Measurement of the first ionization potential of astatine by laser ionization spectroscopy

1] CERN, CH-1211 Genève, Switzerland [2] Institut für Physik, Johannes Gutenberg-Universität Mainz, D-55128 Mainz, Germany.
Nature Communications (Impact Factor: 11.47). 05/2013; 4:1835. DOI: 10.1038/ncomms2819
Source: PubMed
ABSTRACT
The radioactive element astatine exists only in trace amounts in nature. Its properties can therefore only be explored by study of the minute quantities of artificially produced isotopes or by performing theoretical calculations. One of the most important properties influencing the chemical behaviour is the energy required to remove one electron from the valence shell, referred to as the ionization potential. Here we use laser spectroscopy to probe the optical spectrum of astatine near the ionization threshold. The observed series of Rydberg states enabled the first determination of the ionization potential of the astatine atom, 9.31751(8) eV. New ab initio calculations are performed to support the experimental result. The measured value serves as a benchmark for quantum chemistry calculations of the properties of astatine as well as for the theoretical prediction of the ionization potential of superheavy element 117, the heaviest homologue of astatine.

Full-text

Available from: Ephraim Eliav
ARTICLE
Received 21 Aug 2012 | Accepted 27 Mar 2013 | Published 14 May 2013
Measurement of the first ionization potential
of astatine by laser ionization spectroscopy
S. Rothe
1,2
, A.N. Andreyev
3,4,5,6
, S. Antalic
7
, A. Borschevsky
8,9
, L. Capponi
4,5
, T.E. Cocolios
1
, H. De Witte
10
,
E. Eliav
11
, D.V. Fedorov
12
, V.N. Fedosseev
1
, D.A. Fink
1,13
, S. Fritzsche
14,15,w
, L. Ghys
10,16
, M. Huyse
10
, N. Imai
1,17
,
U. Kaldor
11
, Yuri Kudryavtsev
10
,U.Ko
¨
ster
18
, J.F.W. Lane
4,5
, J. Lassen
19
, V. Liberati
4,5
, K.M. Lynch
1,20
, B.A. Marsh
1
,
K. Nishio
6
, D. Pauwels
16
, V. Pershina
14
, L. Popescu
16
, T.J. Procter
20
, D. Radulov
10
, S. Raeder
2,19
, M.M. Rajabali
10
,
E. Rapisarda
10
, R.E. Rossel
2
, K. Sandhu
4,5
, M.D. Seliverstov
1,4,5,12,10
, A.M. Sjo
¨
din
1
, P. Van den Bergh
10
,
P. Van Duppen
10
, M. Venhart
21
, Y. Wakabayashi
6
& K.D.A. Wendt
2
The radioactive element astatine exists only in trace amounts in nature. Its properties can
therefore only be explored by study of the minute quantities of artificially produced isotopes
or by performing theoretical calculations. One of the most important properties influencing
the chemical behaviour is the energy required to remove one electron from the valence shell,
referred to as the ionization potential. Here we use laser spectroscopy to probe the optical
spectrum of astatine near the ionization threshold. The observed series of Rydberg states
enabled the first determination of the ionization potential of the astatine atom, 9.31751(8) eV.
New ab initio calculations are performed to support the experimental result. The measured
value serves as a benchmark for quantum chemistry calculations of the properties of astatine
as well as for the theoretical prediction of the ionization potential of superheavy element 117,
the heaviest homologue of astatine.
DOI: 10.1038/ncomms2819
OPEN
1
CERN, CH-1211 Gene
`
ve, Switzerland.
2
Institut fu
¨
r Physik, Johannes Gutenberg-Universita
¨
t Mainz, D-55128 Mainz, Germany.
3
University of York, York YO10
5DD, UK.
4
University of the West of Scotland, School of Engineering, Paisley PA1 2BE, UK.
5
The Scottish Universities Physics Alliance (SUPA), Glasgow,
G12 8QQ UK.
6
Advanced Science Research Center, Japan Atomic Energy Agency, Tokai 319-1195, Japan.
7
Department of Nuclear Physics and Biophysics,
Comenius University, 84248 Bratislava, Slovakia.
8
Centre for Theoretical Chemistry and Physics, The New Zealand Institute for Advanced Study,
Massey University Auckland, Private Bag 102904, 0745 Auckland, New Zealand.
9
Helmholtz Institute Mainz, D-55128 Mainz, Germany.
10
IKS KU Leuven,
Department of Physics and Astronomy, Celestijnenlaan 200D, B-3001 Leuven, Belgium.
11
School of Chemistry, Tel Aviv University, 69978 Tel Aviv,
Israel.
12
PNPI NRC KI, Orlova Roscha, 188300 Gatchina, Russia.
13
Ruprecht-Karls Universita
¨
t, Seminarstrae 2, D-69117 Heidelberg, Germany.
14
GSI Helmholtzzentrum fu
¨
r Schwerionenforschung, Planckstrae 1, D-64291 Darmstadt, Germany.
15
Frankfurt Institute for Advanced Studies (FIAS),
D-60438 Frankfurt am Main, Germany.
16
Belgian Nuclear Research Centre SCK CEN, Boeretang 200, B-2400 Mol, Belgium.
17
High Energy Accelerator
Research Organization (KEK), Oho 1-1, Tsukuba 305-0801, Japan.
18
Institut Laue-Langevin (ILL), 6 rue Jules Horowitz, F-38042 Grenoble, France.
19
TRIUMF, Accelerator Division, 4004 Wesbrook Mall, Vancouver, BC, V6T2A3 Canada.
20
University of Manchester, School of Physics and Astronomy,
Manchester M13 9PL, UK.
21
Institute of Physics, Slovak Academy of Sciences (IP SASc), Du
´
bravska
´
cesta 9, 845 11 Bratislava, Slovakia. w Present address:
Helmholtz-Institute Jena, Fro¨belstieg 3, D-07743 Jena, Germany. Correspondence and requests for materials should be addressed to S.Ro
(email: sebastian.rothe@cern.ch)
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Page 1
A
statine (At) is the rarest naturally occurring element on
earth with an estimated total abundance of 0.07 g (ref. 1).
There is significant interest in the pharmaceutical use of
the isotope
211
At because its decay properties make it an ideal
short-range radiation source for targeted alpha therapy in cancer
treatment
2–5
. As only ultra-trace quantities can be used for
experimental chemical studies, computational chemistry is an
invaluable resource for improving our understanding of At
chemistry
6,7
. Such computations benefit greatly from validation
provided by experimental measurements, as summarized in
Brown
8
and Nefedov et al.
9
One of the most important properties
is the binding energy of the outermost valence electron which
determines the chemical reactivity of an element and, indirectly,
the stability of its chemical bonds in compounds. This energy is
also referred to as the first ionization potential (IP). Of all the
naturally occurring elements, At is the only one whose IP has not
been experimentally deduced. However, the IP(At) had been
calculated through the use of various theoretical treatments
10–15
.
Early extrapolations from the IP data for neighbouring
elements gave values of IP(At) from 9.2 to 10.4 eV
10–13
,
however, recent, more precise theoretical treatments
14,15
cluster
around 9.3 eV (Table 1). Regarding the prediction of IPs of
heavier elements and in particular the superheavy elements,
the inclusion of relativistic and correlation effects are of major
importance. A measurement of the IP for At, the nearest lighter
homologue of the recently discovered element with Z ¼ 117
(ref. 16), therefore provides an experimental basis for developing
theoretical methods which incorporate these effects.
The most precise method for determining IPs is the analysis of
the converging series of atomic energy levels with a high principal
quantum number n, known as Rydberg states. The Rydberg series
of an atom can be observed most easily in the ionization spectrum
of the final transition of a step-wise resonant ionization scheme.
Before this work, the only available data on the optical spectrum
of At came from absorption spectroscopy of a 70 ng sample
of the artificially produced, longest living (up to 8.1 h) isotopes
209–211
At. Two optical lines at 216.225 and 224.401 nm were
observed
17
. This rudimentary knowledge of the atomic spectrum
and the lack of a suitable spectroscopic setup in proximity of an
isotope production facility limited the feasibility of a
measurement of the IP of At.
In this article, we report on in-source laser resonance
ionization spectroscopy studies of At isotopes using the resonance
ionization laser ion source (RILIS)
18,19
of the ISOLDE radioactive
ion beam facility
20
at CERN. As an initial step, the first
experimental value of the IP of the At atom is derived from a
direct measurement of the photoionization threshold. This result
facilitates the search for additional atomic transitions during the
development of an efficient three-colour resonance laser
ionization scheme, necessary for the production of pure and
intense At
þ
ion beams at ISOLDE. The use of a suitable second-
step atomic transition enables resonance ionization spectroscopy
of high-lying Rydberg states, resulting in a more precise
determination of the IP(At). The extracted value serves as an
important benchmark for ab initio calculations in atomic physics
and quantum chemistry. New calculations of the IP of At are
performed in this work which compare well with the
experimental value.
Results
First step transitions. The tentative assignments of the two
spectral lines at l
1
¼ 216 and 224 nm to transitions from the At
atomic ground state, as reported in McLaughlin
17
, were
confirmed by resonance ionization laser spectroscopy in a two-
colour laser scheme denoted as {l
1
; 273}, cf. Fig. 1a.
Figures 2a,b show laser scans across both of the two first step
transitions. The photo-ion rate of
199
At, obtained from a-decay
rates, measured by the silicon detector of the Windmill system
(see Methods section), was recorded for successive laser scanning
steps of l
1
while l
2
was fixed at 273 nm, chosen such that the total
photon energy is higher than most of the theoretical expectations
of the IP of At (cf. Table 1).
Ionization threshold. The two-colour ionization schemes
{216; l
2
} and {224; l
2
}, as illustrated in Fig. 1b, were applied for
scanning the second-step laser wavelength within the range
l
2
¼ 312–335 nm, which encompasses the most recent published
Table 1 | Calculated and experimental values (last two rows)
for the first IP of astatine.
Author Year Method IP (eV)
Finkelnburg et al.
10
1950 Extrapolation 9.5(2)
Varshni et al.
11
1953 Extrapolation 10.4
Finkelnburg et al.
12
1955 Extrapolation 9.2(4)
Kiser et al.
13
1960 Extrapolation 9.5
Mitin et al.
14
2006 DFT 9.24
Chang et al.
15
2010 MCDF, up-shift 9.35(1)
This work MCDF 9.24(15)
This work DC CCSD(T) 9.307(25)
This work Expt. (threshold) 9.315(12)
This work Expt. (Rydberg) 9.31751(8)
Abbreviations: CCSD, Coupled-cluster single and double excitation; DC, Dirac-Coulomb; IP,
ionization potential; DFT, density-functional theory; MCDF, multi-configuration Dirac-Fock.
6p
4
ns
6p
4
nd
75,151 cm
–1
58,805 cm
–1
57,277 cm
–1
57,268 cm
–1
57,157 cm
–1
46,234 cm
–1
44,549 cm
–1
0 cm
–1
6p
4
np
6p
4
7s
6p
5
J = 3/2
J = 5/2
J = 3/2
E
E
IP
610–630 nm
532 nm
532 nm
312–335 nm
273 nm
710–915 nm
216 nm
224 nm
795 nm
Figure 1 | Schematic overview of the investigated ionization paths.
The assignments of the three lowest levels is according to
17
.(a) Verification
of the two transitions from the ground state at l
1
¼ 216 and 224 nm.
(b) Ionization threshold: scan of the ionizing laser wavelength l
2
.
(c) Development of a three-colour scheme: scan of l
2
in the infra-red
region for second excited states starting with l
1
¼ 216 nm first step.
(d) Verification of the levels found by exciting via the l
1
¼ 224 nm first step.
(e) Rydberg series: a wavelength scan of the ionizing laser (l
3
) in the visible
range using the {216; 795; l
3
} excitation path.
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theoretical predictions of the IP including the value from Chang
et al.
15
, privately communicated before publication. Figure 2c
reveals the onset of photoionization at a total excitation energy of
around 75,000 cm
1
. The position of the photoionization
threshold, denoted as the vertical dotted line, was determined
as the intersection of the slope (blue tilted line) at the inflection
point of a fitted sigmoid curve (red line) and its upper level
(blue horizontal line). Using this method we obtain
IP
thr.
(At) ¼ 75,129(95) cm
1
, corresponding to 9.315(12) eV.
The quoted uncertainty arises from the low statistics.
Three-colour ionization scheme. The development of a three-
colour ionization scheme, which required a search for inter-
mediate atomic transitions, resulted in an increased ionization
efficiency and enabled high-resolution laser scans across the
ionization threshold. The search for new second-step transitions
(l
2
) to higher lying atomic states, was performed at the TRIUMF-
ISAC radioactive ion beam facility with the TRILIS laser ion
source
21
. The fundamental output of a widely tunable grating-
based Ti:sapphire (Ti:Sa) laser
22
was used to scan l
2
between 710
and 915 nm in a three-colour scheme {216; l
2
; 532}, see Fig. 1c.
Three new transitions were discovered. These transitions were
subsequently confirmed at ISOLDE where ionization routes via
both the l
1
¼ 224 nm and l
1
¼ 216 nm first steps were studied
by laser spectroscopy of the
205,196
At isotopes. In total six
new transitions between excited states of At have been observed.
An overview is given in Fig. 1c,d, further details of these will be
published elsewhere.
Spectroscopy of Rydberg states. The {216; 795; l
3
} excitation
path was used for the scan of the third step (l
3
) across the
ionization threshold, as given in Fig. 1e. For this study, the
ISOLDE RILIS Ti:Sa laser system was arranged to produce the
two laser beams necessary to populate the E
00
level (cf. Fig. 1). The
transition energies of the intermediate states are n
0
¼ 46,234.0(3)
cm
1
and n
00
¼ 12,571.5(3) cm
1
, obtained from Gaussian fits of
several laser scans across the 216 and 795 nm resonances of
205
At.
From these we obtain E
00
¼ n
0
þ n
00
¼ 58,805.4(5) cm
1
. A syn-
chronized dye laser provided the third step laser radiation and
was set to a tuning range encompassing the previously localized
region of the ionization threshold. The dye laser was scanned
whilst the
205
At ion current was directly measured with a Faraday
cup (FC). A series of more than 30 Rydberg levels was observed in
the l
3
scan range of 15,000–16,300 cm
1
, as shown in Fig. 3a.
The Rydberg atoms were ionized inside the hot cavity ion
source by the residual laser light, black-body radiation or by
collision processes. The Rydberg levels of lower principal quan-
tum numbers reveal resolved multiplet structures, attributed to
different fine structure components, with decreasing splitting as
the ionization limit is approached. A single-Gaussian fit was
applied to each resolved peak and for each unresolved multiplet.
The transition energies (n
n
) of each main peak are plotted
against their principal quantum number n as shown in the
Fig. 3b. These data are sufficient for precise determination of the
ionization limit E
00
lim.
¼ IP(At) E
00
for this excited state using the
Rydberg formula: n
n
¼ E
00
lim:
R
M
ðn dÞ
2
, where n
n
is the transition
1.0
10
1
0.1
Count rate (s
–1
)
0.01
75,000 76,000
Total photon energy (cm
–1
)
77,000
Sigmoid fit
Slope
Upper level
Ionizing threshold
Uncertainty
78,000
0.5
0.0
6
4
2
0
44,548 44,552
Wavenumber (cm
–1
)
46,231 46,234
Wavenumber (cm
–1
)
Count rate (s
–1
)Count rate (s
–1
)
Figure 2 | Laser scans for the two-step ionization scheme. Panels a and b show laser scans across the two optical transitions at l
1
¼ 216 and l
1
¼ 224 nm,
obtained using the ionization path shown in Fig. 1a and a-decay detection of the photo-ionized
199
At. (c) Measurement of the ionization threshold.
The count rate of a-particles from the decay of
199
At is registered for different total photon energies of the applied laser beams used in the
two-colour schemes {216; l
2
} and {224; l
2
} (cf. Fig. 1b).
0
16,150
25 30 35 40 45 50 55
16,200 16,250 16,300 16,350
Wavenumber (cm
–1
)
Principal quantum number, n
10
Ion current (pA)
Wavenumber (cm
–1
)
20
30
26 27 28 29
Principal quantum
number
16,300
16,200
16,100
0.3
0.0
–0.3
56
Figure 3 | Determination of the ionization limit of the excited state. (a)
Laser scan of the ionizing laser across the ionization threshold of At. The
observed Rydberg resonances converge towards the ionization limit. The
indicated principal quantum numbers (n) are based on the assumption of
an nd series. (b) Analysis of the observed Rydberg spectrum. The top panel
shows the position of 31 high-lying Rydberg levels. The Rydberg formula is
fitted to the positions of the main peaks in the multiplets belonging to
different n. The residuals of the fit are shown in the bottom panel.
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energy from the excited state E
00
to a Rydberg level with principal
quantum number n of the valence electron, d is
the quantum defect and R
M
is the reduced-mass Rydberg
constant for
205
At.
Following the trends of quantum defects of nd and ns series
of bromine and iodine
23
, we constrain the integer part bdc to 5 or
3 for ns or nd in At, respectively. The assumption of an nd series
allows us to assign principal quantum numbers to the Rydberg
levels. However, the result of the analysis is unaffected by this
choice. The obtained peak positions were fitted with the two free
parameters E
00
lim.
and d. The result of the fit is shown as the solid
curve in the top panel of Fig. 3b. Residuals with error bars
accounting for the statistical error from the Gaussian fit of each
resonance and the uncertainties of the wavelength measurements
are shown in the bottom panel of Fig. 3b. This approach yields
E
00
lim.
¼ 16,345.4(2) cm
1
and d ¼ 3.16. The sum E
00
þ E
00
lim
gives
the first IP of At as IP
Ryd.
(At) ¼ 75,150.8(7) cm
1
, corresponding
to 9.31751(8) eV. Uncertainties arise from the statistical errors of
0.03 cm
1
from the Rydberg series fit and 0.09 cm
1
for the
determination of E
00
. The systematic error, covering the
wavelength measurements, unresolved hyperfine structure and
data acquisition, is estimated to 0.6 cm
1
. This result is in perfect
agreement with the preparatory direct threshold measurement
but is two orders of magnitude more precise.
Ab initio calculations. Early predictions
10–13
of the IP of At at
9.2–10.4 eV were based on known IPs of other neighbouring
elements and on spectroscopic data of other halogens.
A compilation of all predictions involving recent
calculations
14,15
, as well as theoretical and experimental values
obtained within this work, is given in Table 1. To strengthen this
work, the IP was calculated before the experiment with the use of
two relativistic methods, the multi-configuration Dirac-Fock
(MCDF) and the Dirac-Coulomb (DC) coupled-cluster
approach. The MCDF method provides a conceptually simple
approach for computing excitation energies and IPs of heavy
elements that can be improved systematically and is applicable to
quite different shell structures of the atoms and ions. Although
this method profits from a systematic enlargement of the active
configuration space, (deep) core-valence and core-core
correlations are usually not taken into account. Therefore, in
recent calculations by Chang et al.
15
, the (predicted) IP was
semiempirically up-shifted by 0.3 to 9.35 eV, based on similar
computations for the homologue elements and the comparison
with known IPs from experiment.
In a more systematic treatment, we now took into account all
triple excitations within the {6s,6p,6d,7s,7p} active space as well as
all single and double excitations to the {ns,np,nd,nf,ng; n ¼ 7,8,9}
orbitals, starting from the 6s
2
6p
5
and 6s
2
6p
4
reference
configurations for neutral and singly charged At, respectively.
In addition, core-polarization contributions from the n ¼ 4 and
n ¼ 5 (fully) occupied shells were also incorporated. The most
precise value obtained using this method was IP(At) ¼ 9.24(15) eV
(see Table 1), in reasonable agreement with previous estimates.
The uncertainty quoted indicates that still further correlations are
expected to contribute to the IP.
An alternative approach was based on the solution of the DC
Hamiltonian
24
in combination with the coupled-cluster approach
with single, double and perturbative triple excitations (CCSD(T))
for treatment of electron correlation. The IP was obtained by
taking the energy difference between the calculated energies of the
neutral element and its cation. The Dirac08 program package
25
was used to perform the calculations. The uncontracted Faegri
basis sets
26
were used for At as well as for iodine (I) as a
homologue, consisting of 26s 24p 18d 14f 7g 4h 2i orbitals
for I and 28s 26p 20d 15f 7g 4h 2i orbitals for At. The convergence
of the calculated IPs with respect to the size of the basis set was
verified. The outer 38 electrons of I and 54 electrons of At were
correlated; virtual atomic orbitals with energies higher than 50
Hartree (E1,360 eV) were discarded. The contribution of the
Breit term was assessed by performing Fock-space coupled-
cluster calculations, and was found to be small: 19 cm
1
for I and
5cm
1
for At. Higher order QED effects should thus be
negligible. The final IPs, corrected for the Breit contribution, are
84,095 cm
1
for I and 75,069 cm
1
for At, corresponding to
9.307 eV. The IP value for I closely matches the experimental one
of 84,295 cm
1
(ref. 23), and similar agreement is shown
between the newly measured and CCSD(T) values for At.
Discussion
We apply the technique of in-source laser spectroscopy to
artificially produced At isotopes. The discovered series of high-
lying Rydberg states enables us to deduce the IP with high
accuracy. This first experimental value of the binding energy
of the valence electron of At serves as a benchmark for
the prediction of the chemical properties of At as well as for
predictions of atomic physics properties of the superheavy
elements. The newly available, efficient three-step laser ionization
scheme for At enables further experiments on At isotopes at ISOL
Dipole magnet
Target
Hot cavity ion source
Target material Reaction products Ions
Particle
detection
Laser beams
Extractor
210At
+
205At
+
199At
+
At
60 kV
U
U
U
K
Po
Tl
Fr
Au
U
U
Fr
U
Xe
At
Po
At
At
At
U
U
U
At
199At
+
U
U
At
At
Spallation
Protons
U
U
U
U
Figure 4 | In-source laser spectroscopy at ISOLDE. Protons impinge on a thick target inducing nuclear reactions (for example, spallation) in which
different isotopes of various chemical elements are produced. The reaction products diffuse and effuse towards the hot cavity ion source, into which the
precisely tuned laser beams are focussed. Step-wise resonance laser ionization creates singly charged ions of the desired element. These photo-ionsare
extracted and accelerated by applying a high-voltage potential. The ion beam of the isotope of interest is selected by dipole magnets and guided to a
suitable detection setup.
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facilities world-wide. High-resolution in-source laser spectro-
scopy of isotope shifts and hyperfine structures
27
as well as an
extension of the study of b-delayed fission
28
to the At isotopes is
in preparation. Two theoretical methods are applied to calculate
the IP(At). The higher accuracy of the CCSD(T) result, compared
with the MCDF value is due to better treatment of dynamic
correlation within the former approach. The CCSD(T) approach
is particularly suitable in this case, as the ground states of both At
and At
þ
may be approximated by single determinants. The
dynamic correlation, which is handled better by CCSD(T), is the
dominant effect.
Methods
In-source laser spectroscopy. Astatine isotopes were produced at the CERN
ISOLDE radioactive ion beam facility (see ref. 20) by directing a 1.4-GeV pulsed
proton beam of up to 2 mA from the CERN proton synchrotron booster (PSB) onto
a thick target of uranium carbide (UC
x
) or thorium dioxide (ThO
2
). Figure 4
illustrates the in-source laser spectroscopy method. The products of the proton-
induced nuclear reactions diffuse into a hot (E2,000 °C) metal tubular cavity
within which the neutral atoms are selectively photo-ionized by spatially over-
lapped beams of the RILIS lasers, wavelength tuned to the corresponding transi-
tions of a photoionization sc heme. Ions are extracted and accelerated by an
electrostatic potential of up to 60 kV. The isotope of interest is selected by the
ISOLDE mass separator dipole magnets and transmitted to the detection setups.
The photo-ion signal is recorded as a function of laser frequency.
RILIS laser setup. The RILIS laser system comprises tunable nanosecond dye (type
Sirah Credo) and Ti:Sa lasers, pumped by the second harm onic outp ut of Nd:YAG
lasers at a pulse repetition rate of 10 kHz. The wavelengths required for excitation
and ionization of the atoms were provided by the fundamental output of the lasers
or by the generation of their higher harmonics. A detailed description of the RILIS
laser system can be found in Fedosseev et al.
29
, Rothe et al.
30
and references therein.
Spectroscopy using the Windmill detector. For the initial laser spectroscopy
using the inefficient two-colour scheme (cf. Fig. 1a,b), a sensitive a-decay spec-
troscopy setup (Windmill detector), as described in Andreyev et al.
28
, was used to
detect photo-ion rates in the range of 0.1–1,000 s
1
. The Windmill detector was
installed at the end point of one of the beam-lines of the ISOLDE general purpose
separator. The ion beam was implanted into one of ten carbon foils (20 mgcm
2
),
which are mounted on a rotating wheel. The carbon foil is surrounded by two
Si-detectors to acquire the a-decay spectrum of the implanted sample. The isotope
199
At was chosen for its suitable half-life of 7.2 s and a well-separated a-decay
energy of 6,643 keV. The number of 6,634 keV a-decays was counted for every
wavelength combination of the RILIS dye lasers. This measurement sequence
was synchronized to the PSB supercycle (E60 s), ensuring steady conditions.
Rydberg spectroscopy using FC detection. For the laser spectroscopy of Rydberg
states, the Ti:Sa lasers were used to generate wavelengths required for the first and
second-step transition. The laser powers measured on the laser table were 33 mW
(fourth harmonic) for the first step and 2 W (fundamental) for the second-step
transition. The dye laser (Sirah Credo with DCM dye dissolved in ethanol) was
scanned in the range of 16,599–15,523 cm
1
in two sections at a speed of
0.251 cm
1
s
1
and 0.132 cm
1
s
1
. The laser wavelengths were continuously
measured with a wavelength metre (HighFinesse-Ångstrom WS/7), calibrated with
a frequency stabilized HeNe laser. The three-colour ionization scheme has a higher
efficiency and the photo-ion signal was obtained from a direct ion current mea-
surement with a FC installed in the focal plane of the separator magnet. The best
signal to noise ratio was obtained with the separator magnet set to transmit mass
A ¼ 205. This is because of the relatively high production cross section of the
isotope
205
At, its slow component of release from the ISOLDE target and relatively
long half-life (26.2 min), which ensures quasi-independence from the PSB super-
cycle sequence. The continuous scanning method requires a correction for the
integration time of the FC and potential delays in the data acquisition. The cor-
rection factor was determined separately by spectroscopy of stable manganese
isotopes under the same conditions.
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Acknowledgements
We thank the ISOLDE Collaboration for flexible allocation of beam time. We
acknowledge the support from TRIUMF, which receives federal funding via a con-
tribution agreement with the National Research Council of Canada and support through
an NSERC discovery grant, as well as beam time allocation for experiment S1237 at
TRIUMF. We thank the GSI Target Group for manufacturing the carbon foils.
We acknowledge support by the Wolfgang-Gentner-Programme of the Bundesminis-
terium fu¨r Bildung und Forschung (BMBF, Germany), by FWO-Vlaanderen (Belgium),
by GOA/2010/010 (BOF-KU Leuven), by the IUAP- Belgian Science Policy Office
(BriX network P7/12), by a grant from the European Research Council (ERC-2011-AdG-
291561-HELIOS), by the United Kingdom Science and Technology Facilities Council
(STFC), by the European Union Seventh Framework through ENSAR (contract no.
262010), by the Slovak Research and Development Agency (contract No. APVV-0105-10
and APVV-0177-11) and by the Reimei Foundation of JAEA. We acknowledge the Knut
and Alice Wallenberg Foundation (grant KAW 2005-0121) for funding the RILIS laser
upgrade.
NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2819 ARTICLE
NATURE COMMUNICATIONS | 4:1835 | DOI: 10.1038/ncomms2819 | www.nature.com/naturecommunications 5
& 2013 Macmillan Publishers Limited. All rights reserved.
Page 5
Author contributions
This work constitutes the core of the PhD thesis by S.Ro. A.N.A., B.A.M., Y.K., J.L., S.Rae.,
V.F., K.D.A.W. and S.Ro. conceived the experiment with A.N.A. and V.F. spokespersons of
the At Collaboration. B.A.M., Y.K., N.I., D.V.F., D.A.F., R.E.R., V.F., M.D.S., A.M.S. and S.Ro.
set up the RILIS laser system. M.V., M.M.R., A.N.A., P.V.D., P.V.d.B., D.R., L.G., T.E.C., D.P.,
H.D.W., M.H. and V.L. set up the Windmill detector. B.A.M., T.J.P., M.M.R., Y.K. U.Ko
¨
.,
A.N.A., P.V.D., L.P., K.S., K.N., N.I., L.C., Y.W., D.V.F., D.R., L.G., D.A.F., R.E.R., J.L., S.Rae.,
S.A., V.F., M.D.S., A.M.S., T.E.C., K.D.A.W., D.P., H.D.W., M.H., K.M.L., V.L., J.F.W.L. and
S.Ro. performed the measurements. B.A.M., P.V.D., L.G., V.F., M.H., K.D.A.W. and S.Ro
analysed and interpreted the experimental data. S.F. performed the MCDF calculations. U.Ka.,
A.B.,E.E.andV.PperformedtheDCCCSD(T)calculations.S.F.,U.Ka.,A.B.,E.E.andV.P
discussed and provided the theoretical results. U.Ka., P.V.D., A.N.A., V.F., K.D.A.W. and M.H.
supervised the project. S.Ro. wrote the paper with B.A.M., U.Ka., U.Ko
¨
., A.B., A.N.A., P.V.D.,
J.L., S.Rae., S.A., V.F., T.E.C., K.D.A.W. and M.H. actively contributing to the manuscript.
Additional information
Competing financial interests: The authors declare no competing financial interests.
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How to cite this article: Rothe, S. et al. Measurement of the first ionization potential of
astatine by laser ionization spectroscopy. Nat. Commun. 4:1835 doi: 10.1038/
ncomms2819 (2013).
This work is licensed under a Creative Commons Attribution-
NonCommercial-ShareAlike 3.0 Unported Lice nse. To view a copy of
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ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2819
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