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First Determination of the Ionization Potential of Americium and Curium.

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First Determination of the Ionization Potential
of Americium and Curium""
Riidiger Deissenberger, Stefan Kohler,
Friedhelm Ames, Klaus Eberhardt, Nicole Erdmann,
Heike F u n k , Giinter Herrmann, Heinz-Jurgen Kluge,
Martin Nunnemann, Gerd Passler, Joachim Riegel,
Franz Scheerer, Norbert Trautmann,*
and Franz-Josef U r b a n
Investigations of chemical and physical properties of very
heavy elements have, among other goals, the aim of determining
whether the regularities of the Periodic Table of Elements are
maintained up through the heaviest elements or whether they are
disturbed by relativistic effects, that is, by changes of the electronic structure due to the relativistic mass increase of inner electrons.
The first ionization potential of neutral atoms is one of the
properties which sensitively depends on these relativistic effects.
Plutonium is the heaviest element whose first ionization potential has been determined experimentally. For the elements located at the beginning of the actinide series, ionization potentials
have been measured by techniques such as surface ionization,
electron impact, and laser spectroscopy." - 6 ] Their ionization
potentials have also been derived from thermochemical data by
means of the Born-Haber cycle,r71by calculations,['] and by
extrapolation from the spectral properties of the atoms.['The most accurate ionization potentials have been obtained
from the convergence of long series of highly excited Rydberg
states;L4-61 however, this required a large amount of data.
For elements beyond plutonium, ionization potentials have
been derived only by extrapolation["] based on the systematics
of excited electron states. The reasons why experimental data
cease in this region are illustrated by a precision measurement of
the ionization potential of plutonium by using Rydberg series,
which was published recently,L6' although performed some time
ago: To produce an atomic beam of sufficient density, up to 2 g
of radioactive plutonium-239 had to be used.
We present a method requiring much less material, only about
lo1' atoms ( ~ 0 . ng).
This method opens the possibility for
determining ionization potentials of the heavier actinide elements. Because of their strong radioactivity, these elements can
be conveniently handled only in very small quantities or are
available only in extremely limited amounts. We have used this
new approach to determine the ionization potential of the elements-americium and curium for the first time. In order to test
the method and its precision, the ionization potentials of thorium, neptunium, and plutonium were also measured, for which
precise values are known. As an additional test, the ionization
potential of americium was also determined by convergence of
Rydberg series.
The new method is based on a technique developed for the
determination of ultratrace quantities of actinide nuclides by resonance ionization mass spectrometry." Americium may serve
as an example, with the excitation scheme used shown in
[*] Dr. N. Trautmann, DipLChem. R. Deissenberger. DipLPhys. S . Kohler,
Dr. K. Eberhardt, Dip1.-Phys. N. Erdmann, DipLChem. H. Funk.
Prof. Dr. G. Herrmann. Dr. J. Riegel
Institut fur Kernchemie der Universitit
Fritz-Strassmann-Weg 2. D-55099 Mainz (Germany)
Telefax: Int. code (6131)39-4488
lnstitut fiir Physik der Universitit Mainz (Germany)
This work was supported by the Bundesministerium fur Umwelt, Naturschutz
und Reaktorsicherheit, by the Bundesministerium fiir Forschung und Technologie. and by the Deutsche Forschungsgemeinschaft.
Q VCH Verlu@geseN?ihuffrmhH 0-69451 Wernhemi, 1995
Fig. 2. Ionization threshold (arrows) of americium when applying an electric field
of a) E = 108 Vcm-' and b) E = 120 Vcm-' to the zone of interaction between
atomic beam and laser beams. Recorded is the number of ions N as function of the
Dr. E Ames, Prof. Dr. H.-J. Kluge, Dip].-Phys. M . Nunnemann,
Dr. G. Passler, Dr. F. Scheerer, Dr. F.-J. Urban
Figure 1. An atomic beam
of the element is excited in
two steps by intersection
of two laser beams exactly
tuned to the resonance
(wavelengths 5f77s8s
30884.8 cm-'
I , = 640.52 nm and I , =
654.42 nm). A third laser
beam (A, z 578 nm) then
a2 654.42nm
scans the region of the ionization threshold by further excitation after the
second resonance excita11
640.52 nm
tion step. The threshold
appears as a sudden increase of the photoion
count rate recorded with a
Fig. I . Excitation scheme for americium as
channel plate detector af- used in the determination of the first ionizdter time-of-flight mass tion potential by resonance ionization mass
spectrometry. The two lower excitation steps
analysis. In the interaction A , and I., are kept in resonance, whereas the
zone of the atomic beam
upper step 1, is tuned over the ionization
and the laser beams, an
electric field is applied
whose strength is vaned from measurement to measurement.
With increasing field strength E, the ionization threshold W,, is
shifted to lower values, as Figure 2a,b shows for field strengths of
tuned wavelength I., of the third laser; N is plotted versus the total photon energy
E , , of the three laser beams. In c) the resulting ionization thresholds &, are plotted
versus the square root of the field strength E. A straight line is obtained. as is
expected according to Equation (a). Extrapolation to E = 0 gives the first ionization
potential Inm
of americium.
0570-0833I95j0707-0814 $ 1 0 00+ 2510
Angeh+ Chem Int Ed Ennl 1995. 34, No 7
108 Vcm- and 120 Vcm-I. The arrows mark the respective
ionization thresholds. The peak structure seen beyond the
thresholds is due to the Stark effect.
The quantities W,, and E are related to the first ionization
potential I of the neutral atom as given in Equation (a).r121
= I - l ' o n s r t '€
Figure 2c shows this for americium. By extrapolation to E = 0,
the ionization potential IAm= 48180(3) c m - ' = 5.9736(3)eV is
obtained. In the literature, only an extrapolated value ZAm =
5.993(10) eV is reported,[t01 deduced from the systematics of
5f77s2 and 5f'7s8s states in the lighter actinide elements. Our
additional determination by the convergence of three Rydberg
series['41 gives I,, = 5.9739(2) eV, thus confirming the result
obtained by resonance ionization mass spectrometry in electric
fields. All these data were measured with lo'* atoms of americium-243; with a 7370 year half-life, it is the longest-lived americium isotope. Typical in such an experiment are count rates of
a few Hertz and counting times of 10 to 30 minutes per field
strength setting.
In Table 1 the ionization potentials measured by us are given
and are compared with the most accurate values previously
reported. The agreement is excellent. From this, we conclude
that there is also high reliability for those ionization potentials
that have been determined for the first time.
Table 1 First ioni&+tionpotential I of actinide elements, determined by photoioiiiration In ail electric field (PIE). by convergence of Rydberg series (RSC). by
life-time measurements of Rydberg and autoionizing states (LTM) as well as values
extrapolated from level systematics (Ext).
This work
1 lev1
[I] D . H. Smith, G . R. Hertel. J. Chem. PhyJ. 1969. 5 f . 3105 -3107.
[2] D. L. Hildenbrand. E. Murad, J. Clirm. PIijy. 1974, 61, 5466-5467.
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Paisner. L. J. Radziernski, P1iy.v. Rrr. A 1976. f4,1129 1136.
[4] E. F. Worden, I. C. Conway, J. Opr. Soc. A m . 1979. 69, 7.33 -738.
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[7] L. R. Morss, J. Phjs. Chem. 1971. 75. 392-399.
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[ I l l W. Ruster. F. Ames. H.-J. Kluge. E.-W. Otten, D. Rehklau, F. Scheerer, G .
Herrmann, C. Miihleck, J. Riegel, H. Rimke. P. Sattelberger, N . Trautmann,
N u d . Insrruin. Methods Pliys. R e x Secr. A 1989, 281, 547 -558.
[12] Equation (a) results as follows. According to the saddle point model 1131, the
excitation energy W,,, of an atom with a highly excited electron is given, relative
to its electronic ground state, by I)its Coulomb-energy in the nuclear field with
an effective nuclear charge Z,,,, 2) by its potential energy in the applied external
field of strength E. and 3) by the first ionization potentla1 1. In the one-dimensional approximation [Eq. (b)]:
where r is the distance of the excited electron from the nucleus, e the electron
charge. and B~ the dielectric constant of vacuum. For the ionization threshold
W,,, which is the maximum value of Yr,?Equation (c) results.
[I31 B. H. Bransden, C. J. Joachain, Physics 0f'Arom.s and Mo/ecu/es, Longman.
London. 1984. p. 227.
[14] The energies of the levels Enof Rydberg states of the principal quantum number n can be fitted [15] by Equation (d), to obtain the first ionization poten-
Experinwntcrl Procedure"
The elements were electrochemically deposited as hydroxides on thin metal foils and
covered with a layer of a reducing metal. for example, on rhenium foils withcoating
layers of rhenium or lead-doped platinum [16] or on tantalum with titanium coating.
Thorium was used directly as a thin metal foil. By heating such filaments in the
time-of-llight mass spectrometer, atomic beams were produced.
Laser beams with wavelengths from 520 to 850 nm (260 to 425 nm by frequency
doubling) and bandwidths of several Gigahertz (c1 GHz with an intracavity
etalon) were obtained from dye lasers pumped by two pulsed copper vapor lasers
(repetition rate 6.5 kHz. power 30 W, pulse length 30 ns, wavelengths 510.6 and
578.2 nm with an intensity ratio of 2 : l ) . The wavelengths were determined by a
wavelength meter. occasionally also from absorption bands of molecular iodine.
The laser beams were coupled into the apparatus either by quartz fibers or by
prisms. The ions produced by intersection with the atomic beam were accelerated to
an energy of 2.9 keV. After a drift length of 2.0 m the ions were detected with a
channel plate detector.
= 622.90 nm,
Thorium was excited and ionized in three steps ( A l = 550.42 nm, i.,
E., % 568 nm). neptunium in two steps [I71 (2, = 311.82 nm. i, c 544 nm). plutoni579 nm), and curium in
um in three steps ( i ,= 649.07 nm. I., = 629.75 nm. L,
three steps ( i ,= 655 64 nm. I., = 640.74 nm. 1, c 573 nm).
tiall. Here, RE, is the Rydberg constant of the respective element, and a, h are
constants resulting from the fit. In our measurements of Rydberg series of
americium. these states were populated by tuning the third laser. For ionization
of the Rydberg States. an electric field of 100 Vcm-' was applied 10 ; I S after the
laser pulse.
[I51 A. N. Zherikin. V. 1. Mishin, V. N. Fedoseev. Opt. Specfrosc. 1984, 57. 476479.
[I61 H. Wendeler, R. Deissenberger, F.-J. Urban. N. Trautmann. G. Herrmann,
N u d . Instrum. Methods Phys. Res. Sect. A 1993. 334, 93~--95.
[I71 J. Riegel. R. Deissenberger, G. Herrmann, S. Kohler. P. Sattelberger, N. Trautmann, H. Wendeler. F. Ames, H:J. Kluge, F. Scheerer, F.-J Urban, Appl. P h y s
See!. B 1993, 56, 275-280.
Received: November 10. 1994 [Z74671E]
German version: Angew. Chem. 1995, /[J7, 891
Keywords: actinides . americium curium . ionization potentials
An,qew. Chrwi. In/. Ed. Eiigl. 1995, 34. N o . 7
V C H Verlu~s,qrsrllscli~iJr
mhH. 0.69451 Weinheinl, 1995
S 10.00 + 3 ; O
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potential, first, determination, ionization, americium, curium
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