Chemical Physics 482 (2017) 221–225 Contents lists available at ScienceDirect Chemical Physics journal homepage: www.elsevier.com/locate/chemphys Interatomic Coulombic Decay of HeNe dimers after ionization and excitation of He and Ne H. Sann a, T. Havermeier a, H.-K. Kim a, F. Sturm a, F. Trinter a, M. Waitz a, S. Zeller a, B. Ulrich a, M. Meckel a, S. Voss a, T. Bauer a, D. Schneider a, H. Schmidt-Böcking a, R. Wallauer b, M. Schöffler a, J.B. Williams c, ⇑ R. Dörner a, T. Jahnke a, a Institut für Kernphysik, Universität Frankfurt, Max-von-Laue-Str. 1, 60438 Frankfurt, Germany b Fachbereich Physik, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, Germany c Department of Physics, Reno University, NV 89557, USA article info abstract Article history: We study the decay of a helium/neon dimer after ionization and simultaneous excitation of either the Available online 7 September 2016 neon or the helium atom using Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS). We find that, depending on the decaying state, either direct Interatomic Coulombic Decay (ICD) (i.e. mediated by a virtual photon exchange), exchange ICD (mediated by electron exchange) or radiative charge transfer occurs. The corresponding channels are identified. Ó 2016 Elsevier B.V. All rights reserved. 1. Introduction photoionization of HeNe and identify all decaying states and decay processes involved after shake-up ionization of either the Ne or the After the prediction of Interatomic Coulombic Decay (ICD) in He atom. The decay rate of ICD can be described theoretically (in 1997 by Cederbaum and coworkers [1] it was found that this elec- the lowest pertubation order) by the electron/electron-Coulomb- tronic decay process involving two or more atoms or molecules of a matrix element. In the prototype case of ICD (i.e. ICD after Ne(2s) loosely bound compound of matter is a very common phenomenon innervalence ionization) the decay rate is proportional to 2 [2]. As ICD occurs, an electronic excitation of one atom or molecule jV L2p;R2p;L2s;k À V L2p;R2p;k;L2sj , where the two contributions to the decays by a transfer of the excitation energy to a different, neigh- overall matrix element boring atom or molecule. It was experimentally confirmed in the RR ¼ /Ã ð~ Þ/ ð~ Þ e2 early 2000s by three pioneering experiments employing electron V L2p;R2p;L2s;k L2p r1 L2s r1 j~ À~ j r1 r2 ð1Þ spectroscopy [3], Cold Target Recoil Ion Momentum Spectroscopy /Ã ð~ Þ/ ð~ Þ ~ ~ R2p r2 k r2 dr1dr2 [4] and the lifetime analysis of photoelectron spectra [5]. ICD has been found since then in a manifold of atomic and molecular sys- and tems and different excitation schemes [2,6]. RR ¼ /Ã ð~ Þ/ ð~ Þ e2 ICD of HeNe mixed dimers has been studied already about V L2p;R2p;k;L2s L2p r1 k r1 j~r À~r j 1 2 ð2Þ 6 years ago by Sisourat et al. [7]. Intriguingly, it turned out, that /Ã ð~ Þ/ ð~ Þ ~ ~ R2p r2 L2s r2 dr1dr2 in case of Ne(2s) shake-up ionization it is the nuclear motion that dominates the Interatomic Coulombic Decay. Depending on the are referred to as direct and exchange contribution in the literature exact state populated, Sisourat and coworkers identified, that ICD [8,9]. L2s and L2p denote the 2s and 2p orbitals located at the left may only occur after the dimer stretches to rather large internu- atom and R2p denotes the 2p orbital situated at the right atom. clear distances of up to 7 Å, as the decay is energetically forbidden In direct ICD the excited valence electron fills the vacant hole in for smaller internuclear distances. Accordingly, a drastic increase its own atom. In the case of exchange ICD the electron, which fills of the decay width was observed to occur with the periodicity of the hole, comes from the atom that did not have the initial the vibrational motion of the dimer’s nuclei. In the present article vacancy. Fig. 1 shows a sketch of both contributions to the ICD we give a comprehensive overview over all ICD channels following matrix element. The resulting electrons and ions are indistinguish- able, but the processes depend very differently on the internuclear distance R. As exchange ICD requires an overlap of the electron and ⇑ Corresponding author. hole orbitals both located at different atoms its probability E-mail address: [email protected] (T. Jahnke). increases with smaller internuclear distances. http://dx.doi.org/10.1016/j.chemphys.2016.09.008 0301-0104/Ó 2016 Elsevier B.V. All rights reserved. 222 H. Sann et al. / Chemical Physics 482 (2017) 221–225 the former was investigated already in an earlier work by Sisourat et al. [7]. There it was shown that the contribution to ICD of the decay of the He–Ne+ð2sÀ1) state is very small because the dimer needs to stretch before ICD becomes energetically possible and instead the main contribution comes from He–Ne+ð2pÀ23s) shake-up states. Here we show that, additionally, there is a signif- icant contribution from the decay of the He–Ne+ð2pÀ23p) shake-up states. The transition from Ne+ð2pÀ23p) to the ICD final state Ne+ð2pÀ1) is dipole forbidden. Therefore, the decay rate via direct ICD is very small as a decay of that kind is dominated by a dipole transition due to the virtual photon exchange [17]. Instead exchange ICD may occur. In earlier work by Jahnke et al. on shake-up ICD of Ne2 dimers the two contributions were disentan- gled experimentally by investigating the kinetic energy release (KER) of the two ions after the decay (and the subsequent Coulomb explosion). Within the reflection approximation [16] the kinetic energy release is a direct measure of the internuclear distance of the dimer’s atoms at the instant of the decay. At the large internu- clear distances of interest the potential energy curves of the repul- sive doubly charged states are in good approximation 1/R. Therefore, in atomic units, the internuclear distance R is obtained as R ¼ 1=KER. The decay probability of the two contributions scales differently with the internuclear distance of the two atoms Fig. 1. Sketch of the direct contribution (top) and the exchange contribution involved in the decay: while the direct ICD is dominated by a (bottom) to the ICD transition matrix element as ICD in a neon dimer occurs after = 6 inner valence ionization. dipole/dipole-interaction and thus scales with 1 R [17,18], the exchange ICD depends on an orbital overlap of the contributing In 2007 Jahnke et al. demonstrated that these contributions can electron and hole states. Its probability decreases exponentially be disentangled in an experiment due to the R-dependence of their with increasing R. Accordingly, the two contributions occur within probability [10]. The experimental data presented in this article different KER ranges, namely direct ICD at smaller KER values (as it shows similar signatures of direct and exchange ICD depending on occurs at larger internuclear distances) and exchange ICD after the the parity of the decaying shake-up states. dimer contracted significantly (as compared to its ground state mean internuclear distance) and thus after establishing a sufficient 2. Experimental technique amount of orbital overlap. Fig. 2 shows the corresponding results. We identify two groups of kinetic energy releases. Small KER val- The experiment was performed at the BESSY synchrotron radi- ues correspond to a decay at large internuclear distance (pathways ation source at beamline U125-2_SGM in single bunch operation A1–A4), close to the equilibrium distance of the neutral HeNe. using the COLTRIMS technique [11–13]. A supersonic gas jet con- These events result from direct ICD. The group of events at larger taining 93% He and 7% Ne was expanded through a nozzle of KERs (B1, B2) result from electron exchange ICD. 20 lm diameter and then intersected with a linearly polarized In Fig. 2 we plot the measured energy of either of the two elec- photon beam. The gas jet was precooled to 40 K to create helium/- trons emitted during the process. Horizontal features of a constant, neon mixed dimers. The ions and electrons were guided by homo- KER-independent, electron energy indicate a photoelectron, while geneous electric and magnetic fields towards two position and diagonal lines for which the sum of the KER and the electron time sensitive microchannel plate detectors with delay line read- energy is constant depict measured ICD electrons. This allows to out [14]. The electron arm of the spectrometer employed McLaren identify the corresponding states (see Table 1). The direct ICD con- time focusing [15] with an overall length of 6.8 cm, while the ion tributions occur in a range of 1:5eV< KER < 5 eV. This correlates arm consisted solely of an homogeneous acceleration region of a to a range of internuclear distances of 2.9 Å to 9.6 Å. A1 marks À À length of 2.4 cm. From their times-of-flight and positions of impact the decay of He–Ne+ð2s 1). A2 and A3 mark decays of the 2p 23s we obtained the momentum vectors and thus the energy of all shake up states with symmetry 3P and 1D [19,20]. The energy posi- charged reaction fragments in coincidence. The two singly charged tions can be found in Table 1. The notations used are the ones for ions in the final state are emitted back-to-back with equal momen- the atomic states in the dissociation limit. In our earlier work by tum.