Letter pubs.acs.org/JPCL
Controlling Low-Energy Electron Emission via Resonant-AugerInduced Interatomic Coulombic Decay Miku Kimura,† Hironobu Fukuzawa,† Tetsuya Tachibana,† Yuta Ito,† Subhendu Mondal,† Misaki Okunishi,† Markus Schöffler,‡ Joshua Williams,‡ Yuhai Jiang,¶ Yusuke Tamenori,§ Norio Saito,∥ and Kiyoshi Ueda*,† †
Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Institute for Nuclear Physics, Johann Wolfgang Goethe University Frankfurt, Frankfurt 60438, Germany ¶ Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China § Japan Synchrotron Radiation Research Institute (JASRI), Sayo, Hyogo 679-5198, Japan ∥ National Institute of Advanced Industrial Science and Technology (AIST), NMIJ, Tsukuba 305-8568, Japan ‡
ABSTRACT: We have investigated interatomic Coulombic decay (ICD) after resonant Auger decay in Ar2, ArKr, and ArXe following 2p3/2 → 4s and 2p3/2 → 3d excitations in Ar, using momentum-resolved electron−ion−ion coincidence. The results illustrate that ICD induced by the resonant Auger decay is a well-controlled way of producing energy-selected slow electrons at a specific site.
SECTION: Spectroscopy, Photochemistry, and Excited States
I
n 1997, Cederbaum et al.1 theoretically suggested a new relaxation mechanism of the electronically excited species in the presence of the loosely bound neighboring species. In this mechanism, the excited species decays electronically by ionizing the neighboring species. This new relaxation mechanism is called interatomic or intermolecular Coulombic decay (ICD). Six years later, the first experimental proof for occurrence of ICD was reported for neon clusters.2 Since then, a number of theoretical and experimental studies on ICD have been carried out in many different systems, as recently reviewed.3,4 These studies elucidated that ICD plays an important role in chemistry, transferring the energy and the charge from the excited species to the environment surrounding it. It is worth noting also that ICD in hydrogen-bonded systems and its role as a source of low-energy electrons in the biological medium have been discussed in several recent publications.5−8 Despite the extensive studies of ICD, little attention has been paid to how to prepare the ICD initial states. A usual way of studying ICD is to use monochromatic synchrotron radiation, tuning the photon energy high enough to populate innervalence hole states or satellite states that undergo ICD.2,5,6,9,10 This approach, however, does not specify the site of ionization; the radiation may populate both inner- and outer-valence hole states of any atoms and molecules at the same time. Santra and Cederbaum11 theoretically postulated that some Auger final © XXXX American Chemical Society
dicationic states are also subject to ICD. Following this prediction, experimental investigations for ICD after Auger decay were carried out extensively.12−14 These studies employed a monochromatic radiation with photon energy above the core ionization threshold of a specific atom. This approach may improve, to a certain amount, the selectivity of the atomic photoionization, though the branching ratios to the Auger final states that undergo ICD are on the order of 10% or so. Very recently, Gokhberg et al.15 suggested using resonant Auger decay to populate the ICD initial states. One can selectively excite core electrons not only on chemically different atoms but also on identical atoms occupying nonequivalent sites in the system. Besides this high site-selectivity, the resonant-Auger-induced ICD provides us with the tunability of the ICD electron energy using different resonant core excitations. Noting that the ICD produces low-energy electrons locally at the site where a resonant excitation takes place, they also noted the relevance of the site-specific production of controlled low-energy electrons via ICD with radiation therapy that requires the localized radiation damage. The resonantReceived: March 26, 2013 Accepted: May 7, 2013
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value of 5.0 eV corresponds to the internuclear distance of 2.9 Å, suggesting that the ICD occurs after the bond shortening by ∼1 Å. The sum between the electron energy and the KER is constant for ICD.9 The distributions of the energy sums are depicted in Figure 2a−c for Ar2, ArKr, and ArXe, respectively,
Auger-induced ICD has been very recently independently observed in argon dimers16 and molecular dimers.17 In the present work, we fully explore the controllability and tunability of resonant-Auger-induced ICD electrons using Ar2, ArKr, and ArXe as prototype samples. Namely, we demonstrate how the ICD emission is controlled by the resonant excitation energy of a specific species (Ar) and influenced by the chemical environments (i.e., the neighboring species Ar, Kr, or Xe) of the excited species (Ar). The schematic sequence that we have investigated is as follows. First, a 2p electron in Ar is excited to an unoccupied orbital ArX + hν → Ar(2p−1n l)X
(1)
where X is Ar, Kr, or Xe and nl is 4s or 3d. Then, a spectator Auger decay occurs in the Ar atom Ar(2p−1n l)X → Ar +(3p−2 n′l)X + e−Auger
(2)
−2
populating the spectator final states Ar (3p n′l), where n′ can be n (strict spectator states) or n + 1 (shakeup states). Then, if ICD is energetically allowed, ICD occurs from these states, leading to Coulombic fragmentation +
Ar +(3p−2 n′l)X → Ar +(3p−1) + X +(m p−1) + e−ICD
(3)
where m = 3 for X = Ar, m = 4 for X = Kr, and m = 5 for X = Xe. In the experiment, we record 3D momenta for each of two fragment ions and the ICD electron in coincidence. Figure 1 depicts the result of ArKr forming the Ar+−Kr+ ion pair at the Ar 2p3/2 → 3d excitation. The coincidence
Figure 2. Energy distribution for the sum of the electron energy and the KER, as measured (upper) and as estimated (lower) for Ar 2p3/2 → 3d excitation in (a) Ar2, (b) ArKr, and (c) ArXe. The estimates given by the (green) thin and (red) thick vertical bars are based on the branching ratios for the resonant Auger transitions following the Ar 2p3/2 → 3d excitation reported by Mursu et al.19 ICD is energetically forbidden for the states given by the (green) thin vertical bars and allowed for those given by the (red) thick vertical bars. The (blue) dotted curve is a convolution of the (red) thick vertical bars by Gaussian profiles with a 0.94 eV fwhm (i.e., the average experimental resolution) for comparison with the experimental spectrum. See the text for further details.
Figure 1. The results for ArKr. The excitation photon energy is tuned to the Ar 2p3/2 → 3d excitation at 246.94 eV. ICD occurs following resonant Auger decay in the Ar atom, as described in eqs 1−3. The ICD electrons are recorded in coincidence with Ar+ and Kr+. (a) Relationship between the ICD electron energy and the KER for the fragmentation to Ar+ and Kr+. (b) Energy distribution of the ICD electrons. (c) The KER for the fragmentation to Ar+ and Kr+. See the text for further details.
at the Ar 2p3/2 → 3d excitation. The present result for Ar2 extracted from the Ar/Kr mixture data agrees well with the previous measurement.16 The initial states of ICD are populated by the resonant Auger decay following the 2p3/2 → 3d excitation in Ar (see eq 2). The final states of the ICD are the lowest dicationic states (see eq 3). In the case of ArKr depicted in Figure 1, for example, these states dissociate to Ar+(3p−1 2P1/2,3/2) and Kr+(4p−1 2P1/2,3/2). The positions of the vertical bars in Figure 2 are the differences between the energies of the resonant Auger final states and the energy sum of the two atomic ions relative to the two neutral atoms in the ground state. The energy of the resonant Auger final states can be found in the literature.19 Here, we took account of the energy differences between Kr+(4p−1 2P1/2) and Kr+(4p−1 2P3/2) and between Xe+(5p−1 2P1/2) and Xe+(5p−1 2P3/2) but took the weighted average for unresolved Ar + (3p −1 2 P 1/2 ) and Ar+(3p−1 2P3/2). The heights of the vertical bars are the relative intensities of the corresponding resonant Auger transitions that
measurement for one electron and two ions provides correlations between the electron kinetic energy and the kinetic energy release (KER) between the Ar+ and Kr+ ions for each event, as shown in Figure 1a. The electron energy distribution depicted in Figure 1b was recorded in coincidence with the fragmentation to Ar+ and Kr+. These electrons are the ICD electrons. The distribution of the KER depicted in Figure 1c exhibits two peaks at 3.7 and 5.0 eV. The bond length of ArKr in the neutral ground state is 3.88 Å.18 Calculating the Coulomb repulsion energy between the two ions separated at this distance, we obtain the value of 3.7 eV. Thus, the KER peak at 3.7 eV corresponds to the ICD that occurs at the equilibrium internuclear distance of ArKr. On the other hand, the KER 1839
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can be found also in the literature.19 The ICD is energetically forbidden for the states given by (green) thin bars. The (blue) dotted lines are the results of convolutions for the thick (red) bars corresponding to the states that can undergo ICD. As discussed in the previous report for Ar2,16 the distributions of the energy sums in Figure 2 are correlated well with the intensity ratios of the resonant Auger transitions. This correlation may be explained by assuming that these resonant Auger final states decay predominantly via ICD as long as ICD is energetically open. Comparing the three spectra in Figure 2, we notice the following points. The energy distribution shifts to higher energy when one goes from Ar2 to ArKr and to ArXe. This is explained by the decrease in the ionization potential from Ar to Kr and to Xe, resulting in the increase in the energy to be shared by KER and ICD electron energy. The energy increase is in reasonable agreement with the energy decrease in the ionization potential as one goes from Ar (15.76 eV for 3p−1 2P3/2 and 15.94 eV for 3p−1 2P1/2) to Kr (14.00 eV for 4p−1 2P3/2 and 14.67 eV for 4p−1 2P1/2) and to Xe (12.13 eV for 5p−1 2P3/2 and 13.44 eV for 5p−1 2P1/2). Also, the energy distribution is broadened as one goes from Ar2 to ArKr and to ArXe. This is because the energy difference in the spin−orbit components (2P3/2 and 2P1/2) increases when one goes from Ar (0.18 eV) to Kr (0.67 eV) and to Xe (1.31 eV) and thus contributes to the spread of the energy sum. Furthermore, the fraction of the states for which ICD is energetically forbidden [given by the (green) thin vertical bars in Figure 2] decreases as one goes from Ar2 (7.4%) to ArKr (3.8%) and to ArXe (2.7%). Let us inspect the correlation between Figures 1 and 2, focusing on the ArKr case. The peak at ∼7.5 eV in Figure 2b corresponds to the transition Ar+[3p−2(1D)3d 2D]−Kr → Ar+ (3p −1 )−Kr+ (4p −1 ), whereas the peak at ∼10.5 eV corresponds to the transition Ar+[3p−2(1D)4d 2D]−Kr → Ar+(3p−1)−Kr+(4p−1). In Figure 1a, these two ICD transitions form the islands A and B, respectively. Here, we find that the results of ArKr show similar trends to those of Ar2 previously reported.16 Namely, the ICD Ar+[3p−2(1D)3d 2D]−Kr → Ar+(3p−1 )−Kr+(4p−1) [A in Figure 1a] occurs at the equilibrium internuclear distance of ∼3.9 Å of the neutral ArKr before the nuclear motion occurs, while the ICD Ar+[3p−2(1D)4d 2D]−Kr → Ar+(3p−1)−Kr+(4p−1) [B in Figure 1a] occurs after the bond shortening to the distance of ∼2.9 Å, which is close to the turning point of the classical vibrational motion of ∼2.7 Å calculated by Gokhberg (private communications). According to a recent theoretical study, the lifetimes are estimated to be 13−220 fs for Ar+[3p−2(1D)3d 2D]−Kr and 600 fs−2 ps for Ar+[3p−2(1D)4d 2D]−Kr.15 The difference of these lifetimes accounts for the observations. We note also that the less intense peak at ∼4.3 eV in Figure 2b corresponds to the transitions Ar +[3p −2( 1D)4s 2 D and 3p−2(3P)3d]−Kr → Ar+(3p−1)−Kr+(4p−1) that form the island at an ICD electron energy of ∼0.5 eV and KER of 3.7 eV and evidences another fast ICD at the equilibrium distance of ArKr. These ICD channels are energetically closed for Ar2 but open for ArKr and ArXe. We now compare, in Figure 3, the ICD electron energy distributions recorded in coincidence with two fragment ions for all three dimers Ar2 (a,d), ArKr (b,e), and ArXe (c,f) at two different excitations Ar 2p3/2 → 4s (a−c) and Ar 2p3/2 → 3d (d−f). Comparing ICD emissions (d−f) after the Ar 2p3/2 → 3d excitation in Figure 3, we find that ICD electron energy increases as one goes from Ar2 to ArKr and to ArXe, in a similar
Figure 3. Energy distribution for ICD electrons for Ar 2p3/2 → 4s excitation in (a) Ar2, (b) ArKr, and (c) ArXe, and for Ar 2p3/2 → 3d excitation in (d) Ar2, (e) ArKr, and (f) ArXe.
manner as the energy sum distribution shifts to higher energy as one goes from Ar2 to ArKr and ArXe. The energy spread of the ICD electrons is smaller than that of the energy sum given in Figure 2. This is because a part of the energy sum is transferred to the increase in the KER in the case of the ICD initial states Ar+[3p−2(1D)4d 2D]X, with X being Ar, Kr, and Xe, as discussed in the previous paragraph. Comparing ICD emissions (a−c) after the Ar 2p3/2 → 4s excitation in Figure 3, however, we cannot see similar trends. This is because ICD is closed energetically for a significant part of the resonant Auger final states populated by the Ar 2p3/2 → 4s excitation, 76.2% for Ar2, 42.7% for ArKr, and 21.8% for ArXe. As a result, ICD emissions from different dimers are dominated by different ICD initial states. This sharply contrasts the case of Ar 2p3/2 → 3d excitation, where most of the Auger final states undergo ICD. The most important message of Figure 3 may be revealed by comparing different excitation energies for the same system. Compare the ICD emissions from ArXe at two different excitations (c) Ar 2p3/2 → 4s and (f) Ar 2p3/2 → 3d. By increasing the excitation photon energy by 3 eV, we could shift the ICD emission energy by ∼5 eV. This demonstrates controllability of low-energy electron emission at a specific site. A similar energy shift of ∼4 eV is also observed for ArKr. In the case of Ar2, however, because ICD is closed for most of the resonant Auger final states populated by the Ar 2p3/2 → 4s excitation, we could not control the ICD electron energy much by varying the excitation energy. It is worth noting here that Kryzhevoi and Cederbaum20 suggested theoretically a different method of controlling the energies of ICD electrons by modifying the pH index of the medium. In conclusion, using Ar2, ArKr, and ArXe as prototype samples, we demonstrated that ICD commonly occurs after resonant Auger decay and that the ICD emission is influenced by the neighboring atom (Ar, Kr, and Xe) of the excited atom (Ar). By changing the excitation energy and selecting different core excited states in Ar, we could control the energy of ICD electrons. We demonstrated it using the Ar 2p3/2 → 4s and 2p3/2 → 3d excitations in ArKr and ArXe. This tunability is general, but if ICD is energetically closed for most of the resonant Auger final states, the tunability is suppressed, as seen in the Ar2 case. Due to the site-specific nature of the core excitation followed by resonant Auger decay, the resonant1840
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ACKNOWLEDGMENTS We are grateful to K. Gokhberg, A. I. Kuleff, P. Demekhin, and L. S. Cederbaum for stimulating discussion and K. Gokhberg for providing us with theoretical results on the potential energies prior to publication. The experiments were performed at SPring-8 with the approval of JASRI. The work was supported by a Grant-in-Aid (21244062) from JSPS and by the Management Expenses Grants for National Universities Corporations from MEXT. S.M. acknowledges financial support from JSPS. Y.H.J. acknowledges support from the National Basic Research Program of China (973 Program) (grant 2013CB922200) and the National Natural Science Foundation of China (grant 11274232).
Auger-induced ICD is site-specific in nature. Thus, the present study suggests a new way to produce low-energy electrons at a specific site in a well-controlled manner. Noting that the lowenergy electrons cause effectively the single- and double-strand breaks in DNA, the present study implicates a new way to cause the radiation damage to a specific site in a well-controlled manner and thus is relevant to the radiation therapy, as suggested by Gokhberg et al.15 Note added: The ICD after resonant Auger decay in the ArNe dimers has been very recently independently observed.21
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EXPERIMENTAL METHOD The experiment was carried out on the c branch of the beamline 27SU22−24 at SPring-8. The heterodimers ArKr and ArXe were produced by expanding mixed gas at a stagnation pressure of 0.3 and 0.1 MPa, respectively, through a pinhole of 30 μm diameter. The flow rate ratio of mixed gas was 13:2 for Ar/Kr and 50:3 for Ar/Xe. Under these conditions, the cluster beam contains not only heterodimers (ArKr or ArXe) but also monomers (Ar and Kr or Xe), homodimers (Ar2 and Kr2 or Xe2), and a very small fraction of larger clusters. The signals from specific dimers were selected by applying the momentum conservation law for the ion pairs detected in coincidence (see below). The data set of the Ar/Kr mixture gas was used also for the analysis of Ar2. The cluster beam was directed vertically and crossed with the incident radiation at right angles. The photon energy was tuned to the excitation energy of Ar 2p3/2 → 4s at 244.39 eV or Ar 2p3/2 → 3d at 246.94 eV.25 The photon bandwidth was set to ∼0.05 eV, except for the Ar 2p3/2 → 4s excitation in ArXe, where it was increased to 0.13 eV so that the counts of events were sufficient. Our momentum-resolved electron−ion multicoincidence12,26 is equivalent to cold-target recoil-ion momentum spectroscopy or reaction microscopes27 and is based on recording times of flight (TOFs) for electrons and ions with two position and time-sensitive multihit-capable detectors (Roentdek HEX120 for electrons and HEX80 for ions). Knowledge of position and arrival time on the particle detectors, (x,y,t), allows us to extract information about the 3D momentum of each particle. The electron and ion spectrometers are placed face to face. The spectrometer axis is horizontal and perpendicular to both the incident radiation and the cluster beam. In the present experiments, electric and magnetic fields were applied to the interaction region, so that all of the electrons (with energy up to 13 eV for the Ar/Kr mixed gas and up to 22 eV for the Ar/ Xe mixed gas) and all of the fragment ions were guided to the electron and ion detectors, respectively. Detailed geometric descriptions and typical field conditions of the spectrometers were given elsewhere.26 The TOFs of electrons and ions were recorded with respect to the bunch marker of the light source using multihit time-to-digital converters (Roentdek TDC8HP) by selecting only electron signals synchronized with the single bunches.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 1841
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