Effects of electron irradiation on structure and bonding of SF₆ on Ru(0001)
Electron stimulated desorption ion angular distribution (ESDIAD) and temperature programmed desorption (TPD) techniques have been employed to study radiation-induced decomposition of fractional monolayer SF₆ films physisorbed on Ru(0001) at 25 K. Our focus is on the origin of F⁺ and F⁻ ions, which d...
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irk-123456789-1288172018-01-15T03:03:51Z Effects of electron irradiation on structure and bonding of SF₆ on Ru(0001) Faradzhev, N.S. Kusmierek, D.O. Yakshinskiy, B.V. Madey, T.E. Electronically Induced Phenomena: Low Temperature Aspects Electron stimulated desorption ion angular distribution (ESDIAD) and temperature programmed desorption (TPD) techniques have been employed to study radiation-induced decomposition of fractional monolayer SF₆ films physisorbed on Ru(0001) at 25 K. Our focus is on the origin of F⁺ and F⁻ ions, which dominate ESD from fractional monolayers. F⁻ ions escape only in off-normal directions and originate from undissociated molecules. The origins of F⁺ ions are more complicated. The F⁺ ions from electron stimulated desorption of molecularly adsorbed SF₆ desorb in off-normal directions, in symmetric ESDIAD patterns. Electron beam exposure leads to formation of SFx (x = 0 - 5) fragments, which become the source of positive ions in normal and off-normal directions. Electron exposure > 10¹⁶ cm⁻ ² results in decomposition of the entire adsorbed SF₆ layer. 2003 Article Effects of electron irradiation on structure and bonding of SF₆ on Ru(0001) / N.S. Faradzhev, D.O. Kusmierek, B.V. Yakshinskiy, T.E. Madey // Физика низких температур. — 2003. — Т. 29, № 3. — С. 286-295. — Бібліогр.: 34 назв. — англ. 0132-6414 PACS: 79.20.La http://dspace.nbuv.gov.ua/handle/123456789/128817 en Физика низких температур Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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Electronically Induced Phenomena: Low Temperature Aspects Electronically Induced Phenomena: Low Temperature Aspects |
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Electronically Induced Phenomena: Low Temperature Aspects Electronically Induced Phenomena: Low Temperature Aspects Faradzhev, N.S. Kusmierek, D.O. Yakshinskiy, B.V. Madey, T.E. Effects of electron irradiation on structure and bonding of SF₆ on Ru(0001) Физика низких температур |
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Electron stimulated desorption ion angular distribution (ESDIAD) and temperature programmed desorption (TPD) techniques have been employed to study radiation-induced decomposition of fractional monolayer SF₆ films physisorbed on Ru(0001) at 25 K. Our focus is on the origin of F⁺ and F⁻ ions, which dominate ESD from fractional monolayers. F⁻ ions escape only in off-normal directions and originate from undissociated molecules. The origins of F⁺ ions are more complicated. The F⁺ ions from electron stimulated desorption of molecularly adsorbed SF₆ desorb in off-normal directions, in symmetric ESDIAD patterns. Electron beam exposure leads to formation of SFx (x = 0 - 5) fragments, which become the source of positive ions in normal and off-normal directions. Electron exposure > 10¹⁶ cm⁻ ² results in decomposition of the entire adsorbed SF₆ layer. |
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Article |
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Faradzhev, N.S. Kusmierek, D.O. Yakshinskiy, B.V. Madey, T.E. |
| author_facet |
Faradzhev, N.S. Kusmierek, D.O. Yakshinskiy, B.V. Madey, T.E. |
| author_sort |
Faradzhev, N.S. |
| title |
Effects of electron irradiation on structure and bonding of SF₆ on Ru(0001) |
| title_short |
Effects of electron irradiation on structure and bonding of SF₆ on Ru(0001) |
| title_full |
Effects of electron irradiation on structure and bonding of SF₆ on Ru(0001) |
| title_fullStr |
Effects of electron irradiation on structure and bonding of SF₆ on Ru(0001) |
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Effects of electron irradiation on structure and bonding of SF₆ on Ru(0001) |
| title_sort |
effects of electron irradiation on structure and bonding of sf₆ on ru(0001) |
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Фізико-технічний інститут низьких температур ім. Б.І. Вєркіна НАН України |
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2003 |
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Electronically Induced Phenomena: Low Temperature Aspects |
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http://dspace.nbuv.gov.ua/handle/123456789/128817 |
| citation_txt |
Effects of electron irradiation on structure and bonding of SF₆ on Ru(0001) / N.S. Faradzhev, D.O. Kusmierek, B.V. Yakshinskiy, T.E. Madey // Физика низких температур. — 2003. — Т. 29, № 3. — С. 286-295. — Бібліогр.: 34 назв. — англ. |
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Физика низких температур |
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2025-07-09T09:58:03Z |
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| fulltext |
Fizika Nizkikh Temperatur, 2003, v. 29, No. 3, p. 286–295
Effects of electron irradiation on structure and bonding of
SF6 on Ru(0001)
N.S. Faradzhev, D.O. Kusmierek, B.V. Yakshinskiy, and T.E. Madey
Department of Physics and Astronomy and Laboratory for Surface Modification, Rutgers University
136 Frelinghuysen Rd., Piscataway, NJ 08854-8019, USA
E-mail: madey@physics.rutgers.edu
Received July 22, 2002, revised August 14, 2002
Electron stimulated desorption ion angular distribution (ESDIAD) and temperature prog-
rammed desorption (TPD) techniques have been employed to study radiation-induced decomposi-
tion of fractional monolayer SF6 films physisorbed on Ru(0001) at 25 K. Our focus is on the origin
of F+ and F– ions, which dominate ESD from fractional monolayers. F– ions escape only in off-nor-
mal directions and originate from undissociated molecules. The origins of F+ ions are more compli-
cated. The F+ ions from electron stimulated desorption of molecularly adsorbed SF6 desorb in
off-normal directions, in symmetric ESDIAD patterns. Electron beam exposure leads to formation
of SFx (x = 0–5) fragments, which become the source of positive ions in normal and off-normal di-
rections. Electron exposure > 1016 cm–2 results in decomposition of the entire adsorbed SF6 layer.
PACS: 79.20.La
1. Introduction
As part of a program to study radiation-induced
processes in adsorbed fluorine and chlorine-containing
molecules [1–4], we are examining electron stimu-
lated desorption (ESD) of SF6 adsorbed on a single
crystal metal surface. This study has several compo-
nents, including: 1) the study of the structure and re-
activity of SF6 on a clean Ru(0001) [5] surface; 2) the
influence of coadsorbed atoms and molecules in ESD
of SF6 (including a search for enhancement of the F–
ESD signal [1–3]); 3) electron beam damage pro-
cesses in the adsorbed layer. The present paper focuses
on the latter: the effects of electron bombardment on
the structure and bonding of SF6, as revealed by
changes in F+ and F– ESD yields and angular distribu-
tions.
Sulfur hexafluoride, SF6, is a highly symmetric, in-
organic, chemically inert, man-made molecule. The
sulfur atom resides at the center of a regular
octahedron, whose corners are occupied by the six flu-
orine atoms. SF6 has a positive electron affinity,
whose presently accepted value is � 1.06 � 0.06 eV [6].
Based upon studies of low-energy electron interactions
with gaseous SF6, it is known that gaseous SF6 atta-
ches thermal and near thermal electrons with a very
large cross-section to become SF6
�. This ability to
capture thermal electrons makes SF6 popular for tech-
nical applications as an electron scavenger in high
voltage electrical devices [7]. SF6 is also used as a
dry-etching gas in plasma processing [8] and is known
to be a greenhouse gas.
Previous experiments with SF6 adsorbed on
Ru(0001) [9] and Ni(111) [10] indicate that SF6 is
physisorbed on the metal surface. Based upon the
structure of SF6 and of Ru(0001), it was argued that
the molecule should be oriented so that one set of
three F atoms is in contact with the substrate and the
other set of three F atoms lies in a plane parallel to the
surface facing the vacuum. Since the threefold symme-
try of SF6 coincides to such a large degree with the
symmetry of the (0001) plane of hcp Ru, preferred ad-
sorption sites and some degree of ordering of the mole-
cule are highly probable.
ESD of adsorbed molecules implies desorption of
neutral fragments (atoms and molecules) as well as
both positive and negative ions. Electron stimulated
desorption ion angular distribution (ESDIAD) is a
very useful technique for determining the bonding
structure of molecules adsorbed on single-crystal sur-
faces [11], since the trajectory of the desorbing parti-
cle is determined mainly by the orientation of the
© N.S. Faradzhev, D.O. Kusmierek, B.V. Yakshinskiy, and T.E. Madey, 2003
bond that is broken. ESDIAD also has a great utility
for providing insights into structure and dynamics of
decomposition of adsorbed molecules under electron
bombardment [12]: distinct electron-induced changes
in the ESDIAD patterns and intensities of specific
ions can be monitored and analyzed. In this paper we
concentrate on both F+ and F– ions produced by ESD
of a fractional SF6 layer (0.25 ML) adsorbed on a
Ru(0001) substrate (because of the high electron af-
finity of the F atom, we expect a strong F– signal in
ESD).
In the gas-phase, the low-energy dissociative elec-
tron attachment (DEA) resonance that leads to F–
formation is due to the reaction
e–(2.7eV) + SF6 � F– + SF5 .
The thermodynamic threshold is 0.65 eV [7]. In the
condensed-phase, the F– signal is dominated by two
resonant features, with maxima at 5.8 eV and
� 11 eV. A very weak signal is also detected between
1 and 3 eV [7].
In the gas-phase, electron-induced dissociation of
SF6 leading to the formation of positive ions becomes
significant above � 16 eV, producing SFx
+ (x = 1, 3,
4, 5) and F+ [6]. Measurements of electron impact
dissociative ionization of gaseous SF6 give the thresh-
old energy for F+ formation somewhere in the range of
30–50 eV [13,14], very different from F–. The com-
parison of ESDIAD images for F+ and F– should pro-
vide insights into the mechanisms of ion formation.
In order to understand better the behavior of ad-
sorbed SF6 on Ru(0001) under electron irradiation,
we utilize several surface-sensitive techniques, mainly
temperature programmed desorption (TPD) and
ESDIAD. The major findings of this work include es-
tablishing the dissociation dynamics of fractional
monolayers of SF6 adsorbed on Ru(0001), insights
into the origins of F+ and F– ions, and the difference
in sensitivity to electron exposure of fractional
monolayer and multilayer coverages of SF6. We find
that the F– ions originate primarily from undissociated
SF6 molecular adsorbates, while the F+ ions have a
high yield from both molecular SF6 and adsorbed dis-
sociation fragments.
Section 2 outlines the experimental procedures,
sec. 3 focuses on the results obtained, and sec. 4 pro-
vides a discussion of the results presented and offers
possible explanations.
2. Experimental procedures
Experiments have been carried out in an ultrahigh
vacuum (UHV) chamber equipped with apparatus
for Auger electron spectroscopy (AES), low energy
electron diffraction (LEED) and temperature prog-
rammed desorption (TPD). The chamber houses two
detectors for ESD experiments: a quadrupole mass
spectrometer (QMS) and an electron stimulated
desorption ion angular distribution detector. The
chamber is ion-pumped, reaching a base pressure of
� 5·10–11 torr after system bakeout.
The substrate is a Ru(0001) single crystal mounted
on a copper sample holder connected to a manipulator
and attached to a closed-cycle helium refrigerator,
which cools the sample to � 25 K. The sample can be
heated to 1600 K by electron bombardment of its
backside. Substrate temperature is measured by a
chromel-alumel thermocouple attached directly to the
sample. The crystal surface is cleaned by sputtering
using 1 keV Ar+ ions, heating in oxygen, followed by
annealing in vacuum. The procedure ensures effective
cleaning of the surface, which demonstrates a distinct
1�1 LEED pattern. The absence of contaminants is
monitored by AES and work function measurements.
High purity (99.95%) sulfur hexafluoride (SF6:
«Matheson») is deposited onto the clean surface at 25 K
via a directional capillary array gas doser. Gas purity
is checked by QMS (residual-gas analysis mode) as it
is introduced into the chamber. Coverages are deter-
mined by temperature-programmed desorption mea-
surements. For TPD studies, the Ru sample is heated
by radiation from a hot W filament approximately
� 1 mm from its backside. A negative bias is applied to
the sample to prevent electron bombardment from ei-
ther the heating filament or QMS filament, as the
temperature is increased.
A Kimball Physics series electron gun, providing a
focused beam of electrons in the energy range of
20–1000 eV, is the source of electron irradiation in the
electron stimulated desorption experiments. The inci-
dence angle of primary electrons is 55� with respect to
the sample normal.
ESD mass-spectra of positive ions are obtained us-
ing the QMS to detect ions produced upon electron
bombardment of adsorbed SF6. In this mode of opera-
tion, ESD-mode, the ion-source filament of the QMS
is turned off, and the electron gun is used to bombard
the sample with a focused electron beam, � 1 mm2 in
area. Typically, the energy of the incident electrons is
200 eV and the sample is held at + 20 V.
The ESDIAD detector allows us to perform an-
gle-resolved ESD studies of both negative and positive
ions. Time-of-flight (TOF) mass separation of
desorbing species is accomplished by pulsing the pri-
mary electron beam and gating a retarding grid with a
repetition rate of 10 kHz and duration of 200 ns. Un-
less otherwise specified, angular distributions of de-
sorbing F+ have been measured for incident electron
kinetic energies of 350 eV (250 eV electron gun energy
Effects of electron irradiation on structure and bonding of SF6 on Ru(0001)
Fizika Nizkikh Temperatur, 2003, v. 29, No. 3 287
and sample bias of + 100 eV). Measurements for F–
ions were made for incident electron energies of
250 eV (350 eV electron gun energy and sample bias of
– 100 eV). The sample bias is applied in order to
accelerate escaping ions and achieve a wider collection
angle. The total electron beam exposure used to obtain
the ESDIAD images is � 1012–1013 cm–2. The details
of the experimental technique have been reported else-
where [15–17].
All ESDIAD measurements reported in this paper
are made with the sample cooled to � 25 K. Thermally
induced changes in ESDIAD patterns were «frozen»
after annealing by cooling to � 25 K, before measure-
ments were made.
For experiments involving TPD associated with
electron beam damage of the adsorbed layer, the sam-
ple is held at + 20 V potential and 90 eV electrons
from the QMS filament (I � 1.5 �A, E � 70 eV) bom-
bard the sample. The QMS filament provides a
defocused electron beam that allows for irradiation of
the whole sample, in contrast to the electron gun with
a focused electron beam that irradiates � 1 mm2 of the
target sample.
TPD measurements [5] indicate that the SF6 mole-
cules are primarily physisorbed on the Ru substrate.
We estimate a constant sticking probability (� 1) for
SF6 at 25 K [5]. Based on the packing density of the
basal plane of ruthenium (1.58·1015 atoms/cm2), and
the molecular size of SF6, we identify the saturation
coverage of SF6 in the first adsorbed layer as � 0.33
ML (5.3·1014 molecules/cm2).
3. Results
3.1. ESD mass-spectra
Figure 1 shows typical ESD mass-spectra measured
for positive ions at fractional monolayer (Fig. 1,a)
and multilayer (Fig. 1,b) coverages of SF6 on
Ru(0001). Data are obtained using the QMS
(ESD-mode) with incident electron energies of 200 eV
and a sample bias of + 20 V. Figure 1,c shows the gas
phase spectrum for comparison.
In the fractional monolayer regime, the conden-
sed-phase spectrum (Fig. 1,a) exhibits a strong F+ ion
signal. The yield of other fragments is suppressed. In-
creasing the coverage leads to changes in desorption
yields of positive ions. As the coverage grows
(Fig. 1,b, 2.5 ML SF6), we detect a considerable in-
crease in desorption of singly charged fragments: S+
and SFx
+ ions (x = 1, …, 5). SF6
+ species are not ob-
served. Note the extremely low yield of SF4
+ frag-
ments, which are barely detected in the spectrum.
The gas-phase spectrum for positive ions shown in
Fig. 1,c agrees with the NIST Chemistry WebBook
[18], except that we detect higher relative yields of F+
and S+ ions, which may be due, in part, to the contri-
bution of ESD from species adsorbed on parts of the
QMS. In contrast to thick SF6 layers (Fig. 1,b), the
gas phase SF4
+ signal (although smaller than the SF3
+
and SF5
+ signals), is comparable to yields detected for
the other ions. As for the multilayer SF6 film on Ru
(Fig. 1,b), no SF6
+ is detected in the gas-phase. It is
believed that this ion is unstable both in its ground
state and its excited electronic states [6].
The spectrum in Fig. 1,c also reveals the presence
of doubly charged fragments: SFx
� � (x = 1, …, 4). The
intensities of these fragments are related to the inten-
sities of the corresponding singly charged ions, and are
dependent on x. For even values of x (2, 4), the ob-
served intensities of singly- and doubly charged ions
are approximately equal, whereas for odd values of x
(1, 3) the doubly ionized fragment signals are smaller
by about one order of magnitude. This observation is
consistent with the partial electron-impact ionization
cross-sections reported elsewhere [6].
In spite of the fact that the cracking pattern of
ESD for negative ions contains only a few fragments,
we observe the same tendency [5] as for positive ions.
For fractional monolayer coverages, the F– ion is the
main fragment escaping from the surface. A small frac-
288 Fizika Nizkikh Temperatur, 2003, v. 29, No. 3
N.S. Faradzhev, D.O. Kusmierek, B.V. Yakshinskiy, and T.E. Madey
Fig. 1. Mass-spectra for ESD of positive ions from SF6 on
Ru(0001) for coverages: 0.25 ML (a) and 2.5 ML (b).
Sample bias is + 20V. Incident electron energy is 220 eV.
Gas-phase spectrum for positive ions (equilibrium pressure
of SF6 in chamber is 1�10–7 Torr) (c).
tion of F2
– ions is also observed in accordance with
previous ESD studies reported for other halogenated
molecules on Ru(0001) [4,19]. Increasing deposition
of SF6 leads to changes in the spectrum, mainly the
emergence of the SF5
– ion. The yield of this ion gradu-
ally increases and eventually even surpasses the F2
–
signal.
A more comprehensive treatment of the ESD mass
spectra will be presented elsewhere [20].
In general, one can conclude that for SF6 molecules
in the first adsorbed molecular layer, which are in con-
tact with the ruthenium surface, desorption of rela-
tively massive SFx ions under electron bombardment
is suppressed; the ESD signals are dominated by F+
and F–. This may be attributed, in part, to higher
reneutralization rates for more massive, slow-moving
ions, whereas less massive, fast-moving fluorine ions
can easily escape from the substrate [21].
3.2. Structure and bonding of SF6; thermal effects
Experiments performed using ESDIAD reveal that
after deposition of fractional monolayer coverage
(0.25 ML) of SF6, strong off-normal emission of both
F+ and F– ions are observed. Typical «halo» patterns
for F+ and F– ions are shown in Fig. 2,a and Fig. 3,a,
respectively.
Heating the adsorbed SF6 layer results in a trans-
formation of the ESDIAD patterns for both F+ and F–
ions. As the temperature approaches � 90 K, the ini-
tial «halos» are replaced by distinct six-fold symmetry
patterns for both ions (F+: Fig. 2,b and F–: Fig. 2,d).
This temperature corresponds to the maximum rate of
desorption of molecules from the first adsorbed layer
[5].
Deposition of SF6 coverages higher than 0.33 ML
(saturation coverage) leads to changes in the angular
distributions for both ions. The patterns become quite
broad and featureless, with intensity centered on the
surface normal, representing a random spatial orienta-
tion of molecules in the successive adsorbed molecular
layers.
The «halo» patterns in Fig. 2,a and Fig. 3,a indi-
cate a random azimuthal orientation of SF6 molecules
adsorbed on Ru(0001) by 3 fluorine atoms with the
other 3 pointing away (Fig. 2,c) [5,9]; no S—F bonds
are oriented along the surface normal. Heating the
sample induces a rearrangement and ordering of the
molecules, which results in the hexagonal patterns
seen in Fig. 2,b and Fig. 2,d. The existence of six
beams in Fig. 2,b and Fig. 2,d is attributed to the ad-
sorption of SF6 in two azimuthally-oriented domains
on Ru(0001), rotated by 60� with respect to each
other [5].
Effects of electron irradiation on structure and bonding of SF6 on Ru(0001)
Fizika Nizkikh Temperatur, 2003, v. 29, No. 3 289
Fig. 2. ESDIAD patterns for F+ and F– ions from 0.25
ML of SF6 on Ru(0001). F+ ion halo-like pattern after
deposition at 25 K (a). Heating the sample to ~ 90 K
results in hexagonal patterns for both F+ (b) and F– (d)
ions. The dynamics observed are consistent with
adsorption of the molecule on Ru by 3 fluorine atoms (c).
Incident electron energy Ee is 350 eV for F+ ion patterns
and 250 eV for F– ion pattern.
Fig. 3. ESDIAD patterns for F+ and F– ions illustrating
effects of electron irradiation on adsorbed SF6: F– ions
after deposition of 0.25 ML of SF6 on Ru at 25 K (a); F–
ions after electron exposure of 1015 cm–2 (b); possible
sources of F+ ion emission along surface normal (c), F+
ions after deposition of 0.25 ML of SF6 (d); F+ ions after
electron exposure of 1015 cm–2 (e); F+ ions after electron
exposure of 1016 cm–2 (f). The patterns (d—f) are
measured for 350 eV primary electrons; images (a, b) are
detected for 250 eV electrons.
a
c
b
d
a
b
c
d
e
f
3.3. Electron beam-induced changes
in adsorbed SF6
3.3.1. ESDIAD patterns. Typical transformations
of the F– ESDIAD patterns observed under electron
bombardment of 0.25 ML of SF6 are illustrated in
Fig. 3,a,b. Dynamics of the angular distribution of the
F– ions are as follows: during electron bombardment
the initial «halo» pattern (Fig. 3,a) loses its contrast
very quickly (Fig. 3,b) and then virtually disappears
for electron exposures > 1015 cm–2. Thus, increasing
electron exposure leads to a decrease of the total yield
of F– ions, but does not induce a change in the angular
distribution («halo» pattern) of this fragment; only
heating of the surface causes the F– ESDIAD pattern
to change from a «halo» to a hexagon.
In contrast to F– ions, the angular distributions for
F+ ions change under electron bombardment. Fi-
gure 3,e shows that exposure of 0.25 ML of adsorbed
SF6 molecules to � 1015 cm–2 results in the transfor-
mation of the initial «halo» (Fig. 3,d) into six
off-normal beams and a prominent central peak. The
angular positions of these beams are similar to those
observed after heating (Fig. 2,b). The most noticeable
difference between the hexagonal F+ patterns in Fig.
2,b and Fig. 3,e is in the width of the beams. Further
electron bombardment (� 1016 cm–2) results in the
disappearance of the off-normal beams, and the gra-
dual growth and saturation of the central peak
(Fig. 3,f).
The observed changes in the ESDIAD patterns lead
us to believe that F– ions originate primarily from
undissociated molecularly adsorbed SF6, while the F+
ions originate from SFx dissociation fragments.
3.3.2. Correlation between F+ and F– ion de-
sorption. The behavior of F+ and F– ion desorption
along normal and off-normal directions as a function
of electron exposure is illustrated in Fig. 4. Each data
point represents the integrated signal intensity for the
indicated region of the ESDIAD patterns. For clarity,
the data corresponding to the off-normal desorption of
F+ ions are divided by a factor of 6.
There is evidence for correlated behavior of the
curves in Fig. 4. Each data set demonstrates two dis-
tinct regions: the ion signals change rapidly for elec-
tron beam exposures 2·1014 cm–2 and change more
slowly for exposures
2·1014 cm–2. (An exposure of
� 2·1014 cm–2 corresponds approximately to one inci-
dent electron per two surface molecules.) In the initial
region (at lower exposures), we observe an increase of
the F+ ion yield in the off-normal direction, and the
appearance of the F+ signal along the surface normal
direction. Under the same conditions, the desorption
signal for F– ions in the off-normal direction decreases
very rapidly. The rate of the F– ion drop decreases at
higher exposures, where the F+ ion yield also changes:
in the off-normal direction, the curve goes through a
maximum and then exhibits exponential decay. Along
the surface normal, the F+ yield saturates and appears
to decrease very slowly.
Thus, during electron beam exposures we observe a
different ESD yield of F– and F+ ions in off-normal di-
rections (a) and F+ ions along the surface normal and
off-normal directions (b). This implies that F+ and F–
ions are escaping from different species.
3.3.3. Influence of electron irradiation on TPD
spectra. In order to gain further insight into the disso-
ciation process of SF6 on Ru(0001), we have also stu-
died the influence of electron irradiation on TPD spec-
tra. Electron beam exposure leads to a faster decrease
of molecular SF6 in the first adsorbed layer as com-
pared to that in the succeeding multilayers. As illus-
trated in Fig. 5, irradiation also results in a change of
the shape of the monolayer feature, as well as a shift of
the maximum desorption rate to higher temperatures.
Exposure of 0.25 ML of SF6 to � 1016 cm–2 leads to an
almost complete disappearance of adsorbed molecular
SF6 on the surface. An even more surprising result is
that the transformation of the fractional monolayer
just described also occurs in the presence of succeed-
ing adsorbed layers of SF6, which are not affected as
strongly by electron beam irradiation (data not
shown). In contrast, electron irradiation appears to af-
fect only the amplitude of the multilayer peak in the
TPD spectrum, while preserving the peak shape and
position.
290 Fizika Nizkikh Temperatur, 2003, v. 29, No. 3
N.S. Faradzhev, D.O. Kusmierek, B.V. Yakshinskiy, and T.E. Madey
�1/6
Fig. 4. Emission of fluorine ions from 0.25 ML of SF6 in
two directions: along the surface normal, and off the
surface normal (hexagonal beams), as a function of
electron exposure. The data are derived from ESDIAD
patterns (cf. Fig. 3). Incident electron energy is 350 eV
for F+ ions and 250 eV for F– ions.
Thus, it appears that molecules in contact with the
Ru surface are more sensitive to dissociation than the
molecules from upper adsorbed layers, which are iso-
lated from the metal substrate.
4. Discussion
4.1. Structure and bonding of SF6; thermal effects
The primary issues that we address are the pro-
cesses that take place in fractional monolayers of SF6
adsorbed on Ru(0001) upon electron bombardment.
As already mentioned in the introduction, a number of
studies of the adsorption and ion emission properties
of SF6 adsorbed on various surfaces have been re-
ported [7–10,21–26]. However, a gap in the under-
standing of the results still exists.
Our data indicate that SF6 molecules dosed onto
Ru(0001) at 25 K are primarily physisorbed, based
upon our TPD and ESDIAD results, and previous
studies of adsorption of SF6 on Ru(0001) [9].
Physisorption is indicated by the closeness of the tem-
perature at which the maximum rate of desorption of
fractional monolayers of SF6 occurs (� 90 K) and the
desorption temperature of condensed SF6 (� 80 K).
Physisorption of molecular SF6 has also been estab-
lished on Ni(111) [22] and graphite (HOPG) [8].
The «halo» patterns in Fig. 2,a and Fig. 3,a indi-
cate a random azimuthal orientation of SF6 molecules
adsorbed on Ru(0001) by 3 fluorine atoms with the
other 3 pointing away [5,9]. The «halo» pattern repre-
sents all the possible azimuthal orientations of S—F
bonds, since ESDIAD provides us with information
averaged over a surface area determined by the elec-
tron beam size (� 1 mm2).
Heating to desorption temperatures ( 90 K) leads
to a redistribution of the F– ion intensity from an ini-
tial «halo» to a hexagon [5]. Integration of ESDIAD
patterns for F– ions (Fig. 2) reveals that the total ion
intensity is essentially unchanged; upon redistribution
of the intensity prior to the onset of desorption (from
«halo» to hexagon) the integral signal remains nearly
constant. As the SF6 desorbs thermally, the F– signals
decrease.
The behavior of F+ ions during heating to
desorption temperatures ( 90 K) is virtually identical
to that of F– ions. The initial «halo» pattern trans-
forms into a hexagon and the total F+ ion intensity re-
mains constant. The F+ ion hexagon differs slightly
from the F– hexagon, in that a central beam is ob-
served.
Both F+ and F– ions are believed to escape from
undissociated SF6 molecules, which have two prefe-
rential azimuthal orientations (giving six ion beams,
rather than 3 beams expected for a single molecular
orientation). The central beam in Fig. 2,b is due to
ESD from a thermal decomposition product of SF6, ei-
ther chemisorbed F or another fragment with an S—F
bond perpendicular to the surface. Dissociation of a
small fraction of SF6 is supported by the persistence of
an F+ signal at temperatures > 120 K, when de-
sorption of the molecular SF6 is complete.
We attribute the temperature-induced changes of
the ESDIAD patterns to the rearrangement of
undissociated SF6 molecules, as they move to occupy
energetically preferential adsorption sites. The rear-
rangement of the patterns for both F+ and F– ions
from initial «halos» to hexagons indicates ordering of
the first adsorbed layer.
The orientation of SF6 molecules after heating to
� 90 K on Ru(0001) is established by comparison of
ESDIAD patterns (Fig. 2,b and Fig. 2,d) and LEED
images for the clean substrate. We identity the axis
defined by the S—F bond in the SF6 molecule to coin-
cide with the <10�10> direction of ruthenium. De-
tailed analysis of adsorption properties and surface
geometry of SF6 on Ru(0001) are reported elsewhere
[5]. In this paper we focus primarily on electron
beam-induced changes in the adsorbed layer.
4.2. Electron beam-induced changes
in adsorbed SF6
4.2.1. ESDIAD patterns. Electron bombardment
causes significant changes in F+ and F– ESDIAD pat-
terns. For exposures
1014 cm–2, the «halo» pattern
due to F– ions (Fig. 3,a) loses its contrast very
quickly and then virtually disappears (Fig. 3,b). The
Effects of electron irradiation on structure and bonding of SF6 on Ru(0001)
Fizika Nizkikh Temperatur, 2003, v. 29, No. 3 291
–
Fig. 5. Evolution of TPD spectra caused by electron
irradiation of 0.25 ML of SF6 on Ru at 25 K. The electron
source is the QMS filament, Ee = 90 eV.
initial «halo» pattern does not rearrange into a hexa-
gon under electron irradiation. Electron bombardment
leads to a decrease in the total ion yield, but the angu-
lar distribution of the F– ions remains the same. This
observation leads to the suggestion that F– ions origi-
nate primarily from molecularly adsorbed SF6.
In contrast to the F– data, the angular distributions
for F+ ions do undergo significant changes under elec-
tron irradiation. As already mentioned, for exposures
of � 1015 cm–2, the initial «halo» pattern (Fig. 3,d)
transforms into six off-normal beams in the shape of a
hexagon and a prominent central beam (Fig. 3,e),
very similar to the transformation that we observe
upon heating to > 90 K (Fig. 2,b). More precisely, azi-
muthal angles for thermal- and electron beam-induced
hexagon patterns are identical, and polar angles are
very similar. More extensive irradiation (1016 cm–2)
however, results in the disappearance of the off-nor-
mal beams and growth and saturation of the central
beam (Fig. 3,f). In contrast to the F– intensity, the F+
ion intensity increases with increasing electron expo-
sure. These behaviors suggest that F+ ions originate
also from SFx dissociation fragments.
4.2.2. Ordering of dissociation fragments. The
similarity of the transformation of the F+ ESDIAD
patterns from a «halo» to a hexagon upon both heat-
ing and electron exposure, makes it tempting to sug-
gest that electron beam exposure of disordered SF6
molecules («halo» pattern) leads to electron-induced
ordering (hexagon pattern), similar to thermally in-
duced ordering. Electron-stimulated mobility of ad-
sorbed species at low temperatures is well known
[27,28]. However, the differences between F+ and F–
ESDIAD patterns during electron irradiation strongly
suggest that electron bombardment causes dissociation
of the adsorbed molecular SF6, rather than ordering of
the adsorbed layer, and that F– ions escape from
undissociated SF6 molecules. This is supported by the
observation that the F– «halo» (Fig. 3,a,b) simply
disappears as electron exposure increases. This is a
consequence of the dissociation of SF6 under electron
bombardment: upon dissociation of SF6, the F– signal
is reduced, while new features in F+ ESDIAD patterns
emerge.
4.2.3. Origins of F–, F+ ions; nature of molecular
fragments. In the following paragraphs, we discuss
the nature of the molecular species and fragments that
contribute to the observed F+ and F– ESDIAD pat-
terns. As previously mentioned, the excitations lea-
ding to F– and F+ ions are very different: F– is a pro-
duct of DEA excited by low-energy secondary
electrons from the substrate,
e– + SF6 � (SF6*)
– � SF5 + F–
with two resonant features at electron energies of
5.8 eV and � 11 eV for the condensed phase; dipolar
dissociation also contributes at higher energies.
In contrast, F+ ions are due to core excitations
(probably F 2s)
e– + SF6 � (SF6*)
+ � SF5
+ + F+ + 2e–
with a threshold energy of � 30 eV.
Our TPD spectra (Fig. 5) support the proposal
that F– ions originate from undissociated SF6. The
area under a TPD curve is known to be proportional to
the number of molecules desorbed thermally and, in
our case, undissociated SF6 and/or SF6 that forms by
recombination upon heating the surface. Damage ki-
netics of SF6 due to electron bombardment (derived
from TPD measurements, Fig. 5) and the dependence
of the F– ion yield on electron exposure (derived from
ESDIAD signals) are compared in Fig. 6. The data are
normalized for clarity and presented in relative units.
The drop in F– ion yield is correlated with the decay
of the concentration of undissociated SF6 molecules
on the surface. The quantitative differences in the two
data sets of Fig. 6 are attributed to errors in determin-
ing electron beam exposure in the two experiments
(focused beam for ESDIAD vs defocused electron
source for TPD), and to the difference in incident
electron energy (90 eV for TPD, 250 eV for
ESDIAD).
The change in the angular distribution of the F+
ions indicates that after electron irradiation, the F+
ions do not escape from undissociated molecular SF6,
but rather from dissociation fragments SFx. Recently
292 Fizika Nizkikh Temperatur, 2003, v. 29, No. 3
N.S. Faradzhev, D.O. Kusmierek, B.V. Yakshinskiy, and T.E. Madey
Fig. 6. A comparison of negative fluorine ion yields
(derived from ESDIAD patterns, e.g., Fig. 3 and plotted
in Fig. 4; Ee = 250 eV) and relative concentration of
undissociated SF6 molecules (derived from TPD spectra,
Fig. 5; Ee = 90 eV) as a function of electron exposure.
The data are measured for 0.25 ML of SF6 on Ru.
Souda [25], in a study of secondary-ion emission from
SF6 adsorbed on Pt(111) observed an increase of one
order of magnitude in the yield of F+ ions after elec-
tron irradiation. He suggested that the F+ ions arise
from chemisorbed F adatoms or dissociation fragments
(SFx). This observation is in agreement with the in-
crease in F+ ion yields upon electron exposures shown
in Fig. 4. (See Sect. 4.2 for discussion of Fig. 4). The
angular resolution of our ESDIAD images allows us to
distinguish between ions that escape normal to the
surface, i.e., from SFx species with an S—F bond per-
pendicular to the surface or fluorine atoms bound di-
rectly to the surface (Fig. 3,c), and those that escape
from SFx (x = 2–5) with S—F bonds directed in
off-normal directions.
The hexagon (Fig. 3,e) that is clearly seen after ex-
posures of � 1015 cm–2 is formed by F+ ions from dis-
sociation fragments (SFx), which desorb from the sur-
face along an off-normal direction. The F+ beams
observed in the pattern after irradiation (Fig. 3,e)
look broader and more asymmetric than the beams of
the hexagon detected after heating (Fig. 2,b), sug-
gesting that several SFx species with S—F bonds in
off normal directions could be contributing to the six
F+ beams. It is most interesting that these fragments
tend to be ordered on the surface (we observe a hexa-
gon instead of a «halo»), in a manner very similar to
the results obtained during irradiation-induced de-
composition of PF3 molecules adsorbed on Ru(0001)
[27,28]. Based upon theoretical equilibrium structures
of SFx (x = 1–6) [29] and simple bonding symmetry
considerations similar to those for domains of PF2
[12], it is possible that the hexagon originates, for ex-
ample, from 3 domains of SF4 species, or
bridge-bonded SF2 species, azimuthally rotated by
120� as shown in Fig. 7. Other fragments, including
inclined SF, could also contribute to the off-normal
F+ ion yields.
The central beam, caused by F+ ions desorbing nor-
mal to the surface, most probably contains contribu-
tions from chemisorbed F adatoms. However, frag-
ments such as SF, SF3 and SF5 might also contribute
to the central beam [29].
At present the identity of the dissociation frag-
ments is an open issue, as we lack direct knowledge
about the stoichiometry and the structure of SFx frag-
ments adsorbed on the Ru substrate. Additional exper-
iments (e.g. high resolution soft x-ray photoemission
spectroscopy [30]) and theoretical guidance are
needed to resolve these issues.
Figure 4 is a plot of ESDIAD intensity as a func-
tion of electron exposure. Both the «along normal»
and «off-normal» F+ curves grow rapidly at low elec-
tron exposures. The rapid initial rise is probably due
to very efficient dissociation in the initial stages of the
decomposition of molecular SF6. In the initial stages
(at low electron exposures) there are many adsorption
sites available for the dissociation fragments of SF6 on
the Ru(0001) surface. Being in contact with the metal
surface enhances the reactivity of fragments and
makes them more prone to dissociate. As the dissocia-
tion process continues at higher electron exposures,
the Ru(0001) surface becomes «filled», and the rate of
further dissociation is greatly reduced. This suggests
that low coverages of SF6 have a higher dissociation
rate than higher coverages.
In concluding this part of the discussion, we note
that there is previous evidence from ESD studies of
PF3 and (CF3)2CO to support the contention that
negative ion desorption arises mainly from molecu-
larly-intact adsorbates, while positive ion desorption
can be dominated also by dissociative fragments, when
they are present [27]. We suggest that the tempo-
rary-negative-ion states formed during DEA of mole-
cules such as SF6, etc., are more weakly coupled to the
surface than similar states associated with more
strongly chemisorbed fragments. Thus, the lifetime for
DEA of intact molecules is expected to be longer than
for possible DEA of chemisorbed dissociation frag-
ments, making the probability of F– desorption from
molecules higher than from fragments.
4.2.4. Estimation of cross-sections for desorption
and dissociation. In the discussion of the origin of the
Effects of electron irradiation on structure and bonding of SF6 on Ru(0001)
Fizika Nizkikh Temperatur, 2003, v. 29, No. 3 293
Fig. 7. Schematic representation of bridge-bonded SF2 and
SF4 species on ruthenium surface (on the left) and
corresponding simulated ESDIAD pattern (on the right)
(a); illustrates formation of hexagonal ESDIAD pattern
(simulated image is on the right) from 3 domains of SF2
species, azimuthally rotated by 120� (shown on the left)
(b).
a
b
F– ion, we made reference to Fig. 6, which clearly
shows a correlation between the drop in F– ion yield
with a decrease in the number of undissociated SF6
molecules left on the surface after irradiation. As de-
scribed elsewhere [31], exposure-dependent surface
coverage � can be roughly expressed by the exponen-
tial law:
� = �0 exp (–
esdFe
) ,
where
esd is the total cross-section (cm2) of mole-
cular SF6 for electron-stimulated desorption and/or
dissociation and Fe is electron exposure (cm–2).
Recent measurements [20] indicate that the F– sig-
nal is directly proportional to �SF6, and we approxi-
mate the total cross-section for electron induced disso-
ciation of a fractional monolayer (0.25 ML) of SF6 on
Ru(0001) from the F– intensity curve in Fig. 4.
We find that for 250 eV incident electron energy,
esd is � (6 ± 2)·10–15 cm2 for electron exposures
< 2·1014 cm–2 and � (8 ± 2)·10–16 cm2 for electron ex-
posures > 2·10–14 cm–2, an order of magnitude diffe-
rence. A qualitatively similar result has been derived
from damage kinetics of molecular SF6 shown in
Fig. 6 (primary electron energy � 90 eV): � (2 ±
± 1)·10–15 cm2 for exposures < 2·1014 cm–2 and � (4 ±
± 1)·10–16 cm2 for higher electron exposures. As
already indicated in the previous section,
esd is ex-
pected to be greater at initial electron exposures, since
there are many adsorption sites available to the disso-
ciation fragments, allowing them to interact with the
Ru substrate and increase their reactivity.
Our rough calculations of
esd indicate that the to-
tal dissociation/desorption cross section for frac-
tional SF6 adsorbed on Ru for low electron exposures
is about 1 order of magnitude greater than for gaseous
SF6. In the case of DEA it has been shown that the
cross-section is often greater for molecules condensed
on a substrate than for the gas-phase analogue. The in-
crease is explained in terms of the induced polariza-
tion energy due to the substrate, which stabilizes a
negative ion against autodetachment [32].
In our experiments there are two sources of elec-
trons, primary and secondary, that can cause dissocia-
tion: 1) energetic primary electrons can initiate
dissociative ionization and/or dipolar dissociation
and 2) low-energy secondary electrons (originating
from the Ru substrate) can lead to DEA, and to a
lesser extent, dissociative ionization. Secondary elec-
trons have a maximum yield at 0–2 eV with a high
energy tail extending to 20–30 eV.
There have been several measurements of electron
impact ionization cross-sections for gaseous SF6.
Dissociative ionization (which results in the formation
of positive ions) of gaseous SF6 becomes significant
for electron energies > 16 eV, and exhibits a maxi-
mum cross-section of � 7·10–16 cm2 for electron
energies of � 100 eV [6].
DEA (which results in the formation of neutral
fragments and negative ions) is appreciable only for
low energy electrons, 0–15 eV. The maximum
cross-section of 2·10–16 cm2 occurs for electron ener-
gies of � 0.1–0.5 eV, and is due mainly to the forma-
tion of SF5
� [6].
The different behavior of the monolayer and
multilayer features in TPD spectra during electron ir-
radiation (see explanation of Fig. 5 in previous sec-
tion), leads us to believe that the average cross-section
for electron-induced dissociation of SF6 fractional
monolayer coverages is approximately one order of
magnitude greater than for multilayers. The higher
rate of dissociation of SF6 molecules in contact with
the Ru(0001) surface provides indirect evidence of the
important contribution of DEA to the total dissocia-
tion process of SF6 on Ru(0001). Low-energy secon-
dary electrons are known to play a major role in DEA
[3], which is an extremely efficient process in colli-
sions of low-energy electrons with gas- and con-
densed-phase halogenated molecules. In addition, an
electronically excited species in contact with the
metal tends to react more easily than when it is iso-
lated from the metal.
4.3. ESD mass-spectra
The interaction of adsorbed SF6 molecules with the
Ru(0001) substrate is responsible for the substantial
differences in the ESD mass-spectra (Fig. 1) between
fractional monolayer and multilayer coverages. As
mentioned in the description of Fig. 1, the ESD
mass-spectra for fractional monolayer coverages ex-
hibits only a strong F+ ion signal; the yield of other
fragments is suppressed. However, the spectrum for
2.5 ML of SF6 displays a variety of desorbing species.
A very similar effect was observed for the adsorption
of C6H12 on Ru(0001): ESD of a C6H12 monolayer
was observed to yield only H+, while multilayer
coverages yielded many more ionic fragments [33]. It
was suggested that for the monolayer species in direct
contact with the substrate, the probability of elec-
tron-induced dissociation is high, but that de-excita-
tion processes involving electron tunneling from the
substrate also occur with high probability. This leads
to suppression of desorption of slow-moving, massive
ion fragments (see discussion of mass effects in ESD
by Madey et al. [34]). Our ESD mass-spectra are con-
sistent with these considerations: for fractional
monolayer coverages we observe only a strong F+
yield, while multilayer coverages yield more ionic
fragments.
294 Fizika Nizkikh Temperatur, 2003, v. 29, No. 3
N.S. Faradzhev, D.O. Kusmierek, B.V. Yakshinskiy, and T.E. Madey
5. Conclusion
We have studied ESD of F+ and F– ions from frac-
tional monolayers of SF6 on Ru(0001) at 25 K as a
function of electron irradiation. The origin of these
ions appears to be strongly dependent on the electron
exposure.
For exposures less than � 1013 cm–2 both F+ and F–
ions observed in off-normal directions arise from un-
dissociated SF6 molecules. Higher electron exposures
(
1014 cm–2) lead to stepwise decomposition of the
parent SF6 molecule, with the F+ and F– ions escaping
from different species: whereas the F– ion is still emit-
ted by undissociated SF6 molecules, the F+ ions escape
in off-normal directions from SFx (x = 2–5) frag-
ments. SFx fragments are ordered on the surface,
which is indicated by the observed hexagonal
ESDIAD patterns for F+ after irradiation. Along di-
rections normal to the surface, the F+ ions arise most
likely from dissociation fragments with an S—F bond
normal to the surface, and chemically bonded F
adatoms. Adsorbed SF6 molecules and SFx fragments
decompose almost completely after electron exposures
of � 1016 cm–2. The total cross section for electron-im-
pact dissociation of molecular SF6 on Ru (incident
electron energy � 250 eV) is found to be � (6 ±
± 2)·10–15 cm2 for electron exposures < 2·1014 cm–2
(which is about 1 order of magnitude greater than for
gaseous SF6), and � (8 ± 2)·10–16 cm2 for higher elec-
tron exposures.
Acknowledgments
We acknowledge valuable discussions with Prof.
E. Carter. This work has been supported in part by the
US National Science Foundation, Grant No. CHE
0075995.
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