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A Hollow-Ion Resonance of Unprecedented Strength Print

A so-called hollow ion is formed when core electrons are removed or excited to higher energy levels, leaving an empty inner shell. Such states can be produced in He-, a fundamental three-electron system and prototypical negative ion. The nuclear Coulomb attraction is efficiently screened in negative ions, greatly enhancing the effects that the electrons have on each other and providing an ideal opportunity to verify and further motivate theoretical models of electron correlation. Our understanding of these basic interactions can elucidate processes of importance in many fields, from the interpretation of cosmic spectra to x-ray lasing efforts using inner-shell ionization and hollow-ion formation. At the Ion-Photon Beamline at the ALS, researchers have detected in negative helium ions a resonant simultaneous double-Auger decay of unprecedented strength, evidence of a triply excited hollow-ion state that has eluded observation for 25 years.

A Tangible Challenge

Our understanding of how electrons move within an atom is, in general, based on the assumption that individual electrons are sensitive only to the average positions of the other electrons in the atom. While it is true that electron correlation (the effect that one electron has on another) is usually insignificant compared to the nuclear attraction, it becomes especially important in negative ions that have more than their normal contingent of electrons (three instead of two for He-). These electron–electron interactions can be the key to determining the physical properties of an atom and are thus of basic importance in many fields. For example, in astronomy, the interpretation of cosmic spectra relies heavily on atomic and ionic data that must be obtained from experiments or state-of-the-art calculations. Negative ions are important in the development of gas lasers, gas discharge devices, and plasma chemistry. In addition, inner-shell ionization and hollow-atom formation are attractive for use in x-ray lasers. Here, researchers investigate what happens when photons are used to excite electrons deep inside a negative helium ion's inner (1s) shell. The resulting hollow ion, with all three of its electrons in excited states, is a particularly valuable test system, as it offers a challenging yet tangible target for future theory development.

Top: Absorption of a photon by an He- ion in the 1s2s2p 4Po ground state boosts a 1s electron into an empty 2p orbital, forming the triply excited hollow-ion 2s2p2 4P state. Bottom: In double-Auger decay, one electron decays to the 1s orbital, while the other two electrons are simutaneously ejected, forming He+.

In contrast to valence-electron excitations, decay pathways of core excited states are highly correlated phenomena, typically involving multi-electron processes such as Auger decay. States located above the double-ionization limit (such as triply excited hollow-ion states) can decay via two-electron emission in a single step if one electron is demoted with the simultaneous emission of two others (double-Auger decay). The significant challenges presented by these complex and exotic multi-electron processes to high-level theoretical models make detailed studies in computationally accessible three-electron prototype systems of prime interest.

The first experimental investigation in He- indicated that some of the observed structure was inconsistent with predictions and stimulated renewed theoretical interest. Included in these new ab initio calculations were detailed investigations of hollow-ion resonances. The positions, widths, and cross sections of these resonances present sensitive parameters for evaluating the calculations; however, such measurements remained unavailable. In addition, while the lowest triply excited quartet state in He- (the 2s2p2 4P state) was predicted 25 years ago, until now it has eluded observation. In fact, this state has not been observed in photoexcitation of any three-electron system.

At the Ion-Photon Beamline at the ALS, researchers were able to measure, for the first time, the photoexcitation widths, line shapes, and absolute cross sections of He- triply excited (hollow-ion) states. A rubidium-vapor charge-exchange ion source was used to produce a 9.96-keV He- beam in the 1s2s2p 4Po ground state. A 60-nA beam of He- was merged with a counter-propagating photon beam from ALS Beamline 10.0.1, leading to excitation of the He- from its ground state to the 2s2p2 4P, 2p3s3p 4D, and 2p3s3p 4P states. Subsequent Auger decay in the merged region led to two-electron loss. The resulting He+ ions (the signal) were deflected by a demerging magnetic field and counted to obtain the cross sections (i.e., probabilities) of the various resonances versus incident photon energy.

Measurement of double-Auger decay from the 2s2p2 4P state with the best-fit profile. The filled circle is the absolute cross-section measurement.

He+ formation following photoexcitation/photodetachment near the triply excited (a) 2p3s3p 4D and (b) 2p3s3p 4P resonances. Dotted line shows calculated values [Sanz-Vicario et al., Phys. Rev. A 65, 060703 (2002)]. Inset: Higher-statistics scan (magnified by a factor of 6) showing a previously unresolved feature (c) with the best-fit Lorentzian profile (solid curve). Filled circles are absolute cross-section measurements.

Because the 2s2p2 4P state lies below the 2s2 threshold, there is no intermediate state to accommodate sequential (two-step) Auger decay. The observed signal must therefore be due to a double-Auger process, involving all three electrons of the ion simultaneously. This represents the first observation of double-Auger decay from a photoexcited negative ion, and its strength is three to four orders of magnitude larger than similar observations in other systems. Calculations of double-Auger decay in He- are not yet available and would be of great value in improving our understanding of this unexpectedly strong resonance. Otherwise, theory is in good general qualitative agreement with the new data, except for differences in the position and shape of certain features. A fourth feature in the spectrum, resolved for the first time, is observed to be Lorentzian in shape, contrary to predictions. The researchers conclude that, while our understanding of this three-electron system has advanced considerably in the past few years, further improvements in theory are still needed.

Research conducted by R.C. Bilodeau and G. Turri (Western Michigan University and ALS); J.D. Bozek and G.D. Ackerman (ALS); A. Aguilar (ALS and University of Nevada, Reno); and N. Berrah (Western Michigan University).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES). Operation of the ALS is supported by BES.

Publication about this research: R.C. Bilodeau, J.D. Bozek, A. Aguilar, G.D. Ackerman, G. Turri, and N. Berrah, "Photoexcitation of He- hollow-ion resonances: Observation of the 2s2p2 4P state," Phys. Rev. Lett. 93, 193001 (2004).