves for the one-site states Ar2+(3p2 3P, 1D, 1S)-Ar (upper) and the two-site state Ar+(3p1)Ar+(3p1) (lower) of Ar2. Note that more states than explicitly indicated exist, but these are essentially degenerate with those shown here. For details see Ref. [19].
The lowest PECs shown in Fig. 3 correspond to the dissociative two-site states Ar+(3p1)–Ar+(3p1) (31.52 eV in the dissociation limit [20]), while the higher PECs correspond to the one-site states Ar2+(3p2 3P, 1D, 1S)–Ar. The energy differences between one-site and two-site states in Fig. 3 are 8 eV and thus the energies of the ‘interatomic’ Auger transitions to two-site Ar+(3p1)–Ar+(3p1) states are expected to be 8 eV higher than the energies of the atomic LM23M23 Auger transitions. In the Auger spectrum of the Ar2 dimers in Fig. 1, however, we cannot see a trace of such high energy components. Furthermore, if the ‘interatomic’ Auger transitions to two-site Ar+(3p1)– Ar+(3p1) states took place, direct dissociation would take place and the expected KER would be 4 eV. This again contrasts with our observation of the KER distribution. These findings clearly indicate that the ‘interatomic’ Auger decay is negligible. The displayed PECs show also that there is no two-site satellite state that crosses the one-site Ar2+(3p2)–Ar states. The PEC of the lowest two-site satellite state Ar+(3p23d)–Ar+(3p1) is 16 eV higher than the PES of two-site state Ar+(3p1)–Ar+(3p1). The only possible way to produce the Ar+–Ar+ ion pair is thus a radiative charge transfer, i.e., the radiative decay from the one-site states Ar2+(3p2 3P, 1D, 1S)–Ar to the two-site state Ar+(3p1)–Ar+(3p1). If the radiative charge transfer takes place at the equilibrium internuclear distances of the one˚ , the expected site states, which range from 2.75 to 3.10 A KERs are from 5.24 to 4.65 eV and thus well agree with our observation. 4. Summary We have applied the Auger-electron–ion–ion coincidence momentum imaging technique to the investigation of the electronic decay processes in argon dimers after the creation of the a 2p inner-shell vacancy. We found that
N. Saito et al. / Chemical Physics Letters 441 (2007) 16–19
the atomic LM23M23 Auger decay to the one-site states followed by the radiative charge transfer is the dominant channel to produce the Ar+-Ar+ ion pair. The measured KER agrees well with the estimate of KER based on the ab initio potential energy curves and the assumption that the radiative charge transfer takes place in the vicinity of the equilibrium internuclear distances of the one-site states. We emphasize that our observation gives the first convincing evidence for the occurrence of the radiative charge transfer in an isolated rare gas dimer. Acknowledgement The experiments were performed at SPring-8 with the approval of the program review committee (2006A1216NSb-np and 2006A1757-NSb-np). The Letter was partly supported by Grants-in-Aid for Scientific Researches from Japan Society for the Promotion of Science (JSPS) and by the Budget for Nuclear Research from Ministry of Education, Culture, Sports, Science and Technology, based on screening and counseling by the Atomic Energy Commission. The authors are grateful to the staff for the SPring8, especially to Y. Tamenori and J.R. Harries, for their help in the course of the experiment. X.-J.L. acknowledges JSPS for financial support. Financial support by the DFG is gratefully acknowledged. References [1] M. Thompson, M.D. Baker, A. Christie, J.F. Tyson, Auger Electron Spectroscopy, Wiley, New York, 1985.
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[2] See, for example T. Gonza´lez-Lezana, J. Rubayo-Soneira, S. MiretArte´s, F.A. Gianturco, G. Delgado-Barrio, P. Villarreal, J. Chem. Phys. 110 (1999) 9000, and references therein. [3] See, for example F. Tarantelli, L.S. Cederbaum, Phys. Rev. Lett. 71 (1993) 649, and references therein. [4] C. Cachoncinlle, J.M. Pouvesle, G. Durand, F. Spiegelmann, J. Chem. Phys. 96 (1992) 6085. [5] A. De Fanis et al., J. Phys. B: Atom. Mol. Opt. Phys. 37 (2004) L235. [6] Y. Morishita et al., Phys. Rev. Lett. 96 (2006) 243402. [7] K. Ueda, X.-J. Liu, G. Pru¨mper, H. Fukuzawa, Y. Morishita, N. Saito, J. Electron. Spectrosc. Relat. Phenom. 155 (2007) 113. [8] R. Johnsen, M.A. Biondi, Phys. Rev. A 18 (1978) 996. [9] H. Ohashi et al., Nucl. Instrum. Meth. A 467–468 (2001) 529. [10] K. Ueda, J. Phys. B: Atom. Mol. Opt. Phys. 36 (2003) R1. [11] G.C. King, M. Tronc, F.H. Read, R.C. Bradford, J, Phys. B: Atom. Mol. Phys. 10 (1977) 2479. [12] K. Ueda, J.H.D. Eland, J. Phys. B: Atom. Mol. Opt. Phys. 38 (2005) S839. [13] N. Saito, A. De Fanis, I. Koyano, K. Ueda, J. Electron. Spectrosc. Relat. Phenom. 144–147 (2005) 103. [14] Y. Morishita et al., Radiat. Phys. Chem. 75 (2006) 1977. [15] K. Siegbahn et al., ESCA Applied to Free Molecules, NorthHolland, Amsterdam, 1969. [16] H. Pulkkinen, S. Aksela, O.-P. Sairanen, A. Hiltunen, H. Aksela, J. Phys. B: Atom. Mol. Opt. Phys. 29 (1996) 3033. [17] J. Schirmer, A. Barth, Z. Phys. A 317 (1984) 267. [18] F. Tarantelli, Chem. Phys. 329 (2006) 11. [19] S.D. Stoychev, A.I. Kuleff, L.S. Cederbaum, in preparation. [20] A.A. Radzig, B.M. Smirnov, Reference Data on Atoms, Molecules, and Ions, Springer-Verlag, Berlin, 1985.
Evidence of radiative charge transfer in argon dimers N. Saito a, Y. Morishita a, I.H. Suzuki a,b, S.D. Stoychev c, A.I. Kuleff c, L.S. Cederbaum c, X.-J. Liu d, H. Fukuzawa d, G. Pru¨mper d, K. Ueda d,* a
National Institute of Advanced Industrial Science and Technology (AIST), NMIJ, Tsukuba 305-8568, Japan b Photon Factory, Institute of Materials Structure Science, KEK, Oho 1-1, Tsukuba, 305-0801, Japan c Theoretische Chemie, Physikalisch-Chemisches Institut, Universita¨t Heidelberg, D-69120 Heidelberg, Germany d Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Received 13 February 2007; in final form 23 April 2007 Available online 27 April 2007
Abstract Auger electron spectra recorded in coincidence with two Ar+ ions produced from Ar2þ 2 suggest that the bound one-site two-hole state, Ar (3p2)–Ar, decays further only via radiative charge transfer to the dissociative two-site two-hole states Ar+(3p1)–Ar+(3p1). The measured kinetic energy release of Ar2þ 2 agrees well with the theoretical estimate based on this process. Ó 2007 Elsevier B.V. All rights reserved. 2+
1. Introduction Inner-shell ionization of molecules leads to ion formation with an energy well above the double ionization thresholds. Then the inner-shell ionized states can decay by electron emission. This process is known as the Auger process [1]. The Auger final states are dicationic ‘two-hole’ states. In case of diatomic molecules such as N2, these twohole states dissociate into N+ and N+ due to Coulomb explosion independently of whether the two holes are created on one atom (i.e., N2+–N), or on two atoms (i.e., N+–N+). The situation is significantly different for van der Waals rare gas dimers such as Ar2, where the internuclear distance is significantly larger [2], say, three times larger than in N2. The Auger spectra are generally considered as ‘self imaging’, i.e., fingerprinting images of the atoms where the inner-shell hole is created (see for example [3] and references therein). If we apply this self-imaging picture to Ar2 after 2p ionization, the ‘atomic’ Ar LMM Auger takes place, creating two holes in one Ar site. In Ar2, two holes
*
Corresponding author. Fax: +81 22 217 5380. E-mail address: [email protected] (K. Ueda).
0009-2614/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2007.04.077
can be located on a single Ar site and such dicationic states are not necessarily dissociative [4]. Previously, De Fanis et al. [5] and Morishita et al. [6] observed the Ar+–Ar+ formation using electron-ion-ion coincidence, to a certain branching ratio, after 2p ionization of Ar2 ½Ar–Ar þ hm ! ½Arþ ð2p1 Þ–Ar þ e photo ! Arþ þ Arþ þ e þ e photo : Ueda et al. [7] suggested three possible processes leading to the Ar+–Ar+ dissociation: 8 2þ 1 1 > > > ½Ar ð3s 3p Þ–Ar þ e > þ þ > 2 > ! Ar ð3p n‘Þ þ Ar þ e ðaÞ > > > < ½Ar2þ ð3p2 Þ–Ar þ e ½Arþ ð2p1 Þ–Ar ! > ! Arþ ð3p1 Þ þ Arþ ð3p1 Þ þ e þ hm ðbÞ > > > > > ½Arþ ð3p1 Þ–Arþ ð3p1 Þ þ e > > > : ! Arþ ð3p1 Þ þ Arþ ð3p1 Þ þ e ðcÞ In pathway (a) the one-site states Ar2+(3s13p1), in which 3s and 3p holes are located on one Ar site after L23M1M23 Auger decay, couple with the two-site satellite states Ar+(3p2n‘)-Ar+(3p1) with n‘ such as 3d, 4s, 4p, 4d, 5s,
N. Saito et al. / Chemical Physics Letters 441 (2007) 16–19
The experiment was the same as the one previously reported [6]. Thus only a brief account is repeated here. The experiment has been carried out on the c branch of the soft X-ray photochemistry beamline 27SU [9,10] at SPring-8. The storage ring was operated in the 35 singlebunches + 6/42 filling mode, which provides a single-bunch separation of 114.0 ns. Argon dimers were produced by expanding argon gas at a stagnation pressure of 2 bar cooled to a temperature of 170 K through a pinhole of 30 lm diameter and 0.25 mm thickness. The coincidence measurements described below were performed at a photon energy of 257.05 eV, i.e. 8.4 and 6.3 eV above the atomic Ar 2p1 2P3/2 and 2p1 2P1/2 ionization thresholds [11]. Our electron–ion–ion coincidence momentum imaging is based on recording the electron and ion times-of-flight (TOFs) with multi-hit two-dimensional position sensitive detectors [12–14]. Knowledge of position and arrival time on the particle detectors, (x, y, t), allows us to extract information about the linear momentum (px, py, pz) for each particle. The two TOF spectrometers are placed face to face. In the present setting of the electric and magnetic field conditions, all the electrons up to 12 eV in kinetic energy and all the Ar+ ions up to 10 eV kinetic energy, both ejected in 4p sr, were accelerated onto the MCP detectors, whereas the energetic Auger electrons with 200 eV only ejected towards the detector in 4p/20 sr could be detected.
3.1. Auger electron spectra We recorded coincidence events between one electron and two ions, as well as those between one electron and one ion, in a list mode. In Fig. 1, the Auger spectrum of the Ar2 recorded in coincidence with the Ar+–Ar+ ion pairs is compared with the Auger spectrum of the Ar atoms recorded in coincidence with the Ar2+ ions. The energy resolution is only 20 eV. Counts of the Ar dimer Auger spectrum is much lower than those of the monomer Auger spectrum. Within the statistical uncertainties, however, these two Auger spectra coincide. In the figure, the energies of the atomic Auger transitions to 3s13p1 (L23M1M23 at 190 eV) and to 3p2 (L23M23M23 at 205 eV) are indicated by vertical dashed and solid lines, respectively [15,16]. It is clear that Ar+–Ar+ ion pair can be produced not only from the one-site Ar2+(3s13p1)–Ar states but also from the one-site Ar2+(3p2)–Ar states and that the latter is predominant. 3.2. Kinetic energy release The KERs in the fragmentation into Ar+–Ar+ are shown in Fig. 2. We plot the KERs coincident with photoelectrons (open circles) and with Auger electrons (closed symbols) selected with the three regions in Fig. 1. The profiles of the KERs do not depend on the Auger electron energies. The KER distribution is peaked at 5.3 eV and exhibits a longer tail in the low energy side. The corresponding internuclear distance estimated from the two-hole ˚ , is significantly shorter than Coulomb repulsion, 2.7 A Auger3 Auger2 Auger1
L23M1M23
L23M23M23
3000
40
Ar + - Ar +
Ar 2+
2000
20 1000
0 160
200
Counts for Ar 2+
2. Experiment
3. Results and discussion
Counts for Ar + - Ar +
5p and thus may dissociate to Ar+ and Ar+. Pathway (b) represents a radiative charge transfer [8], i.e., the radiative decay from the one-site Ar2+(3p2)–Ar states, populated by the atomic L23M23M23 Auger decay, to two-site Ar+(3p1)–Ar+(3p1) ion-pair states. Besides the selfimaging ‘atomic’ Auger decay, ‘interatomic’ Auger decay, i.e., the core-hole decay in which an electron in the neighboring atom participates, can also take place and then the two holes are created directly at the two different Ar sites – the pathway (c). With the limited knowledge experimentally obtained, Ueda et al. speculated that the potential coupling mechanism (a) may be most likely [7]. The decisive conclusion on the prominent channel to the Ar+–Ar+ ion-pair formation can, however, be extracted from the Auger-electron–ion–ion coincidence hidden in the previous data set [6]. We thus have reanalyzed the data. The Auger electron energy spectra recorded in coincidence with two Ar+ ions indicates that the single-site two-hole state Ar2+(3p2)–Ar that is supposed to be bound can produce the Ar+–Ar+ ion-pair, suggesting that these bound states further decay via radiative charge transfer to the dissociative two-site two-hole states Ar+(3p1)–Ar+(3p1), i.e., process (b). Measured kinetic energy release (KER) gives reasonable agreement with the predictions based on ab initio adiabatic potential energy curves of one-site and two-site states in Ar2 and the radiative charge transfer (b) between them.
17
0 240
Electron Energy (eV) Fig. 1. (color online) The Auger spectra of Ar2 dimers recorded in coincidence with the Ar+-Ar+ ion pairs and of the Ar atoms recorded in coincidence with the Ar2+ ions. Vertical solid and dashed lines are the energies of the L23M23M23 and L23M1M23 atomic Auger transitions, respectively. The kinetic energy release is analyzed in coincidence with the electrons with the kinetic energies in the region of the labels of Auger1, Auger2 and Auger3 above the figure. The scales for the counts were adjusted so that the areas of the counts are the same in the figure.
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N. Saito et al. / Chemical Physics Letters 441 (2007) 16–19
50
45
E (eV)
1000
Counts
1
All Auger1(x21) Auger2(x25) AUger3(x42)
500
3
4 5 Kinetic Energy Release (eV)
6
Fig. 2. (color online) The total kinetic energy release (KER) in the Ar2þ 2 fragmentation into the two Ar+ ions. Open circles show the KER coincident with photoelectrons; closed squares, circles, and triangles denote the KERs coincident with Auger electrons selected in the regions of Auger1, Auger2, and Auger3, respectively. The scales for the counts were adjusted so that the areas of the counts are the same in the figure.
the equilibrium bond length in the neutral ground state ˚ ). It implies that nuclear motion (bond shortening) (3.8 A takes place in the final state of the Auger decay, Ar2+(3p2)–Ar, before a radiative transition to the two-site states Ar+(3p1)–Ar+(3p1) occurs. The Auger states are thus only intermediate states in this pathway to the Ar+ + Ar+ ion production. Let us briefly discuss the relation between the coincident count ratio of Ar+–Ar2+ to Ar+–Ar+ and the Auger transition branching ratios. The coincident count ratio of Ar+– Ar2+ to Ar+–Ar+ is 0.12 after the detection efficiency correction. On the other hand, Auger transition branching ratios are 0.092 for the Auger electron energies of 177– 180 eV (3p33d), 0.139 for 187–193 eV (3s13p1), and 0.727 for 201–208 eV (3p2) [16]. Our previous Letter [6] suggests that Ar2 in the 3p33d state decays into Ar+– Ar2+. The present Auger-electron–ion coincident results suggest that Ar2 in the 3s13p1 and 3p2 states decays into Ar+–Ar+. Then the Auger transition ratio of 3p33d to 3s13p1 and 3p2 is 0.11, which is very close to the coincident count ratio of Ar+–Ar2+ to Ar+–Ar+. 3.3. Radiative charge transfer We have performed ab initio calculations to obtain the potential energies of Ar2. The calculations were done by use of the formalism of Green’s functions in the so-called algebraic diagrammatic construction (ADC) scheme, in particular the ADC(2) for the particle–particle propagator [17] as implemented by Tarantelli [18]. The resulting double ionization potentials are added to the potential energy curve (PEC) of the ground state of Ar2 calculated via the CCSD(T) method. A full account for the calculations will be given elsewhere [19]. It should be noted that all computed ionic state PECs displayed in Fig. 3 have been shifted such that they match the experimental ionic energies at large R.
Ar2+(3p-2 1D)-Ar
Σ-u
Ar2+(3p-2 3P)-Ar
3 + Σu
Πu Σ-u 3
0
Ar2+(3p-2 1S)-Ar 1
3
35
1 + Σg
1 + Σg
3
40
Σ-u
Πu
1 + Σg
Ar+(3p-1 2P)-Ar+(3p-1 2P) 3
4
5
6
R (Å) Fig. 3. (color online) Calculated potential energy curves for the one-site states Ar2+(3p2 3P, 1D, 1S)-Ar (upper) and the two-site state Ar+(3p1)Ar+(3p1) (lower) of Ar2. Note that more states than explicitly indicated exist, but these are essentially degenerate with those shown here. For details see Ref. [19].
The lowest PECs shown in Fig. 3 correspond to the dissociative two-site states Ar+(3p1)–Ar+(3p1) (31.52 eV in the dissociation limit [20]), while the higher PECs correspond to the one-site states Ar2+(3p2 3P, 1D, 1S)–Ar. The energy differences between one-site and two-site states in Fig. 3 are 8 eV and thus the energies of the ‘interatomic’ Auger transitions to two-site Ar+(3p1)–Ar+(3p1) states are expected to be 8 eV higher than the energies of the atomic LM23M23 Auger transitions. In the Auger spectrum of the Ar2 dimers in Fig. 1, however, we cannot see a trace of such high energy components. Furthermore, if the ‘interatomic’ Auger transitions to two-site Ar+(3p1)– Ar+(3p1) states took place, direct dissociation would take place and the expected KER would be 4 eV. This again contrasts with our observation of the KER distribution. These findings clearly indicate that the ‘interatomic’ Auger decay is negligible. The displayed PECs show also that there is no two-site satellite state that crosses the one-site Ar2+(3p2)–Ar states. The PEC of the lowest two-site satellite state Ar+(3p23d)–Ar+(3p1) is 16 eV higher than the PES of two-site state Ar+(3p1)–Ar+(3p1). The only possible way to produce the Ar+–Ar+ ion pair is thus a radiative charge transfer, i.e., the radiative decay from the one-site states Ar2+(3p2 3P, 1D, 1S)–Ar to the two-site state Ar+(3p1)–Ar+(3p1). If the radiative charge transfer takes place at the equilibrium internuclear distances of the one˚ , the expected site states, which range from 2.75 to 3.10 A KERs are from 5.24 to 4.65 eV and thus well agree with our observation. 4. Summary We have applied the Auger-electron–ion–ion coincidence momentum imaging technique to the investigation of the electronic decay processes in argon dimers after the creation of the a 2p inner-shell vacancy. We found that
N. Saito et al. / Chemical Physics Letters 441 (2007) 16–19
the atomic LM23M23 Auger decay to the one-site states followed by the radiative charge transfer is the dominant channel to produce the Ar+-Ar+ ion pair. The measured KER agrees well with the estimate of KER based on the ab initio potential energy curves and the assumption that the radiative charge transfer takes place in the vicinity of the equilibrium internuclear distances of the one-site states. We emphasize that our observation gives the first convincing evidence for the occurrence of the radiative charge transfer in an isolated rare gas dimer. Acknowledgement The experiments were performed at SPring-8 with the approval of the program review committee (2006A1216NSb-np and 2006A1757-NSb-np). The Letter was partly supported by Grants-in-Aid for Scientific Researches from Japan Society for the Promotion of Science (JSPS) and by the Budget for Nuclear Research from Ministry of Education, Culture, Sports, Science and Technology, based on screening and counseling by the Atomic Energy Commission. The authors are grateful to the staff for the SPring8, especially to Y. Tamenori and J.R. Harries, for their help in the course of the experiment. X.-J.L. acknowledges JSPS for financial support. Financial support by the DFG is gratefully acknowledged. References [1] M. Thompson, M.D. Baker, A. Christie, J.F. Tyson, Auger Electron Spectroscopy, Wiley, New York, 1985.
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[2] See, for example T. Gonza´lez-Lezana, J. Rubayo-Soneira, S. MiretArte´s, F.A. Gianturco, G. Delgado-Barrio, P. Villarreal, J. Chem. Phys. 110 (1999) 9000, and references therein. [3] See, for example F. Tarantelli, L.S. Cederbaum, Phys. Rev. Lett. 71 (1993) 649, and references therein. [4] C. Cachoncinlle, J.M. Pouvesle, G. Durand, F. Spiegelmann, J. Chem. Phys. 96 (1992) 6085. [5] A. De Fanis et al., J. Phys. B: Atom. Mol. Opt. Phys. 37 (2004) L235. [6] Y. Morishita et al., Phys. Rev. Lett. 96 (2006) 243402. [7] K. Ueda, X.-J. Liu, G. Pru¨mper, H. Fukuzawa, Y. Morishita, N. Saito, J. Electron. Spectrosc. Relat. Phenom. 155 (2007) 113. [8] R. Johnsen, M.A. Biondi, Phys. Rev. A 18 (1978) 996. [9] H. Ohashi et al., Nucl. Instrum. Meth. A 467–468 (2001) 529. [10] K. Ueda, J. Phys. B: Atom. Mol. Opt. Phys. 36 (2003) R1. [11] G.C. King, M. Tronc, F.H. Read, R.C. Bradford, J, Phys. B: Atom. Mol. Phys. 10 (1977) 2479. [12] K. Ueda, J.H.D. Eland, J. Phys. B: Atom. Mol. Opt. Phys. 38 (2005) S839. [13] N. Saito, A. De Fanis, I. Koyano, K. Ueda, J. Electron. Spectrosc. Relat. Phenom. 144–147 (2005) 103. [14] Y. Morishita et al., Radiat. Phys. Chem. 75 (2006) 1977. [15] K. Siegbahn et al., ESCA Applied to Free Molecules, NorthHolland, Amsterdam, 1969. [16] H. Pulkkinen, S. Aksela, O.-P. Sairanen, A. Hiltunen, H. Aksela, J. Phys. B: Atom. Mol. Opt. Phys. 29 (1996) 3033. [17] J. Schirmer, A. Barth, Z. Phys. A 317 (1984) 267. [18] F. Tarantelli, Chem. Phys. 329 (2006) 11. [19] S.D. Stoychev, A.I. Kuleff, L.S. Cederbaum, in preparation. [20] A.A. Radzig, B.M. Smirnov, Reference Data on Atoms, Molecules, and Ions, Springer-Verlag, Berlin, 1985.