Photon Factory Activity Report 2017 #35 (2018)

BL-27A/2017G507

Ion Desorption from Cesium Chloride and Cesium-adsorbed Soilby Surface Ionization
Yuji BABA1,2* and Iwao SHIMOYAMA2
Sector of Fukushima Research and Development, Japan Atomic Energy Agency, 7-1 Aza-Omachi,Taira, Iwaki-shi, Fukushima, 970-8026, Japan
2
Materials Sciences Research Center, Japan Atomic Energy Agency, Tokai-mura, Naka-gun, Ibaraki,319-1195, Japan
1

1 Introduction
When a solid is heated in vacuum, a part of the surface
layer desorbs as ions. This phenomenon, known as“surface ionization” (or thermal ionization), was alreadyfound in 19th century, and then its basic mechanism wasformulated [1]. After that, surface ionization has been
investigated in various fields such as production of ions inaccelerator and sample preparation technique in mass
spectrometry.
The mechanism of surface ionization has been
investigated for simple systems such as adsorbed atoms
and molecules on metal substrates. It has been clarified
that the ionic species are considerably hard to be desorbed
compared with neutrals [2]. On the other hand, surface
ionization from bulk compounds has scarcely been
investigated due to the complex process of the surface
ionization.
In this report, we present the results for surface
ionization of bulk cesium chloride (CsCl). This material
was chosen, because it is expected that Cs easily desorbs
as Cs+ ions due to the lowest ionization potential (Ip=3.9eV) among all the elements. As an application of surface
ionization of Cs, we also investigated the desorption ofCs from Cs-adsorbed soil. This experiment wasconducted aiming to develop a method to desorb
radioactive Cs from soil, which was contaminated by theaccident of the Fukushima Daiichi Nuclear Power Station
occurred in March 2011.

can be applied to the crucible.
The desorbed species weremeasured by a quadrupole
mass spectrometer. For ion
measurements, the filament in
the mass spectrometer was
shut off. The base pressure of
the chamber was 10-6 Pa.
To compare the desorption
yield of ions with that ofneutrals, a movable aluminum
Fig. 1: Schematic
plate was put above theview of the electron
crucible. Desorbed ions orbombardment (EB)neutrals were deposited on theheating system.
aluminum plate for a certaintime, and then the surface of
the aluminum plate was measured by X-ray photoelectron
spectroscopy (XPS). For Cs-adsorbed clay, the chemicalstates and contents of Cs before and after heating weremeasured by XPS. The XPS measurements wereperformed at the BL-27A of the Photon Factory. The
typical photon energy was 3000 eV, which enables bulksensitive XPS measurements.
3 Results and Discussion
3.1 Surface ionization of CsClFigure 2 shows the mass spectrum of neutral species
desorbed from CsCl at 460ºC. The H2O peak originatingfrom adsorbed water on
the powder is the mostintense among all masses.
Among the fragments ofCsCl, the intensity of theCs peak is the highest
reflecting
thelowionization potential of Cs.
The other fragments areFig. 2: Mass spectrum for
Cl and double charged
desorbed neutrals fromCs. This mass spectral
CsCl at 460ºC.
pattern is different fromthat of ion desorption(Fig. 3).

2 Experiment
The cesium chloride (CsCl) was powder of special
grade. It was used without any pre-treatments. For Csadsorbed soil, we used vermiculite (clay) collected from
Ono-machi, Fukushima Prefecture. It was stirred in a 5mM water solution of nonradioactive CsCl for 24 hseveral times. The excess CsCl was washed with ethanol.
The content of Cs was 4.7 wt%.
For the measurement of thermal desorption, an electron
bombardment (EB) heating system was used. The
schematic of the EB system is shown in Fig. 1. The
sample of 5 mg was put in the crucible made of tantalum.
The crucible was heated by the bombardment of electrons
from the tungsten hot filament. The crucible waselectrically isolated from the ground, and positive voltage-1-Photon Factory Activity Report 2017 #35 (2018)

Fig. 3 shows the mass spectra of positive ions desorbed
from CsCl at 460ºC. In this measurement, various
positive potentials were applied to the crucible, which are
shown in Y axis. When the crucible was grounded, nodesorbed ions were observed. On the other hand, when
the positive potential was applied to the crucible, Cs+ ionsbegan to appear fromaround +50 V. The
intensity of the Cs+ions almost saturates
at +300 V (Thecrucible potential of
100 V corresponds tothe electric field of670 V/m at theFig. 3:
Mass spectra forsurface.).
desorbed ions from CsCl at
The effect of such
460ºC.
Variouspositive
low electric field on
potentials were applied to thethe ion desorption
crucible, which are shown in
was reported by
the Y axis.
Butman et al. [3] forthin films of alkali
metal compounds. They discussed the enhancement of theion desorption from KCl single crystal by positive electricfield in terms of the excess charge depending on thesurface topography such as steps and kinks [3]. However,
the ion emission they treated plays a minor part in thetotal desorption, so the present results cannot beexplained only by the surface topography. At present, weconsider that the enhancement of ion emission by electricfield is explained by the space charge effect as follows.
The electric field applied at the surface form a spacecharge zone. This space-charge first hinders the ionemission. With increasing the potential of the extraction
field, ions are extracted from the surface more efficiently
resulting in the increase in the ion desorption. Thisphenomenon is known as Langmuir–Child behavior [4,5].
Actually, in the study of ion source, it was reported thatthe emission of potassium and cesium ions from leucitetype materials is enhanced by the space charge following
the Langmuir–Child law under small electric fields(<1000 V/cm) [6]. Although further study is needed toclarify the effect of surface electric field on ion emission,
we consider that the observed enhancement of Cs + ion
emission by the electric field is due the space charge
effect produced at the surface.
Next, we compare the desorption yield of ions andneutrals. Figure 4 shows the intensity ratios of ions toneutrals (ICs+/ICs) desorbed from CsCl as a function of thetemperature. The ICs+/ICs value shows a maximum ataround 415ºC. This temperature is about 230ºC lower
than the melting point of CsCl. To the best of ourknowledge, such high yield of ion desorption from alkalimetal halides at low temperature has not been reported.
Let me consider the mechanism of high ion desorption
yield at such low temperature.
When the CsCl molecules are firstly dissociated in thesurface and then atomic neutrals and ions desorb, the ratioof ions to neutrals (YCs+/YCs) desorbed from a surface isexpressed by the Saha-Langmuir equation [7],YCs+/YCs = (gCs+/gCs)･exp(-(Ei-)/kT)(1)

where g is the statistical weight (degeneracy), Ei is theionization potential of cesium,  is the surface work
function, k is the Boltzmann constant, and T is the Kelvintemperature. The subscripts Cs+ and Cs indicate the ionsand neutrals, respectively. (In case the molecules firstly
desorb and then they are decomposed and ionized, thevalue of Ei is substituted by a dissociative ionizationenergy [8].)The experimentally obtained ICs+ and ICs values are
expressed as
ICs+=Cs+･YCs+
ICs=Cs･YCs

(2)
(3)

where  is the detection efficiency of the mass
spectrometer. Thus, the ICs+/ICs value is shown as
ICs+/ICs = A･exp(-(Ei-)/kT)(4)

where A is the constantcorrespondingto(Cs+/Cs) ･ (gCs+/gCs).
The equation (4) showsthat when Ei is larger
than , the value ofICs+/ICs increases withthe temperature, andwhen Ei is smaller thanFig. 4:
Temperature
, the value of ICs+/ICs
dependence of the desorbed
decreases with theintensity ratio of ions totemperature. Since theneutrals(ICs+/ICs).
Thevalues of Ei are
crucible voltage for ion
constant (Ei=3.9 eV),measurements was +150 V.
the observed maximumTwosimulationlinesof the ICs+/ICs value incalculated by the SahaFig. 4 seems to be due
Langmuirequationareto the changes in theshown for two work function
workfunction
values ( and  eV).
dependingonthetemperature. In thecase of simple metals,
the surface work function changes depending on
temperature because of the thermal vibrations of theatoms. Compared with simple metals, work functions ofcompounds are difficult to determine both theoreticallyand experimentally. For alkali-metal halides, the reported
values of the work function are widespread [9].
Considering the gradient of the ICs+/ICs values, it isdeduced that the work function at the temperature lower
than 415ºC is smaller than Ei, and that at the temperature
higher than 415ºC is larger than Ei. In Fig. 4, the slopes ofthe experimental data are simulated using equation (1) bysetting arbitral A value and two different  values. At the-2-

Photon Factory Activity Report 2017 #35 (2018)

temperature lower than 415ºC, the slope is fitted by
setting =2.0 eV. This value is close to the work functionof metallic Cs. At the temperature higher than 415ºC,
=4.6 eV well reproduces the slope. Although the reasonof such changes in the work function is not clear, it can bequalitatively explained as follows.
As to the structure of CsCl, it has two polymorphic
form; the CsCl-type and NaCl-type. The crystal structureof CsCl changes from CsCl-type structure to NaCl-type
one around 445 °C [10]. Also, the molar volume of CsCl
increases about 16% at this temperature [11]. With thisphase transformation, the standard enthalpy of CsClformation i.e., lattice energy increases at the transition
temperature [9]. The increase in the lattice energy induces
the increase in the work function. Although thetemperature of the ICs+/ICs maximum is tens of degree
lower than the phase transition temperature, we consider
that the changes in the structure from CsCl-type to NaCltype is the origin of the work function change at thismoderate temperature region, which induces themaximum of ICs+/ICs value.
It should be noted that the values of ICs+/ICs shown inFig. 4 are only relative values. A question may be raised
as to which of ions or neutrals desorb with higher
desorption yield in these temperature range. So, we
compared the absolute desorption yield of ions with thatof neutrals using the aluminum plate located above thecrucible shown in Fig.1. Desorbed ions or neutrals weredeposited on the aluminum plate for 30 s at 460ºC. Then,the surface of the aluminum plate was measured by XPS.
By comparing the ICs3d5/2/IAl1s ratio, it was revealed thatthe desorption yield of Cs+ ions with a crucible potentialof +200 V is about ten times higher than that of neutrals.
This is surprising result, because the desorption yield ofions by surface ionization is generally few orders lessthan that of neutrals [12]. Also, desorption of ions bysurface ionization happens at the temperature higher thanthe desorption temperature of neutrals [12]. At present,
we consider that this phenomenon is specific to cesium,
because it has unique property that the ionization
potential is the lowest in all the elements.
3.2 Desorption of Cs+ions from Cs-adsorbed
soil
Fig. 5 shows the mass
spectra for neutrals andions desorbed from Csadsorbed clay at 460ºC.
Desorption of Cs was
not observed in neutral
mass spectrum. WhileCs+
wasclearlyobserved in ion mass
spectrum. In order toverify the applicabilityof surface ionization tothe decontamination ofcesium, XPS spectra forclay was measured before
and after heating.
Fig. 6 shows the XPS
spectra in Cs 3d region forclay before and after
heating at 460 º C for twohours.
Althoughthechemical states did notchange after heating, theFig. 6: XPS spectra forintensity of the Cs 3d5/2
Cs-adsorbed clay before
peakdecreased.
Theand after heating at 460
quantitative
analysisºC for 2 hours. A
revealed that the contents ofpotential of +200 V was
cesium was decreased byapplied to the crucible
13% after heating. The
during heating.
present results show thatthe a part of cesium can bedesorbed as Cs+ ions at low temperature by applying
electric field.
References[1] P.A. Redhead, J. Vac. Sci. Technol. A 16, 1394 (1998).
[2] G.D. Alton, Rev. Sci. Instrum. 59, 1039 (1988).
[3] M.F. Butman, and H. Dabringhaus, Surf. Sci. 540, 313(2003).
[4] C.D. Child, Phys. Rev. 32, 492 (1911).
[5] I. Langmuir, Phys. Rev. 2, 450 (1913).
[6] T. Kolling, A. Schlemmer, C. Pietzonka, B. Harbrecht,and K.M. Weitzel, J. Appl. Phys. 107, 014105 (2010).
[7] M.J. Dresser, J. Appl. Phys. 39, 338 (1968).
[8] T. Suzuki, H. Iwabuchi, K. Takahashi, M. Nomura, M.
Okamoto, and Y. Fujii, Int. J. Mass Spectrom. Ion
Proc. 145, 131 (1995).
[9] L.S. Kudin, A.M. Dunaev, M.F. Butman, and A.S.
Kryuchlov, Russ. J. Phys. Chem. A 85, 260 (2011).
[10] M. Watanabe, M. Tokonami, and N. Morimoto, Acta
Cryst. A 33, 294 (1977).
[11] J.M. Christian, and D.W. Lynch, J. Chem. Phys. 40,248 (1964).
[12] H. Kawano, Y. Zhu, and S. Sugimoto, Appl. Surf. Sci.
175-176, 105 (2001).

* baba.yuji@jaea.go.jp

Fig. 5: Mass spectra forneutrals
andions
desorbed from Csadsorbed clay at 460ºC.
For ion desorption, apotential of +200 V wasapplied to the crucible.
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