Photoelectron recapture as a novel tool for the spectroscopy of ionic Rydberg states A. De Fanis, 1 G. Pr¨ umper, 2 U. Hergenhahn ∗ , 2 M. Oura, 3 M. Kitajima, 4 T. Tanaka, 4 H. Tanaka, 4 S. Fritzsche, 5 N. M. Kabachnik † , 6 and K. Ueda 2, ‡ 1 Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo 679-5198, Japan 2 Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan 3 RIKEN, Spring-8 Sayo-gun, Hyogo 679-5198, Japan 4 Department of Physics, Sophia University, Tokyo 102-8554, Japan 5 Fachbereich Physik, Universit¨at Kassel, D–34132 Kassel, Germany 6 Fakult¨atf¨ ur Physik, Universit¨at Bielefeld, D–33615 Bielefeld, Germany We use recapture of near-threshold photoelectrons by post-collision interaction with Auger elec- trons as an effective method for population of the high Rydberg states of sub-valence ionized Ne ions. The subsequent intermultiplet Auger transitions are detected by high-resolution electron spec- trometry. The series of transitions 2p 4 ( 1 D)np 2 D, 2 F → 2p 4 3 P up to n = 20 are observed and identified with the help of the MCDF ab initio calculations. Spectroscopic data on atomic ions are required for modelling of stellar atmospheres as well as for laboratory plasmas with their relevance to fusion reactors. Exper- imentally the spectroscopic study of ions, in particular their electron spectroscopy is a challenging task. In spite of big progress in this field (see, for example, a review by West [1]) there is still a need for development of new methods of spectroscopic investigation of atomic ions. In the past decade particular interest was attracted by a special class of autoionizing states of atomic ions, the so-called inner-valence excitations [2]. Consider, for ex- ample, a Ne + ion with a vacancy in the 2s subshell. A relaxation of this ion via “normal” Auger decay is ener- getically not allowed. However, if in the process of 2s ionization another electron is excited to a 3p or higher Rydberg orbital, such “two-hole–one-particle” states may decay via the “participator” Auger transition [2, 3]. Es- pecially interesting are the transitions in which the two- hole configuration is not changed but only the core mul- tiplets are different in the initial and final states. For example, in Ne + , due to a significant singlet-triplet split- ting, the 2s2p 5 ( 1 P )3p states lie above the 2s2p 53 P level of Ne 2+ and can decay to it. Such transitions, called valence multiplet-changing or intermultiplet Auger tran- sitions, are predominant in the low kinetic energy part of the spectrum and determine the large width of the inner-valence excited ionic states [4–9]. Spectroscopic information about these “two-hole–one- particle” states so far was mainly obtained by studies of the satellite structure in the sub-valence shell photoion- ization [10]. The state-of-the-art of such ionization + ex- citation experiments has recently been demonstrated by ∗ On leave from Max-Planck-Institut f¨ ur Plasmaphysik, 85748 Garching, Germany † On leave from Institute of Nuclear Physics, Moscow State Univer- sity, Moscow 119899, Russia Bolognesi et al. [11]. They applied the technique of high- resolution threshold photoelectron spectroscopy to study the Ne + 2p 4 nℓ satellite states and successfully observed several Rydberg series, up to n = 26 in some series. The energy positions and the quantum defects δ were deter- mined. Though threshold photoelectron spectroscopy is indeed powerful, its application is limited to states which are coupled to the direct photoionization channel. In principle, singly charged inner-valence excited con- figurations can also be reached by resonant Auger de- cay of a neutral core-excited state, such as the 1s -1 3p photoexcited state in Ne. As an advantage, by reso- nant Auger decay states can be populated, which are hardly accessible via outer-shell photoionization. The exploitation of resonant Auger decay for spectroscopic purposes so far has been limited by the broad photon bandwidths and low resolution of energy analysis for high energy Auger electrons. However, recent develop- ments of high-resolution soft X-ray monochromators and of electron energy analyzers have improved this situation dramatically. Recent experiments employing a Doppler- free resonant Raman Auger technique [12] for studying Ne + 2s2p 5 np states have reached a resolution comparable with the natural life-time width of the states considered [13, 14]. In this experiment, the multiplet structure of the inner-valence states has been resolved. Some of these states lie above the double ionization threshold and there- fore can autoionize, emitting Auger electrons of compar- atively low energy (10-30 eV). The spectroscopy of these second-step Auger electrons provides independent infor- mation about the inner-valence excitations competing in quality with the study of the first-step Auger decay [15]. The only drawback of the resonant Raman Auger tech- nique is that in this way it is not possible to excite higher members of the Rydberg series. Highly excited ionic Rydberg states can be populated by a recapture process in near threshold inner-shell pho- toionization. When the atom is photoionized just above