J. Phys. B: At. Mol. Opt. Phys. 33 (2000) L685–L689. Printed in the UK
PII: S0953-4075(00)16077-8
LETTER TO THE EDITOR
High resolution angle-resolved measurements of Auger
emission from the photo-excited 1s−1 3p state of Ne
Y Shimizu†‡, H Yoshida§, K Okada, Y Muramatsu†, N Saito¶, H Ohashi‡,
Y Tamenori‡, S Fritzsche+ , N M Kabachnik∗ • , H Tanaka# and K Ueda†◦
† Research Institute for Scientific Measurements, Tohoku University, Sendai 980-8577, Japan
‡ Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo 679-5198, Japan
§ Department of Physical Sciences, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
Department of Chemistry, Hiroshima University, Higashi-Hiroshima 739-8526, Japan
¶ Electrotechnical Laboratory, Tsukuba 305-8568, Japan
+ Fachbereich Physik, Universität Kassel, Heinrich–Plett–Str. 40, D–34132 Kassel, Germany
∗ Fakultät für Physik, Universität Bielefeld, D–33615 Bielefeld, Germany
# Department of Physics, Sophia University, Tokyo 102-8554, Japan
Received 2 August 2000, in final form 6 September 2000
Abstract. We present measurements of the resonant Auger electron emission at an electron kinetic
energy of ∼810 eV in the Ne atom following 1s → 3p excitation at an excitation photon energy
of 867.12 eV, with a photon band pass of 60–68 meV, an electron energy resolution of 13 meV,
and the Doppler width due to thermal motion of the sample Ne atoms of 79 meV. Under these
conditions, the overall resolution of 100–105 meV (FWHM) is much smaller than the natural
width, 250 meV, of the Ne 1s hole state, and the resonant Auger final states 2p−2 (1 D2 )3p (or 4p)
2 P, 2 D, 2 F are completely (or partially) resolved. The obtained β values and branching ratios are in
good agreement with first principles calculations by means of the multi-configuration Dirac–Fock
methods within the framework of the two-step model.
With the dramatic increase in the resolution of soft x-ray monochromators installed in
synchrotron radiation facilities in the last decade, it is now possible to carry out resonant photoemission experiments with the excitation photon band pass smaller than the natural width of
the inner-shell excited states. In this situation, the experimental width of the Auger lines is not
determined by the natural width of the inner-shell excited states but is, in general, determined
by the convolution of the excitation photon band pass and the band pass of the electron energy
analyser (Kivimäki et al 1993, Åberg and Crasemann 1994). This line-narrowing effect, often
called the ‘Auger resonant Raman’ effect, has been used for spectroscopic investigations of
the Auger final states of heavier rare gas atoms such as Ar, Kr and Xe (Aksela et al 1996a, b,
1997; see also a review article by Armen et al 2000).
In the present experiment, we use the Auger resonant Raman line-narrowing effect to
resolve the multiplet structures of the Auger final Ne+ states 2p−2 (1 D2 )np 2 P, 2 D and 2 F for
the transitions from the photo-excited 1s−1 3p state of Ne. These resonant Auger spectra of
Ne were observed by several groups with moderate resolution (Aksela et al 1989, Rubensson
et al 1996, Yoshida et al 2000, Saito et al 2000). In the present experiment, we set both the
excitation photon band pass (60–68 meV) and the band pass of the electron energy analyser
(13 meV) smaller than the Doppler width (79 meV) due to thermal motion of the sample Ne
atoms at room temperature; thus the Doppler width is a major source of the experimental
• On leave from: Institute of Nuclear Physics, Moscow State University, Moscow 119899, Russia.
◦ Author to whom correspondence should be addressed.
0953-4075/00/200685+05$30.00
© 2000 IOP Publishing Ltd
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Letter to the Editor
width (100–105 meV) of the Auger lines. From such high resolution spectra, we extract
the branching ratio and the anisotropy parameter β and compare them with those obtained
from first principles calculations using the multi-configuration Dirac–Fock methods within
the two-step model, where the excitation and decay of the resonant state are considered as
independent processes. We demonstrate that, although the line-narrowing effect used in the
present experiment is a typical example of a breakdown of the two-step model, the angular
distributions extracted from the experimental spectra are well described within the two-step
model.
The experiment has been carried out on a high-resolution plane grating monochromator
(Ishiguro et al 1999) installed on the c-branch of the soft x-ray figure-8 undulator beamline
27SU at SPring-8, the 8 GeV synchrotron radiation facility in Japan. The excitation photon
band pass Ehν is 60–68 meV at Ehν = 867.12 eV. The direction of the linear polarization
vector for the first-order harmonic photon generated by the figure-8 undulator is horizontal,
whereas that of the 0.5th-order harmonic photon is vertical (Tanaka and Kitamura 1996). Thus
one can perform angle-resolved electron spectroscopy with an electron spectrometer fixed in
the horizontal direction, switching the direction of the polarization vector from horizontal to
vertical and vice versa.
The angle-resolved high-resolution electron spectroscopy system employed in the present
experiment consists of an SES-2002 hemispherical electron spectrometer, a GC-50 tunable
gas cell and a differentially pumped chamber manufactured by Gammadata Scienta. The
analyser is fixed in the horizontal direction and operated at a pass energy of 20 eV, resulting
in an electron energy resolution Ee ∼ 13 meV. The Auger emission from the photo-excited
1s−1 3p state of Ne as well as from the photo-ionized 1s−1 state of Ne+ is recorded in series for
both horizontal and vertical directions of polarization. Knowing that the Auger emission from
the photo-ionized 1s−1 state of Ne+ is isotropic (Kabachnik and Sazhina 1984), we obtain the
angular distribution for the Auger emission from the photo-excited 1s−1 3p state of Ne. The 2s
and 2p photoelectrons of Ne are also measured and the degree of light polarization is confirmed
to be better than 0.98 for both directions.
Figure 1 shows a portion of the electron spectra of the resonant Auger transitions from the
Ne 1s−1 3p state to the final Ne+ 2p−2 (1 D2 )3p 2 D, 2 P and 2 F states. The spectra are recorded at
a photon energy of 867.12 eV for horizontal (upper) and vertical (lower) polarizations. Similar
spectra are recorded several times in two separate beamtimes and all of them fit well to three
Gaussian profiles of 100–105 meV FWHM, as shown in figure 1. This experimental width
is in fact a convolution of the three Gaussian components: the incident photon band pass of
60–68 meV, the band pass of the electron energy analyser, 13 meV, and the Doppler width,
79 meV, due to thermal motion of the sample Ne atoms. We have extracted the branching ratio
for the 2p−2 (1 D2 )3p 2 D, 2 P and 2 F final states as well as the angular anisotropy parameters β
from several sets of data similar to figure 1. The resulting values are given in table 1.
Figure 2 shows a portion of the electron spectra of the resonant Auger transitions to the
Ne+ 2p−2 (1 D2 )4p 2 D, 2 P and 2 F states. Here 2p−2 (1 D2 )4p 2 D and 2 P completely overlap, but
2p−2 (1 D2 )4p 2 F can be seen separately from 2 D and 2 P. Note also that there are some unknown
weak components on the higher kinetic energy side of 2p−2 (1 D2 )4p 2 F. Similar spectra are
recorded several times and again all of them fit well to four Gaussian peaks with 100–105 meV
FWHM, which represent one 2 F peak, one overlapping peak 2 D + 2 P, and two unknown peaks.
The extracted branching ratios and β are given in table 1. The angular anisotropy parameter
β of the resonant Auger transition to the Ne+ 2p−2 (1 S0 )3p 2 P state is also given in table 1.
In order to analyse the experimental findings further, we have carried out first principles
calculations for the branching ratios and angular anisotropy parameters β using the multiconfiguration Dirac–Fock (MCDF) approach within the two-step model. The Auger decay
Letter to the Editor
L687
1.2
horizontal
2
1.0
F
0.8
2
Intensity (arb. units)
0.6
P
0.4
0.2
2
D
0.0
vertical
0.8
0.6
0.4
0.2
0.0
811.0
811.2
811.4
811.6
811.8
Kinetic energy (eV)
Figure 1. A part of the electron spectra of the resonant Auger transitions from the Ne 1s−1 3p state
to the final Ne+ of the 2p−2 (1 D2 )3p 2 D, 2 P and 2 F states recorded at a photon energy of 867.12 eV
with horizontal (upper) and vertical (lower) polarizations.
Table 1. Energies, branching ratios and angular anisotropy parameters β for some resonant Auger
electron transitions from the initial 1s−1 3p state of Ne
Experiment
Ek (eV)
Final state
Ratio
811.53
811.29
811.17
2p−2 (1 D)3p 2 F
2p−2 (1 D)3p 2 P
2p−2 (1 D)3p 2 D
807.70
2p−2 (1 S)3p 2 P
806.27
806.17
2p−2 (1 D)4p 2 F
2p−2 (1 D)4p 2 D, 2 P
LS
MCDF
β
Ek (eV)
Ratio
0.47 ± 0.02
0.27 ± 0.05
0.18 ± 0.01
0.98 ± 0.07
0.35 ± 0.02 −0.95 ± 0.06
812.52
812.34
812.19
0.47
0.19
0.34
0.07 ± 0.08
808.02
0.55 ± 0.03
0.33 ± 0.06
0.45 ± 0.03 −0.42 ± 0.07
β
0.286
0.95
−0.98
Ratio
0.47
0.2
0.33
0.013
β
0.286
1.0
−1.0
0.0
0.47
0.53
0.286
−0.24
amplitudes are calculated using the computer program package RATIP (Fritzsche 2000)
based on the MCDF atomic structure code GRASP92 (Parpia et al 1996). Both initial state
configuration interaction (ISCI) and final ionic state configuration interaction (FISCI) are
taken into account. In the initial state there are two strongly mixed resonant states with
J = 1, with dominant configuration 1s−1 3p, which may be excited from the ground state of
the Ne atom. According to our calculations, however, the dipole excitation strength for one
of the two states is 500 times larger than that for the other. Therefore, we ignored the second
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Letter to the Editor
4
horizontal
3
Intensity (arb. units)
2
1
0
vertical
2
D + 2P
2
3
F
2
1
0
806.0
806.2
806.4
806.6
Kinetic energy (eV)
Figure 2. A part of the electron spectra of the resonant Auger transitions from the Ne 1s−1 3p state
to the final Ne+ of the 2p−2 (1 D2 )4p 2 D, 2 P and 2 F states recorded at a photon energy of 867.12 eV
with horizontal (upper) and vertical (lower) polarizations.
one and considered the decay of only one resonance to different channels. The initial state
wavefunction expansion consists of 422 configuration state functions (CSF) which include all
possible single and double excitations within the configuration space of 1s, 2s, 2p-, 2p, 3s,
3p-, 3p, 3d-, 3d orbitals. For the description of the final ionic states (J = 1/2, . . . , 7/2) with
the dominant configuration 2p−2 3p we took into account 433 CSF which include all single
and double excitations within the configuration space of 2p-, 2p, 3s, 3p-, 3p, 3d-, 3d, 4p-, 4p
orbitals. The continuum wavefunctions for the Auger electron were calculated in the potential
of the final ion. The results of calculations are compared with the experimental values in table
1. For the first three transitions, which correspond to the 2p−2 (1 D2 )3p states, the calculated
values of the transition energies, branching ratios and anisotropy parameters are in excellent
agreement with experiment. The last two columns in table 1 contain the branching ratios and
β parameters calculated in pure LS coupling, assuming that the d-wave electron emission
strongly dominates (this follows from our detailed calculations). In this case, the branching
ratios are purely statistical and β values are independent of the Auger amplitude. One can
see that for these particular transitions the predictions of the LS coupling are very close to the
more elaborate calculation. Similar agreement is obtained for the transition to the 2p−2 (1 S0 )3p
state for which the s-wave strongly dominates resulting in β = 0.
It is interesting to compare the above ‘diagram’ transitions with the shake modified ones
to the states with the dominant configuration 2p−2 (1 D2 )4p. The latter states are strongly
perturbed by the configuration interaction. The 2 P and 2 D states are practically overlapped.
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L689
The branching ratio as well as the β parameters deviate considerably from those for the diagram
transitions. A comparison with the prediction of the pure shake model in the LS approximation
(last two columns of table 1) confirms that FISCI is important for describing these states. Direct
calculations within the MCDF approach show that in order to reproduce the energies of the
transitions it is necessary to also include the 4d and 4d- orbitals into the active set. This
corresponds to the final state wavefunction expansion of 5918 CSF. Unfortunately, the present
version of our program does not permit us to calculate the branching ratios and β values for
these transitions. This work is in progress.
In conclusion, using the facilities described above, we have measured the first high
resolution angle-resolved spectra of resonant Auger electrons from the 1s−1 3p excitation in
Ne. The results are in excellent agreement with the ab initio MCDF calculations.
The experiment was carried out with the approval of the SPring-8 program advisory committee
(proposal no 2000A0010-NM-np). The authors are grateful to the staff of SPring-8 for their
help and F Koike for discussion. NMK is grateful to Bielefeld University for hospitality.
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