Download Ejection of Sodium from Sodium Sulfide by the Sputtering of the

ICARUS75, 233--244 (1988)
Ejection of Sodium from Sodium Sulfide by the Sputtering of the
Surface of Io 1
D. B. C H R I S E Y , 2 R. E. J O H N S O N , J. W. BORING, AND J. A. P H I P P S
Department of Nuclear Engineering and Engineering Physics, University of Virginia,
Charlottesville, Virginia 22901
Received September 11, 1987; revised December 23, 1987
Measurements have been made of the sputtering yields, the mass spectra of ejected
molecules, and ejection rates for various kiloelectronvolt ions incident on sodium sulfide
(Na2S). The sputtering yields were small compared to those measured earlier for the
more volatile sulfur ($8) and SO2 due to the strong ionic bonding in the solid. The
mechanism of sputtering for the corotating sulfur and oxygen ions in Jupiter's magnetosphere is due to a cascade of quasi-elastic collisions initiated by the incident ion. The mass
spectrum indicated that sodium is ejected predominantly as a molecule with a lesser
amount ejected as atomic sodium. Making several assumptions it seems unlikely that the
sputtering of Na2S by magnetospheric ions can maintain the observed neutral cloud
densities. Instead, the sodium probably exists as a larger polysulfide for which we show
that the sputtering yield should be greater. © 1988AcademicP..... Inc.
I. INTRODUCTION
The discovery of sodium D-line emission
from Io (Brown 1974) and that these atoms
form an extended cloud (Trafton et al.
1974) was the first evidence that the surface
of Io might supply material to the Jovian
magnetospheric plasma. It is now clear that
the material which makes up the plasma,
predominately sulfur and oxygen ions,
was in fact originally ejected from Io as
neutrals (Matson et al. 1974, H a f t et al.
1981, Cheng 1982). Although the alkali
metals, sodium and potassium, are not the
major species in the plasma (Bagenal and
Sullivan 1981, Gehrels and Stone 1983, Belcher 1983), and are often left out of largescale material injection processes (Kumar
1984, Cheng 1984), they are easily observed
and features of their existence can provide
strong constraints on larger scale magnetosphere-satellite interactions (Schneider et
al. 1987).
A. Implications for Atmospheric Column
Density
The injection of material from Io into the
torus region requires that at some point material be ejected from the surface of Io either directly into the magnetosphere or into
an atmosphere requiring subsequent ejection (Sieveka and Johnson 1984). In either
case, material must be ejected from the surface and this presents a dilemma due to the
low volatility of possible sodium-containing
compounds (Matson et al. 1974). Since sodium is usually contained in the form of low
vapor pressure compounds, at the surface
temperatures characteristic of Io, these
i This work was supported by the NSF Astronomy compounds will not be present in an atmoDivision under Grant AST-85-11391 and the NASA sphere that is not, at least partially, sputter
Geology and Geophysics Division under Grant produced (Fanale et al. 1977, H a f t et al.
NAGW-186.
2 Current address: Code 4673, Radiation Effects 1981, Summers et al. 1983). Therefore,
Dept., Naval Research Laboratory, Washington, DC sputtering of sodium-containing com20375-5000.
pounds on the surface of Io is the favored
233
0019-1035/88 $3.00
Copyright © 1988 by Academic Press, Inc.
All rights of reproduction in any form reserved.
234
CHRISEY ET AL.
mechanism to explain the presence of the
neutral sodium cloud (Matson et al. 1974).
This mechanism, though, is improbable if
there is a collisionally thick atmosphere
which does not allow the majority of the
plasma ions to penetrate to the surface. The
sputtering mechanism, therefore, puts constraints on the atmospheric column density
(Haft et al. 1981, Matson and Nash 1983)
and thus the means of supplying the sulfur
and oxygen required to maintain the plasma
torus (Sieveka and Johnson 1985, McGrath
and Johnson 1987).
B. Implications f o r Surface Composition
The existence of the sodium cloud, in
particular its size and stability, also imposes strong constraints on Io's surface
composition. The alkali metal-containing
material must be sufficiently plentiful in order to supply enough material to maintain
the neutral cloud in a steady state (Nash et
al. 1987). By stepping backward in the ejection process, this material must then also be
a participant in the rapid volcanic resurfacing that occurs on Io (Johnson and Soderblom 1982). Furthermore, if we assume,
as is generally accepted, that the surface is
ultimately the source of all plasma material
(Matson et al. 1974), then this requires that
the host contain only atoms which are contained in the plasma. This qualitative relationship between the surface composition
and plasma composition can also be quantitatively correct if the volcanic resurfacing
rate in the regions which supply most of the
sodium is sufficiently slow so that sputtering of a number of monolayers of material
has occurred. That is, in spite of the possible initial nonstoichiometric sputtering of
sodium-containing compounds on the surface, in time the sputtering rate for individual components must approach the stoichiometric composition of the original
material (Sigmund 1981, Haff et al. 1981).
For example, the absence of a signature
for chlorine eliminates NaCl as a possible
surface host and the detection of carbon
was only consistent with it being of solar
wind origin rather than from Io, thus eliminating the carbonates (Fanale et al. 1982,
Gehrels and Stone 1983). Therefore, in order to be consistent with plasma data, the
alkali metals are likely to exist on the surface in a compound containing sulfur or oxygen or both. Possible examples of these
compounds must also satisfy available
spectral data. The sulfates would supply
Na, S, and O to the magnetosphere but are
unlikely surface constituents since their
deep absorption band at 4.4 /zm is absent
from Io's spectrum (Fanale et al. 1982). Alternatively, sodium (and potassium) can be
an adsorbed species (Hapke 1979), deposited from the volcanic gases or produced by
ion bombardment of the surface (Hapke
1986, McGrath et al. 1986). The important
difference is that adsorbed sodium is
thought to be weakly bound and thus
should sputter monatomically with a larger
yield than chemically bound sodium. Experimental results for the sputtering of
weakly bound adsorbed sodium do not exist.
C. Na2S as a Surface C o m p o n e n t
An often-mentioned family of compounds in which the sodium may exist on
the surface are the sulfides, Na2Si (Nash
and Fanale 1977, Fanale et al. 1977, Lunine
and Stevenson 1985). The monosulfide of
sodium, Na2S, is especially promising since
it also exhibits several important features
found in Io's spectrum (Fanale et al. 1979,
Nash and Nelson 1979, Nash 1987, Nash
and Van Hecke 1987). The most common
method of preparation of Na2S is by reacting the elements, although it is also possible
to dehydrate the hydrate (Brauer 1963,
Walker 1979). The production of sodium
sulfide on the surface of Io could take place
within a hot spot through a similar reaction
of the elements. This scenario not only provides a mechanism of production, but incorporates itself into the volcanic resurfacing process as required.
To date little has been written about the
chemical or physical properties of the so-
THE SPUTTERING OF SODIUM SULFIDE ON IO
dium sulfides, possibly due to the extreme
difficulty involved in handling these unstable and hygroscopic compounds. The most
comprehensive investigation of the chemistry of sodium polysulfides of any importance to these measurements was done by
Oei (1973a, 1973b) in investigating the reactive species in a sodium-sulfur battery. At
room temperature, NazS is a white crystalline solid, as are all alkali metal sulfides
(Tegman 1974). Its high melting point (mp
= 1453°K) is evidence for the strong binding of the molecules occurring within the
solid and its unknown boiling point is evidence for its chemical instability. It decomposes to the elements upon heating (eliminating the possibility of evaporating thin
films), rapidly hydrolyzes to NaOH and
NariS upon exposure to moisture, and is
converted to H2S and Na2CO3 upon exposure to air (Nash 1987).
D. Sputtering of Na2S
In this paper we describe the sputtering
of NazS. The production of thin films of
Na2S, which are necessary to produce any
meaningful sputtering measurements, accounted for the majority of time spent in
researching this system. We chose NazS in
this study because it is the least volatile of
the sulfides and because of its commercial
availability and purity as compared to those
of the other polysulfides. The experimental
results are used to explore the possibility of
ion-induced ejection of sodium from NazS
in terms of supplying the necessary injection rate of sodium atoms. We also incorporate our experience with sulfur sputtering in
order to consider other possible sulfides or
sodium-containing surface materials.
II. E X P E R I M E N T
The production of thin films of NazS was
accomplished by a spray coating procedure. Due to the extreme hygroscopic nature of sodium sulfide any handling of the
sodium sulfide powder, the target substrates, or the final targets took place in a
dry nitrogen atmosphere. The target forma-
235
tion procedure began with producing the
spray coating solution. In a dry nitrogen
filled glove box approximately 1 cc of NazS
powder (Aesars 99.9% pure) was mixed in a
beaker with about 50 ml of anhydrous methanol. Due to the solute's polar nature most
of the powder dissolved easily. To remove
any large particles which did not go into
solution the mixture was then filtered once.
With a small hobby spray painter or atomizer this solution was then sprayed onto a
nickel substrate. The spraying process took
on the order of 1 sec to complete and resulted in a very thin film of solution on the
substrate. The solvent then quickly evaporated leaving behind the Na2S solute as a
thin film. Approximately 900 such " c o a t s "
were applied over a period of 90 min. The
film formed slowly and in small regions.
The film was considered complete when the
regions overlapped and the substrate was
completely covered. Once the spraying was
completed the targets were dried for 2 hr by
warming slightly above room temperature
and increasing the dry nitrogen flow rate on
the target. The final targets were then
mounted in a vacuum chamber and pumped
on at a pressure of 10-6 Torr for 1 hr before
cooling to low temperatures. As a check of
the stoichiometry of the NazS films quantitative X-ray analysis was done on the Na2S
powder and a Na2SO4 standard, verifying
the elemental composition (two Na to one
S).
The nickel substrate, on which the NazS
film was deposited, had 1 /.tCi of Po-210
electrochemically deposited on the center.
By measuring the energy loss of the 5.305MeV a-particle emitted by the Po-210 as it
passed through the NazS film a density
thickness could be measured. The sputtering yield could then be determined by measuring a change in film thickness for a given
amount of integrated beam current. The
above procedure of target formation resulted in an extremely nonuniform film.
The variation in the thickness, as determined by the width of the a-particle spectrum, was on the order of 3/zm for a 8-/zm-
236
CHRISEY ET AL.
thick film. B e c a u s e of the variation in
thickness we determined the yield below by
measuring the change in the centroid of the
a - p e a k . Further details of the sputtering apparatus, the thickness m e a s u r e m e n t technique, and the neutral mass s p e c t r o m e t r y
are described elsewhere (Chrisey et al.
1987, 1988).
III. RESULTS
A. Sputtering Yield Results
The sputtering yield for 34-keV Ar + incident on NazS at 15°K was m e a s u r e d for two
separate films. Typical particle fluxes during the irradiation were - 5 / ~ A / c m 2 (or 3 x
1013 ions/cm2-sec). The total particle flux
hitting the target or dose was - 5 x 1018
ions/cm 2. The total amount of material rem o v e d , as described above, was found to
c o r r e s p o n d to 0.5 NazS molecules per incident ion. This yield value, expressed in
terms of the n u m b e r of parent species
ejected, a s s u m e s stoichiometric sputtering
and is very small c o m p a r e d to solid sulfur,
as is to be e x p e c t e d due to its large surface
binding energy.
B. Mass Spectrum of Ejected Species
Since Na2S is a two c o m p o n e n t molecular target there can be a variety of molecular species ejected. The measured values
for different neutral species detected by a
quadrupole mass s p e c t r o m e t e r (QMS) are
given in Table I and normalized to S, the
largest signal measured. It is seen that significant quantities of molecular Na2S are
ejected, which was not initially expected
for this material. It is important to note that
the values given in Table I are not relative
yields but rather an uncorrected counting
rate f r o m the QMS. In order to m a k e a
quantitative c o m p a r i s o n of different species ejected, these signals must be corrected for various instrumental sensitivities
such as the cracking fractions of the species
ion the ionizer and the m e a n time spent in
the ionizer. T h e s e data do not exist for
NazS.
Molecular Na2S, upon entering the elec-
TABLEI
RELATIVE QMS SIGNALS FOR EJECTED MASSES
FROM SPUTTERING OF A NaES TARGET BY 3 4 - k e V
Ar t IONS
Species
M
(amu)
Escape
energy
(eV)
QMS
signal
Na
S
Na2
NaS
$2
Na3
Na2S
$3
23
32
46
55
64
69
78
96
0.8
1.1
1.6
1.9
2.2
2.3
2.6
3.3
57
100
14
40
90
0
46
0
Note. The QMS signal shown is the relative number
of counts the QMS detects for a given number of incident ions hitting the target. Also shown is the escape
energy, Eesc, in electronvolts for the various species
calculated for the escape velocity of Io (Eeoc= 1/2 M
(Ve~)2, where V~s~ 2.6 km/sec).
tron impact ionizer, can produce fragment
ions such as N a ] , S +, N a +, and N a S + in
addition to NazS + ; the exact composition of
the sputtered ejecta is not clear from the
values given in Table I. S o m e of the species
of ions o b s e r v e d can be fragments of one or
m o r e parents. H o w e v e r , a species such as
$2, which based on previous m e a s u r e m e n t s
of the sputtering of sulfur-containing compounds always appears (Boring et al. 1986),
must be produced in the ion b o m b a r d m e n t
process. Thus the detection of $2 indicates
chemical modification induced by the incident ions. If we ignore these corrections
and interpret the n u m b e r s in Table I as
a p p r o x i m a t e relative n u m b e r densities
present in the sputter ejecta, then there
is an apparent o v e r a b u n d a n c e of sulfur
ejection. This m a y be due to the aforementioned instrument sensitivities or a beaminduced migration p h e n o m e n o n in which a
slightly charged surface can cause the N a +
ions in the solid to migrate (Miotello and
Mazoldi 1985). H o w e v e r , since the thin
films were originally c o m p o s e d of Na2S and
m a n y m o n o l a y e r s of material were removed, stoichiometric sputtering is as-
THE SPUTTERING OF SODIUM SULFIDE ON IO
4000
=
=
I
i
J
237
i
1.0
0.8
2000
0.6
.._=
0.4
¢~
1000
-g
,g
v
Z
800
600
0.2 ~
>-
//
400
/
>-
/
//
520eV
S+
0.1
///
200
10
210
q / 4oi
260eV
O÷
6o 8'o,
FD(~)
0.08
i
200
o]o
(eV
i
400
I
600 800
~2)
FXG. I. The relative yield of neutral sputtered species detected as m/q = 23 versus the surface
deposited energy, FD(0), for incident 34-keV Ne +, Ar +, Kr +, and Xe ~ from left to right, respectively.
The fitted line is for a linear dependence as expected for collision cascade sputtering. The values of
FD(0) have been divided by 3 to represent the target as a "quasi-monatomic" solid. The values of
FD(0), and thus the yield, expected for corotating ions (520-eV sulfur and 260-eV oxygen ions) are
indicated. On the right-hand axis we give the sputtering yield which would be expected for collision
cascade sputtering and normalize vertically based on the measured 34-keV Ar + sputtering yield value.
sumed. Table I simply indicates that both
molecular and atomic species occur and below we use one of the species (Na) to monitor the sputtering yield.
C. Mechanism o f Ejection
In order to understand the behavior of
the sputtering yield as a function of ion energy and ion type we m e a s u r e d the signal
for m/q = 23 (Na) with the QMS for four
different ions (34-keV N e +, Ar +, K r +, and
Xe +) and plot the results in Fig. I versus
the surface-deposited energy, FD(0). The
surface-deposited energy is often written as
aSh. In this expression o~ describes the anisotropy of the cascade and S, is the nuclear
stopping cross section. F o r collisional sputtering of m o n a t o m i c solids (Sigmund 1969,
1981) the sputtering yield is proportional to
FD(0) (see Eq. (2)). The linear d e p e n d e n c e
of the sputtered signal as a function of the
surface-deposited energy, as indicated by
the line in Fig. 1, is like that in the standard
theory for the collisional sputtering of monatomic solids. This justifies the method of
extrapolating the e x p e c t e d sputtering yield
for corotating sulfur and oxygen ions used
here and also used for the sputtering of sulfur (Chrisey et al. 1987). Assuming the
sputtering yield (Y) also has a linear dependence like that seen for m/q = 23 we plot on
the right-hand axis the sputtering yield
which would be expected for collisional
sputtering based on the m e a s u r e d 34-keV
Ar + sputtering yield value. The arrows in
Fig. 1 indicate the values of the surfacedeposited energy e x p e c t e d for corotating
sulfur and oxygen ions. In Table II we
present values of the surface-deposited energy and sputtering yield that were measured for 34-keV Ar + b o m b a r d m e n t and
also those which would be expected for
corotating sulfur- and oxygen-ion b o m b a r d ment.
The m e c h a n i s m of sputtering ocurring
here must be of collisional origin (Chrisey
et al. 1987) and not of electronic origin. In
order for electronic sputtering of insulators
to be efficient, the surface binding energy
must be small c o m p a r e d to the stored electronic excitation energy (i.e., the band gap
in solids), as is the case for the condensed
238
CHRISEY ET AL.
TABLE II
obtain 2 x l 0 7 sodium atoms/cm2-sec leaving
the b o m b a r d e d surface. N o t e that the
NUCLEAR STOPPING CROSS SECTIONS, Sn, AND
YIELD VALUESFOR 34-keV Ar+ AND COROTATING value of 1% used for ~c is the same as that of
SULFUR AND OXYGEN IONS
H a f t et al. (1981) but is less than that obtained f r o m m e a s u r e m e n t s of the sodium
Ion
Ion a"
S,
FD(0)
Yield
mixing ratio in the cold p l a s m a torus near
energy
(eV /~k2) (eV ,~2) (Na,S/ion)
Io or < 5 % (Bagenal and Sullivan 1981) and
(keY)
3% (Gehrels and Stone 1983). The n u m b e r
34
Ar 0.23
1073
247
0.5
presented a b o v e c o r r e s p o n d s to the n u m b e r
0.52
S 0.24
419
103
0.2
of sodium a t o m s which leave the surface,
0.26
O 0.32
182
58
0.1
regardless of the form in which they leave.
As
the o b s e r v e d cloud is atomic sodium,
Note. The nuclear stopping cross sections are from
Ziegler (1984) and are given as a mean cross section sodium ejected in a molecular form must
per atom. The extrapolated yield values were obtained dissociate following ejection in order to
by assuming the total yield is proportional to the sur- contribute.
face-deposited energy FD(0) (where FD(0) = aS,).
The previous results for the sputtering of
Values of a for various species were obtained from
SO2
and sulfur showed that only a small
an empirical curve by Sigmund (1981).
percentage ( - 5 % ) of the species ejected
would escape Io directly (Johnson et al.
gas solids (Brown et al. 1984). F o r other 1984, Chrisey et al. 1987). That is, the disexpected surface constituents of Io, such as tribution in energy of species sputtered
SO2 and sulfur, electronic sputtering was from SO2 and sulfur indicated that the mapossible (Lanzerotti et al. 1982, Torrisi et jority of the species ejected f r o m the sural. 1987, Chrisey et al. 1987). For the face of Io would be gravitationally bound
tightly bonded s y s t e m Na2S, it seems very and thus form a sputter-produced corona. I f
unlikely that any significant electronic sput- we were to a s s u m e that the escape fraction
tering could take place at the electronic was nearly the same for Na2S (Thomas
excitation densities produced by typical
1986) then subsequent ejection and dissociplasma-ion b o m b a r d m e n t .
ation from this sodium-containing corona
would have to be very efficient in order to
D. A p p l i c a t i o n to the P r o d u c t i o n o f the
maintain the sodium cloud. But, as pointed
Sodium Cloud
out in the Introduction, Na2S is a solid very
The a m o u n t of material escaping the different f r o m SO, and sulfur. There are
gravitational field of Io necessary to main- strong theoretical reasons to expect the entain the neutral cloud in a steady-state con- ergy spectra of Na2S should also be differdition has been estimated to be about 10 7 ent f r o m those of SO2 and sulfur, as dissodium atoms/cm2-sec (Matson et al. 1974). cussed below.
The amount of material which would be
IV. DISCUSSION
ejected from the surface due to the entire
corotating p l a s m a ion population b o m b a r d The model for the production of the soing the surface is given by
dium cloud to be explored in this p a p e r is
the direct ejection of sodium from some
ejected sodium = 4~ Ym~c,
(I)
form of sodium sulfide on the surface of Io.
where ~b is the corotating ion flux, Ym is the Although the ejected sodium yield calcum e a n yield, and ~c is the fractional coverage lated a b o v e is larger than that required for
of NazS. Substituting q~ --- 10 l° ions/cm2-sec, an injection rate of 107 sodium a t o m s / c m 2the m e a n yield of sodium Ym = 0.2 sodium
sec, the fraction of this ejected sodium
a t o m s / p l a s m a ion f r o m Table II, and as- which e s c a p e s the satellite has not been insuming a rough lower limit, [~c = 0.01, we cluded. F u r t h e r m o r e , f r o m Table I it ap-
THE SPUTTERING OF SODIUM SULFIDE ON IO
pears that most of the sodium is ejected in a
molecular form which will require later dissociation. An estimation of the escape fraction, its implication for the production of
the sodium cloud, and a reevaluation of
Na2S as a possible sodium-containing surface material are discussed below.
A. Collisional Sputtering
The observed yield for 34 keV Ar ÷ is
small compared to that usually measured
for much more volatile targets such as condensed SOz (Johnson et al. 1984). This
value can be put in perspective if we ignore
the difference in the Na and S masses and
treat solid NazS as a monatomic target. The
aforementioned yield of 0.5 Na2S molecules/ion would then correspond to 1.5 atoms/ion. With the yield expressed in these
units we substitute an average nuclear stopping cross section (S,) per atom in the standard sputtering yield formula (Sigmund
1969, 1981) for monoatomic solids,
Y = 0.042 o~S,/(U ~2).
(2)
This can be used to give an effective surface binding energy per atom or U in the
above expression. The result of applying
Eq. (2) to solve for U is a value of 6.9 eV/
atom (or 21 eV/NazS molecule). This value
is very large but agrees well with the calculated value of the cohesive energy by
Agarwal et al. (1977) of 23 eV/Na2S molecule. The cohesive energy of a monatomic
material is often used as a reasonable estimate for U in Eq. (2). A similar agreement
between this "quasi-monatomic" sputter
model and experiment was previously obtained for $8 sputtering. The effective binding energy as calculated from a fit of the
monatomic sputter yield formula to experimental data (as above) gave a value of 0.19
eV/S atom or 1.5 eV/S8 molecule. This
value also agreed reasonably well with the
measured value of 1.1 eV/S8 molecule for
the sublimation energy (Chrisey et al.
1987). These results are somewhat surprising as the expression was developed for
monatomic solids and not molecular solids.
239
B. Electronic Sputtering
The conclusion that electronic sputtering
on Io will not produce a significant sputtering yield is an extremely important result in
terms of determining the energy and type of
plasma ions which might reach Io's surface.
In order to remove the problem of Io having
a thick atmosphere and also having ions
reach the surface it has been suggested that
the plasma ions which produce the sputtering are high energy protons (Matson et al.
1974, Macy and Trafton 1975). This is consistent, for example, with a 1016/cm2 SO2
atmosphere (Sieveka and Johnson 1985;
McGrath and Johnson 1987) because a 1MeV H + ion can penetrate this atmosphere,
losing only about 200 eV. A l-MeV H + ion
incident on an SO2 target will produce significant sputtering and chemical alteration
(Lanzerotti et al. 1982, Johnson et al. 1984,
Moore 1984). An NazS surface, on the other
hand, requires a heavy low energy ion, like
corotating oxygen and sulfur ions, to produce significant sputtering. Furthermore, in
the absence of a secondary mechanism
such as plasma ejection from an atmosphere (Pilcher et al. 1984), the velocity of
the sodium atoms in the cloud is consistent
with the mechanism of sputtering being collisional rather than electronic in origin
(Carlson et al. 1978). This is because the
distribution in energy of species sputtered
by an electronic mechanism has a sharp
cutoff at high energy corresponding to the
maximum impulsive energy input in the
electronic relaxation process less the surface binding energy (Pedrys et al. 1984).
C. Energy Spectra
In considering the sputtering of Na2S on
the surface of Io in terms of contributions
to the injection rate, it has not yet been
determined whether the majority of sodium
is ejected directly into the magnetosphere
or later during its gravitationally bound trajectory (Sieveka and Johnson 1984, McGrath and Johnson 1987). We assume here
that the energy distribution of ejected spe-
240
CHRISEY ET AL.
TABLE 1II
VALUES OF THE SURFACE BINDING ENERGY (U) FOR
VARIOUS PROPOSED SURFACE CONSTITUENTS OF 10,
GIVEN PER MOLECULE
Species
U
(eV)
Source
SO2
$8
Na/lunar rock~
Na2S
0.37
1.1
4.5
23
Sublimationenergy
Sublimationenergy
Sublimationenergy
Cohesive energy
Note that this value is thought to correspond to the
sublimation energy of Na20 (De Maria et al. 1971).
cies from an NazS target displays a Thompson distribution (Sigmund 1969), as expected for collisional-induced sputtering,
of the shift toward higher energy of the distribution in Eq. (3) (Sieveka and Johnson
1984). This value also cautions against the
use of values for the escape fraction and
sputtering yield from the SO2 and sulfur
system for the sputtering of Na2S (Carlson
et al. 1978, Sieveka and Johnson 1984,
Thomas 1986). With this value of about
100% we find from the result of Eq. (1) that
in terms of j u s t material injection, direct
sputtering on the surface can account for
2 x 10 7 sodium atoms/cm2-sec even with a
modest 1% concentration. This is equal to
the lowest estimate for the supply rate
(Matson et al. 1974) but below the largest
more recent estimates of 109 sodium atoms/
cm2-sec (Pilcher et al. 1984, Nash 1987).
D. A n Alternative Scenario
dN
E
dE - K (E + U) 3'
(3)
where d N is the number of a given species
ejected from the surface with an energy between E and E + dE, U is the surface binding energy of a given species, and K is a
normalization constant. The fraction of the
total amount sputtered which can escape
the gravitational field of a satellite, ~esc, is
the integral of Eq. (3) for energies greater
than the escape energy, Eesc, divided by the
integral of Eq. (3) over all energies. The
result is
2x+ 1
l~esc -- (X + 1) 2'
(4)
where x = Eesd U. F r o m the above equation
it is obvious that the number of a given species which can escape directly is dependent
on the relative magnitude of U to Ee,c. Values of U, known for various molecular species relevant to Io, are given in Table lII. If
we choose for the surface binding energy
values of 7 and 21 eV (from the above
quasi-monatomic model) for Na and Na2S,
respectively, we find the fraction for which
escape is nearly 100%. A similar result was
used earlier for sodium by Sieveka and
Johnson (1984). This fraction is much larger
than that found for sulfur and SO2 because
Na2S contains the most sodium of the sodium polysulfides known to exist. Unfortunately this abundance of sodium, coupled
with the favored size and oxidation state of
- 2 for S, causes the strong ionic bonding in
NazS and thus the relatively low sputtering
yield. Although the larger sulfides contain
less sodium their increased sulfur coordination leads to a considerably lower binding
energy (as indicated by their melting
points). Values of the melting point for various sodium and potassium polysulfides,
thought to exist in the melt, are given in
Table IV. This tradeoff of lower sodium
content per molecule for increased volatility, and thus increased sputtering yield,
makes the larger sodium polysulfides attractive in terms of meeting the required
injection rates.
As an example of this idea of lower binding on increased sulfur coordination we plot
in Fig. 2 the melting points of various
known sodium and potassium polysulfides
versus the ratios of Na or K atoms to S
atoms in the molecule. This plot starts with
$8 since it contains the lowest number of
sodium atoms (zero) and ends with Na2S
and K2S since they contain the most (two)
per molecule. The large jump in melting
points from Na2S to Na2S2 is indicative of
THE SPUTTERING OF SODIUM SULFIDE ON IO
241
2Na + S ~ Na2S.
TABLE IV
MELTING POINTS FOR VARIOUS SODIUM AND
POTASSIUM SULFIDES AND 5 8 AS A
ROUGH INDICATION OF THAT SPECIES SURFACE
BINDING ENERGY
Species
No. Na or K atoms/
No. S atoms
mp
(°K)
$8
Na2Ss, K2S~
NazS4, K2S4
Na2S3, K2S3
Na2S2, KzS~
Na2S, K2S
0
0.40
0.50
0.67
1.00
2.00
386
531,479
558,418
~, 525
748,743
1453, 1113
a When NazS3 approaches a melting point it quickly
changes to equal parts of Na2S4 and Na2S2 (Oei 1973a).
the favored size of S 2 versus $22 and larger
ions in the lattice.
At this point it is worth reevaluating the
aforementioned sodium sulfide production
scenario in order to explore the feasibility
of larger sodium polysulfides. The chemical
composition of various amounts of sodium
and sulfur at various temperatures is presented most clearly in the phase diagram
results of Oei (1973a). A mixture of sodium
and sulfur, when heated, will first form
Na2S,
1500
Depending on the stoichiometric ratio of
sulfur to sodium which exists in the melt,
subsequent formation of larger polysulfides
can proceed not through a single reaction as
shown above but rather through an intermediate polysulfide. If additional sulfur exists,
as is likely, sodium pentasulfide is formed,
2Na2S + 4S ~ Na2S + Na2S5--~
Na2S2 + Na2S4.
D
K 2 Si
Na 2 S i
(6)
Further reactions among these products
with sulfur then proceed,
3Na2S2 + 6S ~ 2Na2S5
+ Na2S2 ~ 3Na2S4.
(7)
The times necessary for these reactions
(hours) are negligible compared to that for
which sodium and sulfur will be in contact
with one another in a hot spot on Io. Therefore, for our purposes the final composition
can be determined by the phase diagram of
Oei (1973a) for a given ratio of S to Na.
Although Oei's diagram ends with Na2S5
being the largest polysulfide formed, subsequent work associated with the sodiumsulfur battery state that larger polysulfides
I
•
(5)
I
I
1000
o-
500
D
S8
I
I
I
0.5
1.0
1.5
2.0
÷Na, K
--/molecule
÷S
FIG. 2. The melting points of various sodium and potassium sulfides versus the ratio of sodium or
potassium atoms to sulfur atoms per molecule. Also shown in this plot is $8.
242
CHRISEY ET AL.
up to Na2S20 exist in the melt (Kn6dler
1985).
The method of Na2S formation within a
hot spot given in the Introduction never
stated the stoichiometric ratio of Na to S
which exists in the melt. Instead the only
estimate of sodium presented was for the
Na2S surface coverage, based on sodium
mixing ratios in the plasma torus (Bagenal
and Sullivan 1982, Gehrels and Stone 1983,
Schneider et al. 1987). Clearly, if excess
sulfur exists in the melt, the larger polysulfides will be produced (Lunine and
Stevenson 1985), in which case the amount
of sodium existing on the surface will stay
the same ( - 1 % in our estimates), but its
effective surface binding energy will decrease significantly. That is, we expect that
the effective surface binding energy in the
aforementioned quasi-monatomic model
will decrease as the melting point decreases. Then as the sodium bonds to more
and more sulfur atoms the average binding
per atom which occurs in the solid should
approach that of pure sulfur. As previously
stated, the effective sputtering binding energy of sulfur determined from our earlier
results is 0.19 eV/S atom.
If, on increased sulfur coordination, the
sodium binding energy approaches that of
pure sulfur, then the mean yield, Ym in Eq.
(1), due to a plasma made up equally of
oxygen and sulfur ions would be about 20
sodium atoms/plasma ion, i.e., the same
sputtering yield as for pure sulfur (Chrisey
et al. 1987). However, as the yield increases the escape fraction decreases due
to the large number of slower species. Assuming an idealized situation where the
sputtering yield and escape fraction are
given by Eqs. (2) and (4), respectively, the
direct injection rate for sputtering from the
surface (which is the product of Eqs. (2)
and (4)) becomes independent of U when
the surface binding energy per atom decreases below Ecsc per atom. Based on the
above, a significant amount of material goes
on to form a sputter-produced coronal
atmosphere similar to the case of sulfur
(Chrisey et al. 1987) and SO2 (Sieveka
and Johnson 1985) sputtering. This atmosphere is then a potential source of material
for the torus containing atoms and small
molecules (e.g., NaS). In fact, previous calculations of the sodium supply rate suggested that ejection and dissociation of sodium from a molecule in an atmosphere was
favorable for producing the observed spatial distribution of fast sodium ejection as
well as the correct ratio of fast to slow sodium ejection (Sieveka and Johnson 1984,
Pilcher et al. 1984). Using the sulfur sputtering rates from Chrisey et al. (1987), the
yield in Eq. (i) becomes Ym -- 20 sodium
atoms per plasma ion, giving a surface
source strength of - 2 × 10 9 sodium atoms/
cmZ-sec using a 1% coverage.
The distribution of masses ejected from
the larger polysulfides are unknown. Previous work on the sputtering of sulfur indicated that smaller species (in particular $2)
were primarily ejected from a target originally composed of $8 molecules (Chrisey et
al. 1988). These results also showed that
correlations in the cascade (e.g., multiple
hits) were increasingly necessary to eject
larger whole species. Therefore, we expect
that corotating ion bombardment will eject
primarily small molecules containing sodium with some atomic sodium.
V. CONCLUSION
We have presented measurements of the
collision cascade sputtering of Na2S and extrapolated the yield to that for the corotating ions (stopping powers) relevant to Io.
The yield is much less than that for other
more weakly bound species. As sodium
may appear in the form Na2Si (where i > 1),
then based on our description of the sputter
yield for Na2S and on our previous measurements for $8 and using a rough lower
limit of 1% atomic composition for sodium,
we can constrain the sodium supply to the
sputter corona by corotating ions as 2 ×
107-2 x l09 sodium atoms/cm2-sec depending on the form of sodium on the surface.
These numbers are now available for describing the Io atmospheric corona. It is in-
THE SPUTTERING OF SODIUM SULFIDE ON IO
teresting that a large fraction of sodium is
ejected in molecular form (e.g., NaS,
Na2S). This is in contrast to assumptions
made earlier for evaluating the sodium
sputter supply rate (Haft et al. 1981). However, it is consistent with the calculation of
Sieveka and Johnson (1984) and Pilcher et
al. (1984) in that aspects of the fast sodium
supply rate suggested collisional ejection of
sodium contained in a molecular form from
the upper atmosphere. The mass spectra
also suggest that a fraction of the sodium is
ejected in an atomic form. Because of the
nature of the collision cascade spectrum,
these neutral sodium atoms have large average energies so that in a collisionless region (e.g., polar region, nightside) much of
the sodium so produced escapes. The fact
that some of the ejected sodium occurs as
atomic sodium is evident from the observations of Schneider et al. (1987). That supplied to the atmosphere in molecular form
requires subsequent photo- or plasma-induced dissociation and collisionai ejection
by the incident plasma. Although the
amount of Na required from the surface is
not well established the largest suggested
values required to supply the torus are - 1 0 9
sodium atoms/cm 2. The present study
shows that more than this upper limit of
sodium can be supplied to the coronal atmosphere if Na is ejected from a polysulfide
and if the surface concentration is larger
than the 1% used here. The subsequent
ejection into the torus is also determined by
the incident plasma (Sieveka and Johnson
1984, McGrath and Johnson 1987).
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