Measurement and ab initio calculation of the Ne photoabsorption spectrum
in the region of the K edge
M. Coreno,
1
L. Avaldi,
1
R. Camilloni,
1
K. C. Prince,
2
M. de Simone,
3
J. Karvonen,
4
R. Colle,
5
and S. Simonucci
6
1
IMAI del CNR, Area della Ricerca di Roma, 00016 Monterotondo, Italy
2
Sincrotrone Trieste, 34012 Trieste, Italy
3
Dipartimento di Fisica ‘‘E. Amaldi,’’ III Universita ` di Roma, Roma, Italy
4
Physics Department, University of Oulu, Oulu, Finland
5
Dipartimento di Chimica Applicata, Universita ` di Bologna, 40136 Bologna, Italy
and Scuola Normale Superiore, 56100 Pisa, Italy
6
Dipartimento di Matematica e Fisica, Unita ` INFM, Universita ` di Camerino, Camerino (Mc), Italy
Received 1 July 1998; revised manuscript received 18 September 1998
The Ne photoabsorption spectrum in the region of the K edge has been measured with unprecedented energy
resolution. The results have been interpreted by using an ab initio method that predicts natural linewidths, as
well as relative intensities and positions of the different 1 s →np (3 n 6) transitions. The remarkable
agreement between theory and experiment improves the spectroscopic characterization of these inner-shell
excited states in an energy region where only electron-energy-loss spectroscopies were considered to provide
accurate and high-resolution data. S1050-29479909002-2
PACS numbers: 32.70.-n, 32.80.Hd, 32.80.Dz
In the last decade we have witnessed a tremendous im-
provement in the energy resolution in the vacuum ultraviolet
vuv and soft x-ray regions. This has been achieved by us-
ing new types of monochromators 1,2 at beam lines of
synchrotron radiation sources, equipped with undulators and
wigglers 3–5. Due to this improved energy resolution, that
is comparable with or even better than the natural width of
inner-shell excited states, the spectroscopic information ob-
tained in photoabsorption experiments has significantly su-
perseded the quality of the results of previous high-
resolution electron-energy-loss studies 6. Such detailed
studies are now questioning our knowledge of basic quanti-
ties, such as, for example, the natural linewidths, and suggest
that thorough investigations are needed, where high-
resolution experiments and ab initio calculations of quanti-
ties such as transition energies, natural linewidths, relative
intensities, and line shapes are combined. Here we present a
study in which the combination of high-resolution experi-
mental data and theoretical values calculated ab initio pro-
vide an accurate spectroscopic characterization of the region
near the Ne K edge.
While the energy region up to the nitrogen K edge has
already been explored at very high resolution 7,8, no com-
parable results have been previously reported near the Ne K
edge. Moreover, due to the absence of theoretical predictions
of the natural linewidths of the 1 s →np ( n 3) transitions,
reference was made to the measured 90.270.02 eV
and calculated 100.2410% natural linewidth of the
Ne
+
(1 s
-1
) state in order to evaluate the resolving power of
the different experimental setups. Some confusion on the
topic has been generated moreover by the fact that, in the
first report of a high-resolution x-ray photoelectron spectros-
copy XPS spectrum 11, a value of 0.23 eV was incor-
rectly quoted for this quantity and was later correctly given
9 as 0.27 eV. Furthermore, the low available experimental
resolution has caused a large spread in the measured line-
widths 4,5,12–19 of the 1 s →3 p transitions, anomalous for
such a simple system and definitely larger than the
Ne
+
(1 s
-1
) natural width. On the other hand, the calculated
natural linewidth of the ionic state is a semiempirical value
obtained by combining Scofield’s relativistic relaxed
Hartree-Fock calculations of the radiative transition rates
20 and fluorescence yield from the evaluation of Krause
21. In this paper we calculate ab initio natural linewidths,
relative intensities, and energies of the 1snp (3 n 6) reso-
nant states as well as those of the Ne
+
(1 s
-1
)state using a
method specifically developed for studying Auger and auto-
ionization processes in atoms and molecules 22.
The experiments were performed at the gas phase photo-
emission beam line of the Elettra storage ring. The radiation
from a 4.5-m undulator 2312.5-cm period is deflected to
the monochromator by a prefocusing mirror that focuses the
beam at the entrance slit of the monochromator in the verti-
cal plane and at the exit slit in the horizontal one. The optical
concepts of the variable angle spherical grating monochro-
mator 24 as well as the calibration procedure 25 have
been described previously. The monochromator consists of
two optical elements: a plane mirror and a spherical grating.
This design provides the considerable advantage of a fixed
focus in the experimental chamber. Five interchangeable
gratings cover the energy region 20–1000 eV. In this experi-
ment the fifth grating 1200 lines/mm has been used. Ac-
cording to the ray tracing and depending on the assumed spot
size on the monochromator a resolving power of about 7500
is expected near the Ne K edge.
A windowless ionization cell located at the end of the
beam line has been used for the photoabsorption experiment.
The gas cell is housed in a six-way stainless steel cross
mounted on an x-y manipulator to enable easy alignment on
the incident beam direction. Two plates 100 mm long and 20
mm wide are used to collect ions electrons, depending on
the bias voltage produced by the interaction of the incident
radiation with the target gas. During the present experiment
the cell was operated typically at a bias voltage of 30 V and
at a pressure of 5 10
-2
mbar. A 2-mm bore and 5-mm-long
pipe in front of the six-way cross acts as a first differential
pumping section, resulting in a pressure drop of about a fac-
PHYSICAL REVIEW A MARCH 1999 VOLUME 59, NUMBER 3
PRA 59 1050-2947/99/593/24944/$15.00 2494 ©1999 The American Physical Society