Journal of Electron Spectroscopy and Related Phenomena 155 (2007) 113–118 Electron–ion coincidence momentum spectroscopy: Its application to Ar dimer interatomic decay K. Ueda a,∗ , X.-J. Liu a , G. Pr ¨ umper a , H. Fukuzawa a , Y. Morishita b , N. Saito b a Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan b National Institute of Advanced Industrial Science and Technology (AIST), NMIJ, Tsukuba 305-8568, Japan Available online 18 October 2006 Abstract Electron–ion–ion coincidence momentum spectroscopy, a well-established tool to study the molecular-frame core-level photoelectron angular distribution, has been applied to investigate interatomic electronic decay processes in argon dimers Ar 2 after the creation of a 2p inner-shell vacancy. Some interatomic Coulombic decay (ICD) processes from an Auger-final dicationic state are identified from the coincidence measurement between the kinetic energy of the ICD electron and the kinetic energy release between Ar + and Ar 2+ . The interatomic character of the dissociation processes into Ar + –Ar + is also discussed. © 2006 Published by Elsevier B.V. Keywords: Electron–ion coincidence; Momentum spectroscopy; Interatomic Coulombic decay; Argon dimer; Dissociation 1. Introduction Electron–ion–ion coincidence momentum spectroscopy has been widely used for measurements of molecular-frame pho- toelectron angular distributions (MFPAD) [1–8]. By making appropriate assumptions about the lifetime and dissociation dynamics of the ion state, one can connect the mea- sured photoelectron–photoion vector correlations with MFPAD [9,10]. In the present paper, we demonstrate that this technique is very powerful also for the investigation of the interatomic electronic decay processes. Inner-shell ionization of atoms and molecules leads to ion formation with an energy well above the double ionization thresholds. Then the inner-shell ionized states can decay by elec- tron emission. This process is known as the Auger process [11]. The Auger spectra are generally considered as fingerprinting images of the atoms where the inner-shell hole is created (see for example [12] and references therein). About a decade ago, Cederbaum et al. [13] proposed a new mechanism of electronic decay where the environment plays a role. An isolated atom or molecule with an innerva- lence vacancy is generally not subject to Auger decay but may ∗ Corresponding author. E-mail address: ueda@tagen.tohoku.ac.jp (K. Ueda). be subject to interatomic or intermolecular Coulombic decay (ICD) if it is in close proximity to other species. ICD is rele- vant to numerous physical, chemical, and biological phenomena involving innervalence vacancies in atoms and molecules in various environments and thus of current significant interest [14–20]. In ICD, an atom with an innervalence vacancy can transfer its energy to a neighboring species which subsequently releases its energy by emitting an electron from its outervalence orbital [13,14]. In principle, ICD can take place without having an over- lap of the orbitals, via a transfer of a virtual photon. Averbukh et al., however, showed that, even in loosely bound van der Waals clusters, the orbital overlap is a crucial factor [17]. ICD can be very fast depending on the environments [20]. ICD can emerge also after atomic Auger decay in clusters [15]. Experimentally, Marburger et al. [16] identified the ICD pro- cess in 2s ionized Ne clusters for the first time. Later, Jahnke et al. [18] reported clear experimental evidence for ICD in 2s ionized Ne dimers by identifying the process unambiguously using electron–ion–ion coincidence momentum spectroscopy in which the kinetic energy of the ICD electron and the total kinetic energy release (KER) between the two Ne + ions were measured in coincidence. ¨ Oherwall et al. estimated the dependence of the ICD rate on the neon cluster size [19]. In the present paper, we describe the application of the electron–ion–ion coincidence momentum spectroscopy to the 0368-2048/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.elspec.2006.10.007