Photoelectron recapture as a tool for the spectroscopy of ionic Rydberg states

Abstract

We use recapture of near-threshold photoelectrons by postcollision interaction with Auger electrons as an effective method for population of the high-Rydberg states of subvalence ionized Ne ions. The subsequent intermultiplet Auger transitions are detected by high-resolution electron spectrometry. The series of transitions 2p 4 ͑ 1 D͒np 2 D , 2 F → 2p 4 3 P up to n = 20 are observed and identified with the help of multiconfiguration Dirac-Fock ab initio calculations.Spectroscopic data on atomic ions are required for modeling of stellar atmospheres as well as for laboratory plasmas with their relevance for fusion reactors. Experimentally, 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 the development of improved methods of spectroscopic investigation of atomic ions. The present methods currently existing for ion spectroscopy are always experimentally very challenging. 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 example, a Ne + ion with a vacancy in the 2s subshell. A relaxation of this ion via "normal" Auger decay is energetically 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]. Especially interesting are the transitions in which the two-hole configuration is not changed but only the core multiplets are different in the initial and final states. For example, in Ne + , due to a significant singlet-triplet splitting, the 2s2p 5 ͑ 1 P͒3p states lie above the 2s2p 5 3 P level of Ne 2+ and can decay to it. Such transitions, called valence multiplet changing or intermultiplet Auger transitions, are predominant in the low kineticenergy part of the spectrum and determine the large width of the inner-valence-excited ionic states [4][5][6][7][8][9].Spectroscopic information about these two-hole oneparticle states so far was mainly obtained by studies of the satellite structure in the subvalence shell photoionization [10]. The state of the art of such ionization + excitation experiments has recently been demonstrated by Bolognesi et al. [11]. They applied the technique of high-resolution threshold photoelectron spectroscopy to study the Ne + 2p 4 nᐉ satellite states and observed several Rydberg series, up to n =26 in some series. The energy positions and the quantum defects ␦ were determined. Although 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 configurations can also be reached by resonant Auger decay of a neutral core-excited state, such as the 1s −1 3p photoexcited state in Ne. As an advantage, states can be populated by resonan...

Coherence and correlations in photoinduced Auger and fluorescence cascades in atoms

“…Examples of such cascades have been discussed in previous sections. In recent paper by De Fanis et al (2004b), a method was suggested for populating effectively a large number of highly excited Rydberg ionic states. The cascade decay of these states produces a rich spectrum of Auger transition series converging to various levels of the doubly charged ion.…”

“…Another approach for measuring the angular anisotropy of the emitted electrons was used in the experiments on the resonant Auger and second-step Auger processes in Ne 1s photoexcitation De Fanis et al, 2004bYoshida et al, 2005;Kitajima et al, 2006). These measurements were carried out using the high-resolution electron spectroscopy end-station of the soft X-ray photochemistry beam-line 27SU (Ohashi et al, 2001b) at SPring-8, the third generation SR source in Japan.…”

Isotope shift calculations for open-shell atoms and ions: an extension to the RATIP program

“…Another large group of applications of the Ratip code concerns the (Auger) electron emission from inner-shell excited or ionized atoms [35][36][37][38][39] as well as the coherence transfer in Auger cascades. For the 2 p → 4s resonantly excited argon [40], for example, we were able to show by means of systematically enlarged wave function expansions that the initial and final-state configuration interactions can be taken into account to a large extent and bring the energy and intensity distributions into much better agreement with experiment [41].…”

2pcorrelation satellites in neon clusters investigated by photoemission

“…number of different experimental techniques [1][2][3][4][5][6][7][8][9] and by theory [10], and is well understood today. An important topic of early studies on Ne satellites was the distinction between different satellite production mechanisms [3].…”