Nuclear Instruments and Methods in Physics Research B 193 (2002) 775–780
www.elsevier.com/locate/nimb
Ion beam induced chemistry: the case of ozone synthesis and
its influence on the sputtering of solid oxygen
M. Fam
a, D.A. Bahr, B.D. Teolis, R.A. Baragiola
*
Laboratory for Atomic and Surface Physics, Engineering Physics, University of Virginia, 116 Engineers Way, Charlottesville,
VA 22904, USA
Abstract
To understand chemical effects caused by ion beams in solids we study the simplest case, the formation of ozone in
solid oxygen. We irradiate thin O2 films with 100 keV protons and study the fluence dependence of the sputtering of
solid O2 with a microbalance and a mass spectrometer. The formation of ozone in the film after irradiation is identified
by optical reflectance spectroscopy and thermal desorption. We discuss the processes involved and the implications of
the fluence dependence of the ozone concentration upon the sputtering yield of oxygen. Ó 2002 Elsevier Science B.V.
All rights reserved.
PACS: 79.20.Rf; 82.50.Fv; 82.50.Gw
Keywords: Solid oxygen; Ozone; Electronic sputtering; Radiolysis; Fluence dependence
1. Introduction
The electronic energy deposition by fast ions in
solidified gases can lead to electronic sputtering [1].
This phenomenon is complex; its description requires an understanding of the conversion of
electronic energy into atomic motion. Studies of
the sputtering of solid oxygen [2–5] have revealed
the interesting fact that sputtering yields depend
non linearly upon the electronic stopping cross
section Se (roughly the energy deposited per unit
path length), in a manner which has been described as a transition between linear and qua-
*
Corresponding author. Tel.: +1-804-922-2907; fax: +1-804924-1353.
E-mail address: [email protected] (R.A. Baragiola).
dratic with increasing Se [6]. Proposed models for
nonlinear yields are based upon coulombic repulsion between ionized lattice atoms near the surface
[7] as well as thermal spikes when the excitation
density is above some threshold value [8]. Additionally it has been noted that the sputtering yield
of solid oxygen is more than twice that of solid
nitrogen in the linear region as well as in the
quadratic region [2–4]. Since all sputtering parameters, the stopping power, the sublimation
energy and the energy required to produce an ion–
electron pair are very similar for the two elements
[5] it appears that the conversion from the electronic excitations to atomic motion is much more
efficient in solid oxygen than in solid nitrogen.
On the other hand, chemical alterations produced by the projectiles will be different in both
cases. Radiation-chemical changes, or radiolysis,
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 0 9 0 3 - 5
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M. Fama et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 775–780
are typically produced by a mixture of reactive
intermediates that initially include both ions and
excited molecules and later, free radicals [9]. Radiolytic formation of ozone (O3 ) in gaseous oxygen,
first studied by Lind [10] using a particles, is of
crucial importance in the middle atmosphere
(ozone is the only atmospheric species which effectively absorbs ultraviolet solar radiation). For
this reason, it has received intense scrutiny during
the last 80 years [11]. In contrast, the generation of
ozone in solid oxygen matrices by particle impact is
a rather new field of research [12,13].
The goal of this study is to establish whether the
presence of ozone in solid oxygen affects the electronic sputtering yield of O2 , as suggested by
the similar fluence dependent behavior of the O2
sputtering yield and of the ozone concentration in
solid O2 [13].
Fig. 1. (a) Sputtered flux of molecular oxygen produced by
100 keV protons (incidence angle 45°) irradiating a new condensed O2 film. (b) After the ion beam was turned off for a
period of several minutes, the sputtered flux of O2 returns instantaneously to the saturation value.
2. Experimental approach
All experiments were conducted in a cryopumped, ultrahigh vacuum chamber (base pressure
1 1010 Torr), equipped with a quartz-crystal
microbalance (QCM), an ultraviolet–visible light
spectrometer, and two quadrupole mass spectrometers (MS); one of them set up to observe the
sample through a collimated differential pumping
stage. Oxygen ice films were grown at <2 nm/s by
dosing pure oxygen gas through a micro-capillary
array onto the gold electrode of the microbalance
[14], refrigerated below 10 K using liquid He. The
film thickness was typically 400 nm, and smaller
than the ion range, to avoid problems with electrostatic charging of the film. The bias of the gold
electrode was set to 30 V to suppress positive
ions ejected from the film [15]. Reflectance absorption spectroscopy was carried out with a Xe
arc lamp and a vacuum spectrometer with a CCD
detector. Gases evolved from the film during irradiation and during subsequent thermal desorption were measured with the MS.
3. Results
Fig. 1(a) illustrates the sputtered flux of O2
produced by 100 keV protons at 45° incidence on a
newly condensed film, measured with the differentially pumped MS normal to the film. It is seen
that the flux of sputtered O2 grows with fluence from an initial value (Y0 ) and saturates (YS )
above a fluence FS 3 1014 ions/cm2 . We have
observed this fluence dependence consistently, regardless of any variation in O2 film thickness,
projectile energy (30–200 keV), or temperature
(between 5 and 20 K). At some time after the
saturation value YS is reached, the ion beam was
turned off for a period that was varied between
seconds and several minutes. When the ion beam
was turned back on, the sputtered flux of O2 returned instantaneously to YS , without exhibiting
any fluence dependence, as shown in Fig. 1(b). All
further switching of the ion beam off and on yielded the same result.
Having observed that the sputtering of O2 depends on fluence during the first irradiation of a
newly grown O2 film, but not during subsequent
irradiations, we concluded that some property
of the film was altered during the irradiation.
To identify this property sputtering, we have attempted to manipulate various characteristics of
the film before and after the first irradiation to
determine whether they affected the fluence dependence of sputtering.
M. Fama et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 775–780
777
3.1. Annealing before first irradiation
To determine whether concentration of defects
in the O2 lattice, or the roughness of the film prior
to the first irradiation, caused the observed fluence
dependence, we attempted to anneal these characteristics by raising the film temperature for a given
period before the first irradiation. Annealing took
several minutes at temperatures as high as 30 K,
during which we observed desorption of several
hundred monolayers of oxygen with the QCM, and
a corresponding increase in the O2 partial pressure
in the chamber with the MS. This pre-irradiation
annealing did not change the behavior of the
sputtering yield upon the first irradiation. As in the
case of no previous annealing, the sputtering yield
continued to depend on fluence for just the first
irradiation, as shown in Fig. 1(a).
Fig. 2. Sputtered flux of O2 for a film previously irradiated
(Fig. 1(b)) and then annealed at 30 K, which also resulted in
desorption of several hundred monolayers.
3.2. Annealing after first irradiation
Speculating that the concentration of defects in
the film or surface roughness is altered by the first
irradiation, and that this may be a cause of the
observed fluence dependence, we attempted to
change these characteristics by annealing the film
after the first irradiation. As before, annealing was
to temperatures as high as 30 K, which also resulted in desorption of several hundred monolayers of O2 from the film as seen by the MS. The first
irradiation having already occurred, the film was
irradiated for a second time following annealing.
For these trials, the sputtering yield, as shown in
Fig. 2, behaved differently from that measured in
the second irradiation in the absence of prior annealing (Fig. 1(b)). Unlike the initial behavior, the
sputtered flux decreased from a value Y1 > YS to
the same saturation value YS .
3.3. Deposition of new films
To determine whether the fluence dependence
of sputtering during the first irradiation is related
to changes in the surface or in the bulk (>1000
monolayers) properties of the film, we grew additional O2 layers of varying thickness over a previously irradiated film and then irradiated the film
Fig. 3. Relative variation for the initial sputtering flux of O2
versus the thickness of newly deposited films over a previously
irradiated film.
for a second time. Fig. 3 shows that the relative
value ðYS Y0 Þ=YS resulting from a second irradiation saturates at about 50% when more than
approximately 100 monolayers of O2 have been
deposited over a previously irradiated film.
3.4. Experiments with electrons
We then explored the idea that the changes in
the film might be caused by the creation and
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M. Fama et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 775–780
trapping of electrostatic charges in the film. Electron emission during irradiation will create excess
positive charges in the film, which may self-trap or
trap at defects sites rather than drift to the metal
substrate. However, we could not restore the initial
fluence dependence of the sputtering yield by irradiating the sample with 30–300 eV electrons
(which neutralize the surface charge).
3.5. Ozone synthesis
The fluence dependence may be caused by the
synthesis of ozone, which alters the composition of
the sample during irradiation. A smaller, 25%
increase in total sputtering yield Y with fluence for
recoil sputtering has been seen before [16] but not
in the electronic sputtering regime. Moreover, the
ejection of Oþ
n clusters from solid oxygen by rare
gas atoms with energies in the keV range was also
seen to depend upon fluence [17]. Both cases were
attributed to the formation and storage of ozone,
but these conjectures, though reasonable, were not
supported by evidence of ozone.
Ozone formation starts by generation of oxygen
atoms. This occurs either by direct dissociation of
O2 by the projectile or by secondary electrons or
indirectly via dissociative electron attachment [18]
or dissociative recombination involving secondary
electrons and ions. The number of dissociations is
G 0:5 per 100 eV of electronically deposited
energy [13]. Ozone production occurs when an
oxygen atom combines with an adjacent O2 molecule.
We can demonstrate ozone synthesis in two
different ways. Following the first irradiation, the
optical reflectance of the film, as shown in Fig. 4,
shows a strong absorption band in the ultraviolet
that matches the Hartley band reported for vapordeposited ozone (it is red-shifted by about 15 nm
[19]). Thermal desorption of irradiated samples, as
shown in Fig. 5, also implies ozone synthesis.
Here, a 1:3 1018 O2 /cm2 (500 nm) film was irradiated with 2:5 1015 ions/cm2 100 keV protons
at normal incidence. The film was then desorbed at
1 K/min. Two desorption peaks are seen by the
microbalance and the MS: one at 35 K, which
corresponds to the sublimation of O2 [20] and the
other at 63 K, due to the sublimation of O3 . As O2
Fig. 4. Ratio of reflectance of a 400 nm O2 film after to before
irradiation with 100 keV Hþ to a fluence of 9:2 1014 ions/cm2 .
Fig. 5. Thermal desorption spectra for a 500 nm film irradiated
with 100 keV protons at normal incidence to a fluence of
2:5 1015 ions/cm2 . The heating rate is 1 K/min. The partial
pressures are not corrected for relative detection efficiency of
the mass spectrometer for O2 and O3 .
M. Fama et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 775–780
desorbs, the concentration of O3 increases and
eventually an ozone film ends up soft-landing [21]
on the Au substrate. The much larger O2 than O3
peak at 63 K is explained by the instability of the
ozone molecule. Unlike O2 , which can be detected
directly by the MS or after it bounces off walls in
the target chamber, O3 can only be detected directly, since it is destroyed efficiently in collisions
with the walls, giving rise to gas phase O2 , explaining the 63 K peak appearing in the O2 curve
of Fig. 5.
To determine the effect of the ozone presence
upon sputtering, we altered the ozone concentration in the irradiated oxygen matrix by exposing a
previously irradiated oxygen film, with a saturated
ozone concentration, to light from a Hg(Ar) UV
lamp, for which the most intense line emission is at
253.7 nm. At this wavelength, near the center of the
Hartley band, ozone can be destroyed by photodissociation with a cross section of 1017 cm2
[22]. Using thermal desorption we found that the
amount of ozone in the film decreased by a factor
of three after 20 min of illumination. Fig. 6(a)
shows the flux of sputtered oxygen molecules
produced by 100 keV protons on a newly deposited film, and Fig. 6(b) after this film was illuminated with the Hg lamp for 20 min. The differences
with Fig. 1(a) and (b) are remarkable.
Fig. 6. (a) Sputtered flux of molecular oxygen produced by 100
keV protons (incidence angle 45°) irradiating a new condensed
O2 film. (b) After the ion beam was turned off and illuminated
during 20 min with UV light (253.7 nm).
779
During UV exposure we observed a low
positive current through the sample (1010 A)
due to photoelectron emission from the gold
substrate. To determine whether these electrons
could regenerate the fluence dependence, we induced electron emission from the substrate into
the oxygen film using light in the visible range
with a low UV component that could not destroy
ozone. The substrate was modified by depositing
a thin layer of sodium (1 monolayer) onto the
gold surface to decrease the surface work function thereby increasing the photoemission current by two orders of magnitude, without
altering the ozone concentration. This treatment
did not produce any change in the sputtering
yield of O2 .
4. Discussion
The presence of ozone in the oxygen matrix
strongly affects the electronic sputtering yield of
molecular oxygen by protons. We propose that
this is due to the storage of energy in the film when
O3 is produced, and which can then be liberated by
subsequent ions. The energy per O3 is one half the
potential energy difference between two O3 molecules and the three O2 molecules to which they can
decay (after providing activation energy). Using
the potential energies (total dissociation energies)
[22] U ðO2 Þ ¼ 5:12 eV and U ðO3 Þ ¼ 1:05 eV þ
U ðO2 Þ ¼ 6:17 eV, then the energy stored per O3
molecule is 1=2ð3 5:12 2 6:17Þ eV ¼ 1:51 eV.
For saturation O3 concentration of 1% [13], the
energy stored in the films is 15.1 meV per O2
molecule, which is twice the cohesion energy of
7.5 meV. Work is needed to identify the mechanisms by which the energy stored in O3 would be
available for O2 ejection; a promising source of
information might be contained in the energy
distribution of the ejected molecules. The idea of
enhanced sputtering due to energy storage may
also apply to other materials where a sufficiently
high concentration of energy can be stored by the
incident radiation either in the synthesis of new
molecules or in metastable electronic states (e.g.
trapped electrons).
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M. Fama et al. / Nucl. Instr. and Meth. in Phys. Res. B 193 (2002) 775–780
Acknowledgements
This work was supported by NSF-Astronomy and NASA’s Office of Space Science. BDT
gratefully acknowledges IGERT fellowship support under National Science Foundation Grant #
9972790.
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