X-ray emission–photoion coincidence spectroscopy of the CO 2 molecule at the O 1s edge A. Kivimäki a,⇑ , M. Alagia a , R. Richter b a CNR-IOM, Laboratorio TASC, 34149 Trieste, Italy b Sincrotrone Trieste, Area Science Park, 34149 Trieste, Italy article info Article history: Received 21 November 2011 In final form 7 February 2012 Available online 15 February 2012 abstract Molecular dissociation following radiative decay of O 1s hole states in CO 2 has been studied by detecting photoions in coincidence with undispersed soft X-ray emission. The production of ions after radiative decay is observed to increase from practically nothing at the O 1s ? p / excitation to the appearance of CO þ 2 , CO + ,C + and O + ions when core-valence double excitations and O 1s shake-up transitions are induced. De-excitation pathways involving radiative decay from different core–hole states are discussed in order to interpret the results. A weak CO 2þ 2 signal is attributed to the radiative Auger effect. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction X-ray fluorescence is a minor decay channel of core–hole states for molecules composed of light atoms. The X-ray fluorescence yield of the 1s 1 states in the important carbon, nitrogen and oxy- gen atoms, for instance, has been calculated to be less than 1% [1]. Despite the low probability of X-ray emission and technical diffi- culties in collecting such emission (grazing-incidence optics is needed), soft X-ray emission (SXE) spectroscopy and its resonant variety, resonant inelastic soft X-ray scattering (RIXS), have be- come versatile spectroscopic tools to investigate the electronic structure and dynamics of core-excited molecules [2]. A special advantage is that dipole selection rules govern radiative transi- tions, which makes the RIXS spectrum much simpler than the res- onant Auger electron spectrum from the same core-excited state. Recent technical developments have made it possible to achieve vibrational resolution in RIXS [3], which will further increase the potentials of this research method. The ground-state electron configuration of the CO 2 molecule is 1r 2 g 1r 2 u 2r 2 g 3r 2 g 2r 2 u 4r 2 g 3r 2 u 1p 4 u 1p 4 g 1 R þ g   ; where the 1r g and 1r u molecular orbitals (MO) correspond to the atomic O 1s orbitals, the 2r g orbital derives from the atomic C 1s orbital, and the rest are inner-valence and outer-valence orbitals. Oxygen 1s photoionization, photoexcitation and ensuing soft X- ray emission of the CO 2 molecule display some intriguing effects. Upon O 1s ionization, the almost degenerate orbitals 1r g and 1r u may be coupled via the antisymmetric stretching vibration of r u symmetry [4]. This so-called vibronic coupling leads to a strong excitation of the dipole-forbidden antisymmetric stretching vibra- tion in the O 1s photoelectron spectrum [5] and it also manifests itself, although more subtly, in the O 1s SXE spectrum [6]. Vibronic coupling breaks the D 1h symmetry of the molecule and provides a mechanism for dynamic core–hole localization in the O 1s ionized state. Similar effects also take place in the RIXS process involving the O 1s ? 2p u (p / ) excitation [7] (2p u is the lowest unoccupied MO of CO 2 ). According to the dipole selection rules, only the 1r g ? 2p u excitation should be allowed and the RIXS spectrum should show transitions to gerade final states. The experimental spectrum [7,8], however, shows both gerade and ungerade final states because the 1r u ? 2p u excitation becomes allowed via cou- pling to the antisymmetric stretching vibration. The angle-resolved SXE spectrum was measured at some photon energies across the O 1s edge in order to study whether the symmetry selectivity of X-ray emission can be used to help assign transitions in the X-ray absorp- tion spectrum [8]. Coincidence techniques have become essential in studies of molecular dissociation that occurs after Auger electron emission, see e.g. [9], as they allow to select some specific decay channels among numerous possible ones. Analogous coincidence techniques have hardly ever been practised when the core–hole decays via soft X-ray emission. We are only aware of few X-ray emission–thresh- old electron coincidence (XETECO) studies [10–13]. The main advantage of the XETECO technique is that it allows to study core photoionization process at the threshold without the post-collision interaction effect, which occurs when the core-ionized state decays via Auger electron emission and which leads to the distortion and shift of the photoelectron line. In the present study, we have detected photoions in coincidence with undispersed soft X-rays following core ionization of the CO 2 molecule. This technique 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2012.02.022 ⇑ Corresponding author. Fax: +39 040 3758400. E-mail address: kivimaki@tasc.infm.it (A. Kivimäki). Chemical Physics Letters 531 (2012) 252–256 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett