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.
Measurement of double-Auger decay from the 2s2p2 4P state with
the best-fit profile. The filled circle is the absolute cross-section
measurement.
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.
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).
ALSNews Vol. 254, June 29, 2005 |