The Application of RBS To Investigate The Diffusion of HCl
Into The Near Surface Region Of Ice.
T. Huthwelker(1,2), U.K. Krieger(2), Th. Peter(2), W.A. Lanford(1)
(1)
University at Albany (SUNY), Physics Department, Washington Avenue, Albany, 12222 NY, USA.
(2)
Institute for Atmospheric and Climate Science (IACETH), ETH Zürich, Switzerland.
Abstract. The interaction of trace gases in the near surface region of aerosols (ice, liquid acids, hydrates) is important
for understanding environmental problems, such as the formation of the Ozone-hole or global warming. Direct
measurements of trace gas concentration profiles on materials such as ice can provide key data to understand the
underlying physical chemistry. However, measurement of concentration profiles in the near surface region of volatile
materials presents a significant analytical challenge due to the materials high vapor pressure. We use Rutherford
Backscattering (RBS) to measure in situ elemental concentration profiles on high vapor pressure materials held in
controlled atmospheres of water vapor and trace gases. HCl uptake experiments are presented and the HCl diffusion and
solubility at temperatures around 200 K are determined.
Additional complications arise from the specific
nature of the ice surface. Ice has a high vapor pressure.
At 200 K, the ice vapor pressure is about 1 mTorr, thus
103 monolayers per second are exchanged with the gas
phase. The concept of adsorption sites, as used in
adsorption models, such as the Langmuir or BETisotherms may not be appropriate to the description of
the trace gas uptake on such highly dynamic surfaces.
Also, close to the melting point there is a disordered
surface on ice [6,7]. It is suspected that this disordered
surface layer may affect the uptake up trace gases on
ice.
INTRODUCTION
The trace gas-aerosol interactions are key to
understanding problems, such as the formation of the
ozone hole and global warming. For example,
chemical reactions of halogen compounds on
atmospheric aerosols prime the atmosphere for the
annual ozone hole [1,2]. Also, the trace gas-ice
interactions are important when analyzing polar ice
cores in order to reconstruct Earth’s past climate [3].
The water uptake on salt and ammonium sulfate
aerosols in the atmosphere is important for cloud
formation processes, and may thus influence the
Earth’s radiation budget.
Consequently a direct investigation of the trace gas
uptake and its diffusion in the near surface region will
refine our understanding of the fundamental physical
chemistry of high vapor pressure material. This will
form the base for modeling heterogeneous processes in
natural environments.
For liquid aerosols, heterogeneous reactions are
well understood in terms of gas phase diffusion
towards the aerosol, mass accommodation at the
surface, diffusion and possible chemical reaction in the
condensed phase. On solids, adsorption and reaction
on the surface play an additional or even dominant role
[4]. However, modeling studies indicate, that simple
adsorption models may not be sufficient to understand
the nature of the trace gas-ice interaction [5]. The
relative importance of surface and bulk processes (i.e.
adsorption, bulk solubility and diffusion) remains
unclear.
The direct measurement of elemental concentration
profiles in the near surface region of high vapor
pressure materials is impossible by analytical methods
that require samples held in high vacuum. Recently,
we have demonstrated, that Rutherford backscattering
(RBS) [8] can be used in situ on samples held in
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
400
controlled ambients at pressures up to a few Torr
[9,10].
gas source
In this paper we present preliminary measurements
of the HCl uptake onto ice and derive diffusion
constants of HCl in the near surface region.
Water, HCl
He+
EXPERIMENTAL SETUP
Ice
To study the uptake of trace gases on ice we have
combined a chemical reactor (a ‘Knudsen-cell’) with a
RBS-setup. Because the system is detailed in the
literature [10], only a brief description is given here.
Si-detector
The target chamber consists of a standard ISO-K
100 cross. A target holder can be mounted horizontally
or vertically in this cross. This target holder can be
rotated and is temperature controlled (-130°C-30°C).
The target chamber has a controlled atmosphere of
water vapor and a trace gas such as HCl and is
continuously analyzed by mass spectrometry. This
high-pressure chamber is connected to the SUNY
accelerator via a differential pumping stage. A
standard Si-detector in the high pressure region is used
to record the energy spectrum of the backscattered
ions.
SUNY
accelerator
Mass spectrometer
FIGURE 1. Sketch of experimental setup.
As we seek to profile Cl at a concentration level of
one part per thousand, the ice contamination with other
impurities must be kept well below this level. To
achieve this goal all glassware in contact with the
water is cleaned for several days in a 10-30 wt%
HNO3-water solution, then carefully rinsed with
deionized water. In the vacuum system only silicon
free vacuum grease is used (if at all). With these
procedures ice with no measurable contaminations can
be made. However, since the stainless steel cell is
regularly exposed to gaseous HCl, there is an HClbackground contamination of ~10-8 Torr, resulting in
non-zero HCl uptake even in the fresh ice. Once the
ice is grown, the water partial pressure in the cell is
adjusted to match the ice vapor pressure.
HCl DIFFUSION INTO ICE
When bringing ice samples from air into the
analysis chamber, atmospheric water vapor will
always condense on the ice surface. In our setup, we
avoid this problem, by growing the ice in situ on the
vertically mounted target holder. The sample holder
can be accessed through an opened flange above the
target surface. To avoid condensation of ambient water
vapor a slight overpressure of dry nitrogen is
maintained in the cell, such that a constant gas flow
exits through the opened top flange.
RESULTS
HCl uptake measurements at –70°C, with HCl
partial pressures ~10-6 Torr have been performed. A
time series of RBS spectra showing HCl uptake into
the near surface region of ice is presented in Fig. 2.
Deionized and degassed water is dropped with a
pipette onto the cooled sample holder and frozen at
temperatures above –5°C. Ice prepared this way is
bubble-free and clear. We do not have a method to
measure the grain structure of the ice. However, since
the ice is frozen slowly, our targets should be large
grained and most importantly, similar to the smooth
ice used by other authors (cf. [11]) for HCl uptake
experiments. Thus our results should be comparable
with those study.
The figure shows two main features. First, the HCl
in the surface region rises with time. Secondly, and
most interestingly, the HCl uptake extends roughly
one micron into the depth of the ice. Further, there is
no sharp surface peak as one would expect for the
adsorption of HCl onto a surface. It should be noted,
that these data show the first direct in situ observation
of the diffusion of HCl in the near surface region of
ice, with ice in equilibrium with its vapor.
401
Depth [A]
5000
2500
be used to derive the solubility and diffusivities of HCl
in ice.
0
0.02
Other data on the HCl diffusion at temperatures
~200 K scatter widely. In previous work, we have
determined a value of 5x10-16-3x10-15 m2/s from
evaporating ice, which was frozen from a HCl doped
solution. Livingston and George [13] have determined
a value of 10-14 m2s-1 from vapor grown ice. Wolff and
Mulvaney [14] estimated 10-13–10-17m2/s using an Xray microprobe. Chu et al. [15] suggested 2x10-17 m2/s.
Fluckinger [11] estimated 4x10-18m2/s for single
crystals, and 10-16-10-17 m2/s for polycrystalline ice
using an indirect titration method in a Knudsen-cell
experiment. The result of this study of 1-4x10-17m2/s is
in the lower range of the available data. This may
indicate, that the HCl transport in the weakly
polycrystalline ice sample benefits only to a minor
degree from lattice imperfections, such as grain
boundaries and dislocations.
.005
Cl/O
norm counts
7500
Cl
0.00
0.8
1.0
1.2
.001
0
1.4
E [MeV]
FIGURE 2. Raw data of the time-series of the HCl
diffusing into ice at –70°C (at t=0, 45, 166, 266, 350,
416 min). The lowermost spectrum (t=0min) shows
the initial contamination of the ice due to residual HCl.
Note that the ice is slowly evaporating in the
uppermost spectrum.
To further analyze these spectra we use Henry’s
Law Hxgas = xsolid to describe the HCl solubility. Here,
xgas and xsolid are the HCl:H2O molecular ratios in the
gas and the solid phase respectively. Assuming that
Henrys’ Law is always valid at the surface, the
diffusion profile is given by (e.g. [12])
xsolid(x,t) = x0 erfc (x / ( D t)1/2 )
The HCl uptake onto ice has been studied by
various authors using a variety of methods. Most
experiments have been performed in much shorter
timescales (less than 30 min) than in the experiments
presented here. Thus our measurements cannot directly
be compared with their results. The solubility of HCl
in single crystals has been measured by Thibert and
Dominé [12] at temperatures above -35°C. These
authors used thermodynamic considerations to derive a
parameterization of the HCl solubility as function of
the temperature and HCl partial pressure. This
parameterization can be used to calculate the HCl
solubility in single crystals at the conditions of our
experiments. Our data give solubilities, that are about
a factor of 50 higher than the estimates from the work
of Thibert and Dominé [12].
(1)
Fitting this equation to the data gives x0 (and thus
the Henrys’ Law constant) and also the diffusivity D.
A condition for validity of this equation is that the
gas-phase concentration of HCl is constant during the
uptake. However, during the course of the uptake
experiment of several hours, the HCl partial pressure
increased by about a factor 4 due to very slow wall
equilibration of he stainless steel cell. But, as the HCl
partial pressure changes very slowly, Eq. 1 is still valid
in a quasi-static approximation.
We are not certain of the significance of our
observation of 2 orders of magnitude higher solubility
of HCl than what would be expected from the
extrapolations of the Thibert and Domine work. In
polycrystalline ice trace gases may accumulate in the
grain boundaries and triple junctions of the ice as
suggested by several authors [14, 16]. RBS cannot
distinguish between such different reservoirs. We also
note, that Thibert and Dominé [12] derived the bulk
solubility from HCl diffusion profiles in single crystals
with a spatial resolution of about 20 µm. In contrast,
RBS probes the near surface region with 2 orders of
magnitude better depth resolution. One may speculate,
that the solubility in the near surface region is
enhanced compared to the solubility in the single
crystal matrix due to specific effects. For example, the
volumes of grain boundaries and triple junctions
increase towards the surface [17, 18]. If HCl
Using this analysis, we find for the HCl surface
concentration values of 0.1-0.4 mole %, corresponding
to a Henry’s Law constants of about H=108). From our
data we derive a diffusion constant of about 1-4 x 10-17
m2/s.
DISCUSSION
We have demonstrated that we can observe the
diffusion of HCl into the near surface region of ice
with RBS in situ and non-destructively. Our data can
402
6. Carslaw, K. S., and Peter, Th., Geophys. Res. Lett., 24
(14), 1743-1746 (1997).
accumulates in such reservoirs, this would enhance the
effective surface concentration compared to the bulk
ice in the polycrystalline sample. Another possibility is
that due to the high ice vapor pressure, there is a
continuous burial of HCl molecules in the upper
surface region. As the HCl is buried in concentrations
above the bulk solubility limit, HCl will be expelled
from the ice matrix. The counter play between the
burial of HCl into the ice and the expulsion of HCl
from the bulk ice will lead to an HCl enriched surface
region, in similarity to the physical picture used by
Gertner and Hynes [19]. Alternatively, a sub-surface
disordered layer (quasi-liquid-layer) may lead to the
observed high HCl concentration in the near surface
region. If so, we may observe the diffusion of HCl into
such a disordered layer in the presented experiments or
the formation of a disordered surface layer by
interaction of the gaseous HCl with the ice surface.
7. Dash, J.G., Fu, H., and Wettlaufer, J. S., Rep. Prog.
Phys., 58, 115-167 (1995).
8. Dosch, H., Critical phenomena at surfaces and
interfaces, Springer Tracts in modern Physics, Berlin,
Springer, 1992.
9. Chu, W. K., Mayer, J. M., and Nicolet, M. A.,
Backscattering spectrometry, New York, Academic
Press, 1978.
10. Krieger, U. K., Huthwelker, T., Daniel, C., Weers, U.,
Peter, Th., and Lanford, W. A., Science, 295, 1048-1050
(2002).
11. Huthwelker, T., Krieger, U. K., Weers, U., Peter, Th.,
and Lanford, W. A., Nucl. Instr. Meth., B 190, 47-53
(2002).
While we are far from understanding every aspect
of HCl uptake on ice, we believe RBS has the potential
to
improve
substantially
our
fundamental
understanding of the physical chemistry of high vapor
pressure solids such as ice. We hope that our work will
stimulate others to apply ion beam analysis on ice and
related materials.
12. Fluckinger, B., Chaix, L., and Rossi, M., J. Phys. Chem.
B., 104, 4432-4439 (2000).
13. Thibert, E. and Dominé, F., J. Phys. Chem. B, 101, 35543565 (1997).
14. Livingson, F. E., and George, S. M., J. Phys. Chem. A,
105, 5155-5165 (2001).
15. Wolff, E. W., and Mulvaney, R., Geophys. Res. Lett.,
18(6), 1007-1010 (2001).
ACKNOWLEDGMENTS
16. Chu, L. T., and Leu, M. T., and Keyser, L. F., J. Phys.
Chem., 97, 7779-7785 (1993).
We like to thank Art Haberl and Waye Skala for
extraordinary support concerning all question around
the SUNY accelerator and other technical problems.
We thank Uwe Weers for assistance with technical
problems.
17. Huthwelker, T., Lamb, D., Baker, M. B., Swanson, B.,
and Peter, Th., J. Colloid and Interface Science, 238,
147-159 (2001).
18. Mader, H., J. Glaciology,. 38, 359-374 (1992).
19. Nye, F. J., J. Glaciology,. 37, 401-413 (1991).
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