Article
pubs.acs.org/JPCC
Anomalous Positive Ion Formation in Negative Ion Collisions with
HOPG
Pinyang Liu,†,‡ Lin Chen,*,†,‡ Shunli Qiu,†,‡ Feifei Xiong,†,‡ Haoyu Jiang,† Jianjie Lu,†,‡ Yuefeng Liu,†,‡
Guopeng Li,†,‡ Yiran Liu,†,‡ Fei Ren,†,‡ Yunqing Xiao,†,‡ Lei Gao,†,‡ Bin Ding,†,‡ Yanling Guo,†,‡
and Ximeng Chen*,†,‡
†
School of Nuclear Science and Technology, Lanzhou University, 730000 Lanzhou, China
Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, Lanzhou University, 730000 Lanzhou,
China
‡
ABSTRACT: Charge transfer on graphite, a typical substrate
and one of the fusion first wall materials, is of great importance
in the plasma−wall interaction, thin-film growth, and surface
catalysis. We present an experimental study of 8.5−22.5 keV
energy carbon, oxygen, and fluorine negative ions scattering from
a highly oriented pyrolytic graphite (HOPG) surface at a
scattering angle of 8°. It is found that the positive ion fraction
decreases monotonically with the increase of both incident
velocity and angle. In particular, these dependences are very
different from those presented in previous studies. A molecular
dynamics simulation reveals that, around the critical condition
for planar surface channeling, a number of projectiles may
penetrate into the subsurface and become energetic atoms when
they emerge from the surface. Hence, an exponential scaling
related to the penetration probability has been proposed to well describe the velocity and angle dependences of positive ion
fractions.
surface scattering technique.18−21 In this work, we experimentally study the charge transfer process, i.e., positive ion
formation, for energetic negative ions scattering on HOPG.
Most studies of charge transfer on HOPG up to now have
been confined to proton projectiles. H/HOPG is considered as
a model system since hydrogen is the simplest projectile.22−26
Positive and/or negative hydrogen ions on HOPG surface have
attracted more attention in the past. Goldberg’s group reported
that H+ fraction from HOPG shows a monotonic increase with
incident energy from 7% to 20% with primary energy in the 2−
8-keV range in large angle scattering and backscattering, and
that H− fraction remains almost constant.22,23 Moreover, the
H+ fraction shows an increase with increasing exit angle in the
7−12% range, while H− fraction shows the opposite tendency23
for 4 keV H+ scattering at large angles. H− fraction in grazing
scattering from HOPG has been reported by Esaulov’s group
where the H− fraction increases with exit angle at 4 keV
incident energies,24 but the positive-ion fraction is not available.
These studies make considerable effort to understand the high
H− fraction which surprisingly conflicts with our understanding
because of the low electron affinity of H− (0.75 eV) as
compared to the work function of HOPG (about 4.6 eV).
1. INTRODUCTION
Carbon has been widely used for its various allotropes and
abundant molecular configurations. One of its well-defined
forms, graphite, can be used as a typical substrate for
adsorption,1 growth,2 morphology,3 photochemistry and photocatalysis.4 It is also attractive in tokamak devices, to be used as
the first wall and the divertor plate.5 In recent years,
ferromagnetism in HOPG has been observed.6−9
Charge transfer on HOPG is one of the important topics of
relevant areas, due to not only its basic interests in physics and
surface chemistry but also its importance in the quantification
of surface analytical techniques, such as low-energy ion
spectroscopy (LEIS)10 and secondary ion mass spectroscopy
(SIMS).11 It is also involved in other application fields, such as
plasma-wall interaction in fusion research12 and thin-film
growth technologies.13 In particular, negative (positive) ion
formation involved in charge transfer processes can be used for
particle detection in interplanetary and interstellar space
because of a low detection efficiency of neutrals14 and has
been proposed to be used in space propulsion thrusters,15,16
and also has been chosen to produce a large neutral beam
current for the neutral beam injector (NBI) system in future
fusion technology, such as the International Thermonuclear
Experimental Reactor (ITER).17 At present, the most direct
way to probe positive or negative ions is the measurement of
charge state distribution of scattered particles using the ion© 2016 American Chemical Society
Received: April 11, 2016
Revised: July 27, 2016
Published: July 28, 2016
18538
DOI: 10.1021/acs.jpcc.6b03680
J. Phys. Chem. C 2016, 120, 18538−18547
Article
The Journal of Physical Chemistry C
However, the positive-ion formation has not been the subject of
similar attention as compared to a lot of research for negativeion formation. One of the possible reasons is that the
incomplete memory loss of the initial charge state may
interrupt the final positive-ion formation during positive-ion
surface scattering.27 But it can be avoided by using neutral and
negative ion beams. So far, direct experimental studies using
negative-ion beams have been much scarcer because the
negative-ion source is needed. Hence, it motivated our present
work.
To date, the interest of charge transfer of multielectron
projectiles scattering on HOPG has grown strongly since such
projectiles can be used to examine many-body theory.28−34
However, in some tens of keV energy range, few experimental
investigations have been conducted.34 Therefore, much effort
should be devoted to microscopic understanding of underlying
scenario of positive-ion production on graphite surfaces.
In this work we have adopted carbon, oxygen, and fluorine
negative ions and a scattering angle of 8° that is close to the
critical condition for planar surface channeling. It is found that
both the incident velocity and angle dependences of positiveion fractions differ from our previous studies.35,36 In particular,
positive-ion fractions decrease with increasing incident energy.
This anomalous dependence has contrasted with a lot of
previous results in the low energy range.22,23,36−42 A molecular
dynamics simulation of ion trajectories has been presented and
significantly improved our knowledge of positive-ion production on HOPG.
The organization of this paper is as follows. In section 2, we
describe our experimental apparatus. After a presentation of the
experimental method, we will present the dependences of ion
fractions with both incident angle and energy in section 3. The
full results are discussed in section 4.
angle was varied from 1° to 7° measured with respect to the
surface plane. To avoid particle scattering on the tube wall, the
scattered beam from the surface passed through the two posttarget separated slits with 1 × 2 mm2 apertures. The charge
states of scattered particles were then analyzed by a parallelplate electrostatic deflector. A one-dimensional position
sensitive microchannel plate (PSMCP) detector was located
60 cm downstream of the deflector at the end of the tube. The
detector essentially consisted of a resistive anode placed behind
two MCPs mounted in a chevron configuration. The detector
was well fixed at appropriate bias voltage where the measured
count rate becomes independent of pulse height. The detector
efficiency increased with impact energy and was assumed to be
identical for scattered particles with different charge states.43−45
A multiparameter acquisition system (MPA-3) (FAST
ComTec) was used for data collection.
We appropriately choose the bias voltage of the deflector to
ensure that all scattered particles are well collected by the
detector. The position spectrum of scattered particles is shown
in Figure 2. The biggest peak corresponds to the scattered
2. EXPERIMENT
The experiments of ion surface scattering were performed in
the apparatus as shown in Figure 1. The C− (O−, F−) ions were
Figure 2. Position distribution of scattered particles for 22.5-keV
negative fluorine ions in specular scattering on a HOPG surface.
Figure 1. Schematic diagram of the experimental setup for ion
scattering measurements.
neutrals, and the positive and negative ions are symmetrically
located on its both sides. The positive ion fraction is defined as
Φ+ = N(+)/N(total), where N(+) is the number of scattered
positive ions corresponding to area of the positive ion peak, and
N(total) is the total number of scattered particles corresponding to the sum of areas of three peaks. Because of signals
recorded in coincidence mode for the PSMCP detector, the
signal-to-noise ratio is high, which guarantees the detection
accuracy. The data are reproduced via a series of measurements
done in different days. The experimental error in the fraction is
mainly determined by counting statistics, and is less than 10%.
The HOPG sample was purchased from MaTeck and
manufacturer quoted a mosaicity of less than 0.8°. To obtain
an atomically flat surface, the layered structure of the graphite
surface simplified the preparation procedure by removing
several top layers with adhesive tape prior to insertion to the
UHV chamber. Figure 3 displays three-dimensional atomic
force microscopy (AFM) image over 1 × 1 μm2 at the prepared
HOPG surface. It is observed that about 60% of the total
surface is relatively flat. Atomic size steps are shown in
atomically flat areas. These steps are about 3 Å in height, which
corresponds to a space between the layers in graphite and is
consistent with previous work.46
The HOPG sample was mounted on a sample holder
supported by a 4-dimensional precision manipulator and was
heated by electron bombardment. The extremely low
produced in a cesium sputter negative-ion source and then
deflected by a 45° bending magnet. The extracted ion beam
passed through a pair of electrostatic plates placed between two
slits. The negative component separated from neutrals passed
through the third collimators 30.5 cm downstream of the
second slit, before entering an ultrahigh-vacuum (UHV)
chamber, with a typical pressure of better than 3 × 10−7 Pa.
Three collimators guaranteed the angular divergence of the
primary beam less than 0.1° (full width at half-maximum
(fwhm)) for all measurements via reducing the beam size. For
surface scattering with a scattering angle of 8°, the incident
18539
DOI: 10.1021/acs.jpcc.6b03680
J. Phys. Chem. C 2016, 120, 18538−18547
Article
The Journal of Physical Chemistry C
off Al (111) pronounced increases with energy after reaching a
kinematic threshold of several keV.40,41 Moreover, for semiconductor surfaces, F+ increases linearly with energy in the
range of 8.5−22.5 keV,36 and Ne+ grows rapidly from 2 to 10
keV energies and then becomes almost constant for higher
energies.42 For HOPG surface, Goldberg’s group reported that
H+ fraction also shows a monotonic increase with incident
energy from 7% to 20% with primary energy in the 2−8-keV
range in large scattering angles of 45° and 135°.22,23 To the best
of our knowledge, such an anomalous dependence on incident
velocity in our case has never been observed.
In Figure 5, we show positive-ion fractions of scattered
carbon, oxygen, and fluorine ions as a function of incident
Figure 3. Surface morphology of clean HOPG obtained by AFM
detected under force modulation mode. Typical technical data:
thickness, 3 μm; length, 225 μm; width, 28 μm; resonance frequency,
75 kHz; force constant, 2.8 N/m; coating, none. The surface is
relatively flat and atomic size steps or corrugations are shown.
adsorption coefficient of compounds and absence of oxidation
layer on the surface ensure cleanliness. During the whole
experiment, the sample was prepared by cycles of annealing at
about 773 K for 30 min to prevent the contamination.
Measurements were made for the incident energies varied from
8.5 up to 22.5 keV.
3. RESULTS
In Figure 4 we show positive-ion fractions as a function of
incident velocity for carbon, oxygen and fluorine negative ions
Figure 5. (a) C+, (b) O+, and (c) F+ fractions as a function of incident
angle at different incident energies as indicated. The black solid line is
calculated C+ fraction for 22.5 keV energies. The magenta dash dotted,
navy dotted, and cyan dashed lines correspond to the O+ fraction for
22.5, 18.5, and 14.5 keV energies, respectively. The pink short dotted,
blue short dashed, and green dash dot dotted lines correspond to the
F+ fraction for 22.5, 18.5, and 14.5 keV respectively.
angle. In general, C+ fraction is greater than O+ fraction and O+
fraction is slightly larger than that of F+ for the same incident
angle and energy. All these positive ion fractions decrease
monotonically with increasing incident angle. It is also observed
that positive ion fractions decrease more steeply for incident
angles below 4°. For a given incident angle, the larger the
incident energy, the lower the positive-ion fraction.
Similar experimental results have been reported by Goldberg’s group,23 in which H+ fraction decreases with increasing
incident angle. The descending rate of the H+ fraction decreases
with increasing incident angle for 4 keV H+ scattering on
HOPG at a scattering angle of 45°. It is also found that the
descending rate of the Ne+ fraction scattering from an
amorphous silicon surface gradually decreases when the
incident angle increases.42 However, for F− and Cl− scattering
on Mg and Ag,47 F+ and Cl+ fractions first decrease steeply,
then keep almost unchanged and even slightly increase with
incident angles at a small incident energy of 1 keV, while for
large projectile energies, the descending rate increases with
increasing incident angle. For F− scattering on Si,36 the F+
fraction decreases linearly with increasing incident angle for 9.5
keV, and for energies of 13.5 and 21.5 keV, F+ fractions trend to
be unchanged at small incident angles and then decrease
monotonously at large incident angles.
Figure 4. (a) C+ fractions (b) O+ and F+ fractions from HOPG as a
function of incident velocity for specular scattering. The black solid,
red dashed, and blue dash dotted curves are the calculated results.
scattering on a HOPG surface at a scattering angle of 8°. In
Figure 4a, the C+ fraction is large and up to about 26%. It
decreases sharply with increasing incident velocity. However, in
Figure 4b, O+ and F+ fractions are smaller than C+, and both of
them also decrease with the increase of velocity with small
fluctuations which is attributed to experimental errors. In
addition, F+ fractions are the smallest, as shown in the figure.
In Figure 4, we observe a quite surprising result that the
positive ion fraction decreases with increasing incident velocity.
This is substantially different from what happens on metallic
surfaces37−39 where scattered Ne+, Ar+ and O+ fractions
increase monotonically with primary kinetic energy in the
range of several keV energies. He+ fraction of grazing scattering
18540
DOI: 10.1021/acs.jpcc.6b03680
J. Phys. Chem. C 2016, 120, 18538−18547
Article
The Journal of Physical Chemistry C
we increased the acceptance angle of the detector appropriately.
The simulations were terminated after 200 fs, or the projectiles
located >3 Å above the surface or <10.1 Å below the surface.
The simulations used approximately 105 projectiles.
In Figure 6 we show typical trajectories for 22.5 keV F−
projectiles impinging on the HOPG surface with the incident
In principle, the positive ion fraction in these studies at large
energies36,47 can be understood in terms of the exponential
dependence of the inverse of exit angle for a given energy.48,49
It is associated with the survival probability of positive ions
during Auger and/or resonant neutralization along the
outgoing trajectory proposed by Hagstrum.49 As we can see,
the first two mentioned studies,23,42 as well as ours, are different
from other studies.36,47 The descending rate of positive-ion
fraction as a function of incident angle cannot be well explained
by the exponential scaling as mentioned above. As a
consequence, the reason for new findings about the dependence of positive-ion fraction on incident velocity and angle will
be discussed in details in the next section.
4. DISCUSSION
4.1. Projectile Trajectories. For grazing surface scattering,
both axial and planar channelings play a role. Axial channeling
takes place between strings of lattice atoms and depends on the
crystal structure and its orientation relative to the incident
beam. Planar channeling occurs in front of the topmost surface
layer or between pair of planes in the bulk.50 For planar
channeling, a critical angle is roughly described by Φcrit =
(2πansZ1Z2/E0)1/2, where the screening length a = 0.8854(Z11/2
+ Z21/2)−2/3, ns is the number of surface atoms per unit area, Z1
and Z2 are projectile and target atomic numbers, and E0 is the
projectile energy. The planar surface channeling occurs below
the critical angle, and the projectiles will be scattered directly by
the first-layer atoms due to a collective potential effects of target
surface atoms, i.e., the specular reflection. Therefore, for charge
transfer of keV-ion/atom surface scattering under a small
grazing angle of incidence, i.e., 1°, the scattering event occurs
above the surface because the incident angle is smaller than the
related critical angle. It is easy to understand that projectiles
have a large probability to penetrate into the surface when the
incident angle is larger than the critical angle.51−54 For
scattering on a HOPG (0001) surface [ns = 3.845 × 1015
atom/cm2 55], we find the critical angle is from about 5.18° to
8.88° for C− energies from 22.5 to 8.5 keV. For O− with the
same energies, it is about 5.83° to 10.00°, and for F− ions, it is
about 6.12° to 10.49°. The critical angles are covered in the
incident angle range used in our experiments. As we know,
HOPG has a relatively loose atom configuration of hexagonal
lattice structure. Moreover, for ion surface scattering, its small
atomic number of Z2 = 6 corresponds to a smaller coulomb
repulsion as compared to other high Z2 materials. Thus, in our
case, it is likely that a part of projectile trajectories may come
from the subsurface and can be detected.
It is also further supported by the trajectory simulation
carried out with the Kalypso software package.56 Briefly,
Kalypso is a molecular dynamics software for simulation of
atomic collisions in solid. The target used in this work consists
of four atomic layers, comprising approximately 13000 atoms.
The target was very long in the direction of the ion path to
allow continuous collisions. Thermal vibrations of the target
corresponding to room temperature (T = 300 K) were
considered in the simulation. The projectile impact points
lied at a hexagonal grid in a representative symmetry area of the
surface atoms. The F−C interaction potential was represented
by uncorrected Ziegler−Biersack−Littmark (ZBL) universal
screening function potential.57 To speed up the ion scattering
spectroscopy (ISS) simulation, we did not model an attractive
potential between the projectile and the target, or between
target atoms.56 In order to obtain enough trajectory number,
Figure 6. Trajectories of 22.5 keV F− projectiles impinging on the
HOPG surface with an incident angle of 4°. (a) Side view
corresponding to the projection onto a plane perpendicular to the
surface (XZ plane). (b) Top view to the projection on the surface
plane (XY plane). Typical trajectories were emphasized by thick lines
with different color. Squares represent C atoms.
angle of 4°. In the upper plane of Figure 6, z-coordinate
represents the direction normal to the surface toward the
vacuum, and x-coordinate is parallel to the incident direction of
projectiles. We find that some projectile’s trajectories are still
above the first layer of the surface, and some ones penetrate
into the second, third and even fourth layers of the surface. In
the lower plane of Figure 6, the projectile’s trajectories are
projected into the surface plane where the zigzag trajectories
are well observed. In Figure 6, the incoming particles mainly
follow three types of trajectories: one (marked by I) is singly
scattered by the top sites of the first-layer atoms, corresponding
to particles stay above the surface. This type has the shortest
length of trajectories. The second one (marked by II) is
scattered between two atomic rows and possess zigzag
trajectories, corresponding to particles that also stay above
the surface. The last one (marked by III) corresponds to
particles stay below the first layer of the surface. The related
projectiles penetrate into the subsurface layers and experience
successive reflections before leaving the surface which have the
longest length of trajectories. As we know, the electron density
above the surface is very different from that in the bulk. Thus,
the charge transfer probability will be affected by the trajectory
where the projectile will encounter with different electron
densities.
18541
DOI: 10.1021/acs.jpcc.6b03680
J. Phys. Chem. C 2016, 120, 18538−18547
Article
The Journal of Physical Chemistry C
For ion solid collisions, the so-called shadow cone50 means
that the projectile cannot enter a defined volume behind the
target atoms. It is an excluded volume behind the scattering
center inside which no primary ions can penetrate. The shadow
cone is produced by the Coulomb repulsive potential between
the projectile and the target atom. At a distance d, this cone
1/2
( d)
radius R sc = 2
Z1Z 2
E0
is inversely proportional to E01/2, i.e.
velocity. This radius decreases with the increase of incident
velocity. From grazing incidence to a large incident angle, the
overlapped shadow cones will separate from each other. As a
result, the number of projectiles penetrated into surface will
qualitatively increase with the increase of incident velocity and
angle. On the other hand, it is impossible for the projectiles to
penetrate into surface with a small perpendicular energy that
cannot overcome the surface Coulomb repulsive potential. We
defined a penetration probability δ which is the ratio of the
number of projectile’s trajectories below the first layer of the
surface to the total number of trajectories collected by the
detector. Therefore, the incident perpendicular velocity
dependent penetration probability is as follows,
⎛
⎞
−λ
δ = A × exp⎜
⎟ (for v sin α > v0)
⎝ v sin α − v0 ⎠
Figure 7. Penetration probability δ for F− scattering on HOPG as a
function of the incident perpendicular velocity: (■) for the perfect
surface, incident energy dependence, detector acceptance angle of ±1°,
and (▲) for detector acceptance angle of ±2°; (●) for the perfect
surface, incident angle dependence, detector acceptance angle of ±2°;
(▼) for defect surface, incident angle dependence, detector acceptance
angle of ±2°. The red and green solid lines (δ) are the fitting results
for the perfect surface and defect surface, respectively. The blue dash
dotted line (δ1) is used for the calculation of carbon positive-ion
fraction, and the black dashed line (δ2) is used for the calculation of
oxygen and fluorine positive-ion fractions.
(1)
where A, λ, and v0 are free parameters. v0 is regarded as a
perpendicular threshold velocity. The projectile cannot
penetrate into the surface when its incident perpendicular
velocity is below v0. v is the incident velocity, and α is the
incident angle with respect to the surface plane.
In order to estimate the penetration probability δ, we
performed a series of trajectory simulations for F− particles
scattered from HOPG with different incident angles and
incident energies. In the case of the incident energy
dependence at specular scattering (4°/4°), five different
incident energies were used: 8.5, 12.5, 16.5, 18.5, and 22.5
keV. The scattered particles were collected with an altitudinal
angle of 4 ± 1° and 4 ± 2°, and with a plane angle width of ±1°
(in polar plane). In the case of the incident angle dependence
for a given incident energy of 22.5 keV, five different angles of
2°, 3°, 4°, 5° and 6° were used. The scattered particles were
collected with an altitudinal angle width of ±2°, and with a
plane angle width of ±1° (in polar plane).
It is noted that, for the incident energy dependence, no
penetration trajectory is found for 8.5 and 12.5 keV energies in
both altitudinal angle sets. It indicates that the critical angle for
planar surface channeling is large for low incident energies,
which is generally consistent with the relationship described
with the formula of critical angle mentioned above. For the
incident angle dependence, no penetration trajectory is found
for an incident angle of 2°, while for an incident angle of 6°
only dozens of projectiles are recorded and most of them
escaped past the edge of the built target. In practice, we try to
further increase the size of the target; however, there are still
trajectories out of the boundary of the target. At the same time,
the computation time is so long as to be unbearable.
We display the penetration probability δ as a function of the
incident perpendicular velocity in Figure 7 for different energies
and angles. The simulation data are well described by eq 1 with
the solid red line with A = 0.3243, λ = 0.0023, and v0 = 0.01066.
The penetration probability δ increases slowly with the increase
of velocity and appears to have a saturated trend. With the
increase of the perpendicular velocity, the simulated trajectories
become more complex and the penetration depth is deeper.
In practice, another factor which should be taken into
account is the quality of the surface, which is a key component
in any surface scattering experiment, particularly for a grazingangle geometry that is vulnerable to surface defects and
contamination. As shown in Figure 3, there are apparent steps
and corrugations in HOPG. Thus, the surface topography could
to some extent influence the trajectory of projectiles. Projectiles
can enter into sublayers from the step edge, and a larger
penetration probability would be expected. The topography of
HOPG is analyzed by scanning tunneling microscope (STM)3
and also shows clearly terraces, which means that those defects
cannot be fully ruled out by the current technical preparation of
target. From the point of view of the surface roughness, we
randomly add some steps for a defect surface in the simulation
to compare with the perfect surface. For the defect surface, the
incident angle dependences of 2°, 4°, and 6° at 22.5 keV
energies were simulated and collected with an altitudinal angle
width of ±2°. The related results are also shown in Figure 7,
and the simulation data are well described by eq 1 with A = 1, λ
= 0.0006432, and v0 = 0.007043. It clearly shows a larger
penetration probability and a smaller perpendicular velocity
threshold for the defect surface.
4.2. Positive Ion Fractions. For negative ion scattering on
HOPG, the complete neutralization occurs in the incoming
path and the negative projectiles first become atoms via
resonant electron loss to the conduction band of HOPG prior
to form positive ions. The positive ions must be produced
through inelastic collisions with surface atoms. The inelastic
collisions include both the ionization and electron excitation
processes. To produce positive ions, direct single ionization
process takes place via Coulomb repulsion well described by
the binary encounter approximation (BEA).58−60 The molecular orbital (MO) promotion model has been successfully
18542
DOI: 10.1021/acs.jpcc.6b03680
J. Phys. Chem. C 2016, 120, 18538−18547
Article
The Journal of Physical Chemistry C
For a single-electron promotion process, singly excited state
is formed and then immediately become positive ions via
resonant ionization with unoccupied states of the surface.63−65
For a double-electron promotion process,66−69 C+ (O+, F+)
ions may be produced through the following scheme,
employed to describe the electronic excitation process in ion
surface collisions.61−71 We briefly point out the possible
electron excitation for C−C system are caused by 3dσ MO
promotion that is associated with the 2p level of C.62 Since the
electron energy level ordering of C is identical with O and F
with respect to C target, similar electron promotion for O and
F should also occur. The MO correlation diagram of O−C and
F−C constructed following the Barat-Lichten rules61 is shown
in Figure 8.
ionization
MO
C−(O− , F−) ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ C0(O0 , F0) ⎯⎯⎯→ C*
AI
*(3s2)(O**(2p23s2), F**(2p33s2)) → C+(O+ , F+)
where in approaching the surface, C− (O−, F−) is ionized
through resonant ionization (resonant electron loss) to form C0
(O0, F0) which by double-electron promotion of C 2p3 (O 2p4,
F 2p5) subsequently forms C** (O**, F**) in hard collisions
with surface atoms. These doubly excited atoms then decay far
from the surface via autoionization (AI) to give C+ (O+, F+)
ions.
In particular, another excitation process should be mentioned
here. In collisions of open shell atoms (groups V, VI, and VII),
the ground configuration of its positive ion corresponds to
several states, e.g., 4S0, 2D0, and 2P0 for O+, 3P, and 1D and 1S
for F+. The upper members of the Rydberg series converging to
the higher ionization limits lie above the first (or second)
ionization limits and are therefore autoionizing.72−75 Such
excited atoms could become positive ions via autoionizing after
receding from the surface, and may become atoms in the
ground state by the Auger de-excitation processes. Thus, in the
case of F scattering, the 2pπ4 core may dissociate into a mixture
of 1D and 1S states, at high energies a statistical population
(2:1) should be reached. The excited states for higher
configurations 1D nl and 1S nl series thus are formed. For O,
the 2pπ3 core may dissociate into 2D and 2P O+ states, and
hence one may expect the population of states of the 2D nl and
2
P nl series. For C, the ground configuration of positive ion
corresponds to only one state (2P), such excited states are not
available.
Besides, another possible mechanism for positive-ion
formation is kinematically induced Auger ionization. It is a
kinematic process occurring in small scattering angles. Auger
ionization is a two-electron process, an electron can go from an
occupied state into an empty state of the surface with energy
transferred to lift an atomic electron to an unoccupied state of
the surface.41,48,76 Energy conservation requires kinetic energy
from the projectile to excite the electronic system and therefore
Auger ionization is only possible to occur above a threshold
kinetic energy of the projectile; i.e., a threshold velocity vth is
deduced,
Figure 8. Qualitative MO correlation diagrams for the O−C and F−C
collision systems. a stands for O and F atoms.
Electron promotion occurs only if a critical internuclear
distance (Rmin) is reached. An adiabatic MO correlation
diagram for selected orbitals of C−C has been calculated by
the Hartree−Fock method and the computer code Gaussian.62
From the correlation diagram, we find that the promotion along
the 3dσ MO occurs near Rmin ≈ 1.2 au. In our case for C−, O−,
and F− scattering at a scattering angle of 8°, the distances of
closest approach calculated by Thomas−Fermi−Moliere
potential with the Firsov screening length63 for the maximum
energy of 22.5 keV are about 0.21 au for C, 0.26 au for O and
0.28 au for F, respectively. For the minimum energy of 8.5 keV,
the corresponding distances are 0.40, 0.47, and 0.50 au,
respectively. Therefore, in our case electron promotion occurs.
According to the MO correlation diagram in Figure 8, 3dσ
MO of the quasi-molecule can be highly promoted in energy so
as to cross other MOs, and then excitation via the radial
coupling at the 3dσ-3sσ, 3pπ crossings and the rotational
coupling at 3dσ−3dπ, 3dπ−3dδ crossings occurs where one or
two electrons of the incident projectile may be involved. In
general, at these crossings, the projectile’s electron can transfer
to its Rydberg states, forming an excited state.61,63−69 On the
other hand, the projectile’s electron can also transfer to target
atoms, forming a positive ion which may survive neutralization.
As we know, the distance of merging of the promoted
diabatic level to the continuum for C−C system is about 0.7
au.62 The distance of closest approach in our case is smaller
than this value as mentioned above. Therefore, once the
promotion of 2p level of the projectile is strong, the promoted
electron may go to the surface during a close collision via
resonant ionization with unoccupied states of the surface.70 As
the ionized particle recedes from the surface, electron capture
may occur and excited states will be populated again.71 Thus,
the singly excited state and/or the doubly excited state can be
formed in this process via one and/or two electron capture. On
the other hand, the ionized particle may survive during the
collision process. Such a physical scenario can be also applied to
the case of O and F.
vth = 3vF (1 −
1 − (|Ea| − W )/9εF )
(2)
where ε F is the Fermi energy of the surface, and
Ea(z) =
( 27.2E − 41z ) au
I
is upward shifted from the first
ionization energy EI of the projectile by image potential effects
at a distance z measured from the image plane. With work
function W = 4.6 eV and εF = 22 eV45 for HOPG, from eq 2,
we deduce that for CI 2P2 3P0 (EI = 11.26 eV), vth = 0.065 au or
Eth = 1.3 keV; for OI 2P4 3P3/2 (EI = 13.62 eV), vth = 0.088 au or
Eth = 3.1 keV; and for FI 2P5 2P3/2 (EI = 17.42 eV), vth = 0.126
au or Eth = 7.5 keV at infinity. This threshold velocity decreases
with the decrease of ion surface distance and can be satisfied by
the minimum incident velocity of 0.204 au (equivalent to a
projectile energy of 12.5 keV) for C, 0.177 au (12.5 keV) for O,
and 0.134 au (8.5 keV) for F.
18543
DOI: 10.1021/acs.jpcc.6b03680
J. Phys. Chem. C 2016, 120, 18538−18547
Article
The Journal of Physical Chemistry C
the starting point of the effective position of the outgoing
trajectory. According to the discussion on projectile trajectory
and positive-ion production, we can simply obtain P+(z0) = (1
− δ) × Pi. As a consequence, the final positive ion fraction can
be expressed in terms of the production probability multiplied
by the survival probability,
The calculation of Auger ionization rate is a very challenging
task at present for the theoretical difficulty in describing the
involved four different electronic states and the long-range of
the Coulomb electron−electron interaction.76 There is only the
qualitative explanation41 for the scattering of He atoms from Al
(111) under grazing angles of incidence. It illustrates that Auger
ionization rate increases with increasing velocity and decreases
with increasing ion surface distance, so that a remarkable effect
of Auger ionization can only be seen near the surface.
On the basis of the discussion of electron promotion and
Auger ionization, in our case the initial formation of positive
ions may be produced. However, the production of positive
ions involving these processes is more complicated and the
relative weight of these processes is not available at present.
Our setup is not yet equipped with a time-of-flight spectroscopy system or an electron energy spectroscopy system that
would allow us to detailedly investigate the electron excitation
process. Therefore, the clarification of these electronic
processes has not yet been achieved so far. In this respect,
we cannot give a quantitative description but can only assume a
constant ionization probability Pi.
As discussed above, some projectiles penetrate into the
surface and are surrounded by many bulk atoms. A series of
stochastic quasi-binary encounters of projectiles with target
atoms will occur (see Figure 6). Thus, the corresponding
projectiles experience a large neutralization probability in
collisions with target atoms below the surface before they are
detected by the detector. For simplification, we reasonably
assume that these projectiles are fully neutralized. This
assumption could make sense when further considering the
neutralization near the surface. As we know, the transition rate
of neutralization, depending on the coupling between
projectiles and surfaces, shows an exponential increase with
the decrease of ion-surface distance. So projectiles have a large
opportunity to capture electron near the surface. Second,
although Auger neutralization is a two-electron process, it may
be dominant for ion surface distances less than 2 au. The
smaller the distance, the larger the neutralization rate. Third,
although the energy loss is ignored for a lot of scattering
research, in practice the deeper the penetration of the
projectile, the larger the energy loss, and the longer time they
will spend near the surface. Even if a small fraction of positive
ions is formed under the surface, they still have a large
opportunity to be neutralized after emerging from the surface.
On the other hand, the rest projectiles that stay above the
surface are related to the production of positive ions in the
vicinity of the surface. In general, the initially formed positive
ions can be reneutralized by resonant neutralization and/or
Auger neutralization on the outgoing trajectory, which can be
simply given by,
τ
dP+(z)
= − 0 e−αzP+(z)
dz
v sin β
⎛
v ⎞
P+(∞) = (1 − δ) × Pi × exp⎜ − c ⎟
⎝ v sin β ⎠
We proposed a perpendicular velocity dependent penetration
probability in eq 1 which fits well to the simulation data for
both the perfect and defect surfaces. Although we have already
obtained parameter values of fluorine ions from two different
surfaces by simulation, the existence of surface defects and their
uncertainty make us not use the related fitting values directly
for a real surface used in the experiment here. Considering that
a defect surface is mainly related to the change of parameters
rather than the incident perpendicular velocity dependence as
shown in Figure 7, we regard A, λ, and v0 as free parameters,
( v sin−α0.0092
− 0.0001 )
a n d o b t a i n δ1 = 1 × exp
( v sin−α0.006
− 0.001 )
δ2 = 0.69 × exp
(
P+(∞) = P+(z 0) × exp
), v =
c
for both F and O particles
(
0.0001
)
PC+(∞) = (1 − δ1) × 0.5 × exp − v sin β , PO+(∞)
(
P (∞) = (1 − δ ) × 0.153 × exp(−
0.0001
)
) for carbon, oxy-
= (1 − δ2) × 0.165 × exp − v sin β and
+
F
2
0.0001
v sin β
gen, and fluorine, respectively. Apparently, O+ and F+ are well
described by eq 4, while for C+ the general tendency is
reproduced, but slightly large deviations are shown, especially
for small incident angles.
The ionization probability Pi of 0.165 for oxygen ions is
larger than 0.153 for fluorine ions. It indicates that it is easier to
produce O+ ions and agrees with the fact that oxygen has a
smaller first ionization energy. The reason for the largest
ionization probability of 0.5 for carbon is not only due to its
small first ionization energy, but also to the special C−C
symmetric collision system, which leads to a large electron
promotion probability.61
The characteristic velocity vc is rather small and about one or
2 orders of magnitude smaller than λ for these three collision
systems. It suggests that the resonant and/or Auger
neutralization have minor effects on the outgoing trajectory.
It may be attributed to the production of double excited-state
atoms. The lifetime of double excited state is likely to be longer
than the collision time [the Ne** lifetime is >10−14 s,77 and our
collision time is at femtosecond time scale], doubly excited
atoms can leave the surface intactly and subsequently
autoionize to positive ions far from the surface where
reneutralization is unlikely.66
When ignoring the penetration effect of the trajectory, we
(3)
τ0e−αz 0
α
for C and
since they have nearly the same atomic number.
We also display the fitting positive ion fractions as a function
of incident angle and velocity in Figures 4 and 5, which are
described
by
eq
4
with
where the electron transition rate is given by τ = τ0e−αz, and β is
the exit angle. The nuclear energy loss is negligible here
because of the glancing incidence scattering,24,68 so that the exit
velocity is equal to the incident velocity. From eq 3, we obtain
the
final
survival
probability
v
− vsinc β
(4)
(
v
obtain an expression for positive ion fraction Pi × exp − v sinc β
)
for all projectiles. It represents that the positive ion fraction is
expressed in terms of the production probability at a close
collision multiplied by the survival probability on the outgoing
trajectory. It clearly shows a monotonous increase of the
is the so-called
characteristic velocity proportional to the neutralization rate.
P+(z0) represents the initial positive-ion probability at z0, z0 is
18544
DOI: 10.1021/acs.jpcc.6b03680
J. Phys. Chem. C 2016, 120, 18538−18547
Article
The Journal of Physical Chemistry C
with the increase of incident velocity and angle, which is very
different from that presented in previous studies.
These anomalous dependences can be well interpreted by
the effect of projectile trajectory where a lot of projectiles
penetrate into the subsurface and suffer from efficient
neutralization. In addition, the initial formation of positive
ions above the surface is mainly discussed in terms of single
and/or double electron promotion and kinematically induced
Auger ionization. However, the clarification of these electronic
processes is not a trivial task. In this respect, we cannot give a
quantitative description, and only assume a constant ionization
probability.
From the experimental data, we find that the resonant and/
or Auger neutralization on the outgoing trajectory of positive
ions seem to be negligible, which may be attributed to the
formation of doubly excited atoms with long lifetime near the
surface. A measurement of the electron energy spectrum may
help to confirm this mechanism, which is in progress.
positive-ion fraction with increasing exit velocity. Moreover, the
increase is faster at small exit angles, and starts to slow down at
high exit angles. It is consistent with many previous
observations.36−42,47 However, this trend is not consistent
with our experimental data as shown in Figures 4 and 5.
Moreover, as discussed above, the survival probability related to
(
v
)
exp − v sinc β is close to 1 due to the negligible vc.
Now let us return to the penetration trajectory effect. The
production probability is (1 − δ) × Pi where the ionization
probability Pi is a constant. The final positive ion fraction can
be expressed in terms of the production probability multiplied
by the survival probability, as shown in eq 4. As we know, the
survival probability is close to 1, so the change of the final
positive ion fraction is mainly determined by the production
probability (1 − δ) × Pi. The factor (1 − δ) decreases
exponentially with the increase of incident perpendicular
velocity when a threshold velocity is reached, and thus it
reverses the trend of the conventional velocity dependence,
resulting in extraordinary decrease of positive ion fraction with
increasing velocity as shown in Figure 4. It also changes the
dependence of angle, resulting in the different descending rate
as shown in Figure 5.
Carbon has a larger penetration probability δ as compared to
oxygen and fluorine (see Figure 7), which agrees with the fact
that carbon has a smaller coulomb repulsion with surface
atoms, smaller size of shadow cone and smaller critical angle for
planar surface channeling. Moreover, the change of carbon
penetration probability is faster. As discussed above, the
calculated positive ion fraction mainly depends on the
penetration probability, so in Figures 4 and 5 C+ fractions
decrease faster with increasing incident velocity and angle as
compared to O+ and F+.
In Figure 5a, the discrepancy between calculation and
experiment is observed and may be attributed to the constant
ionization probability Pi = 0.5 used in this work. In general,
ionization probability may change with the type of collision
processes and with energy. The ionization probability Pi at close
collisions has been calculated by Rabalais et al.37 based on the
Fano−Lichten78 mechanism, from which we find that the
ionization probability increases with decreasing the distance of
closest approach. In electron promotion model, the ionization
probability during level crossing in a close collision is estimated
by the Landau−Zener formula:63,79 Pi = 2P(1 − P), P =
exp(−νc/νr) where P is the probability for electrons to survive
in the same diabatic state after crossing and vr is the relative
velocity of the particles at the crossing distance. From this
formula, we find that the velocity dependence is relatively
complicated. In addition, surface roughness80 may also
influence Pi. Therefore, the calculation of Pi is a very
challenging task at present and is beyond the scope of this
work. For C+ fractions at small incident angles, the ionization
probability may be smaller than 0.5 which makes a large
discrepancy. In this respect, the ionization probability seems to
present the angle dependence. As the incident angle decreases,
the distance of closest approach increases, and thus the
ionization probability is expected to decrease.
■
AUTHOR INFORMATION
Corresponding Authors
*(L.C.) E-mail: [email protected]. Telephone: 15002672420.
* (X .C.) E-mail: [email protected]. T eleph one:
13519619060.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work is supported by the National Natural Science
Foundation of China (Grant Nos. 11405078 and 11474140),
the Fundamental Research Funds for the Central Universities
(Grant Nos. lzujbky-2014-169 and lzujbky-2015-244), the
project sponsored by SRF for ROCS, SEM, and the National
Students’ Innovation and Entrepreneurship Training Program
(Grant Nos. 201410730069 and 201510730078).
■
REFERENCES
(1) Mücksch, C.; Rösch, C.; Müller-Renno, C.; Ziegler, C.; Urbassek,
H. M. Consequences of Hydrocarbon Contamination for Wettability
and Protein Adsorption on Graphite Surfaces. J. Phys. Chem. C 2015,
119, 12496−12501.
(2) Martínez-Galera, A. J.; Gómez-Rodríguez, J. M. Nucleation and
Growth of the Prototype Azabenzene 1,3,5-Triazine on Graphite
Surfaces at Low Temperatures. J. Phys. Chem. C 2011, 115, 11089−
11094.
(3) Appy, D.; Lei, H.; Wang, C.-Z.; Tringides, M. C.; Liu, D.-J.;
Evans, J. W.; Thiel, P. A. Transition Metals on the (0 0 0 1) Surface of
Graphite: Fundamental Aspects of Adsorption, Diffusion, and
Morphology. Prog. Surf. Sci. 2014, 89, 219−238.
(4) Bergeld, J.; Kasemo, B.; Chakarov, D. Photocatalytic Reactions at
the Graphite/Ice Interface. Phys. Chem. Chem. Phys. 2008, 10, 1151−
1155.
(5) Federici, G.; Skinner, C. H.; Brooks, J. N.; Coad, J. P.; Grisolia,
C.; Haasz, A. A.; Hassanein, A.; Philipps, V.; Pitcher, C. S.; Roth, J.;
et al. Plasma−Material Interactions in Current Tokamaks and Their
Implications for Next Step Fusion Reactors. Nucl. Fusion 2001, 41,
1967.
(6) Esquinazi, P.; Setzer, A.; Höhne, R.; Semmelhack, C.; Kopelevich,
Y.; Spemann, D.; Butz, T.; Kohlstrunk, B.; Lösche, M. Ferromagnetism
in Oriented Graphite Samples. Phys. Rev. B: Condens. Matter Mater.
Phys. 2002, 66, 024429.
(7) Guchhait, S.; Ohldag, H.; Arenholz, E.; Ferrer, D. A.; Mehta, A.;
Banerjee, S. K. Magnetic Ordering of Implanted Mn in HOPG
Substrates. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88,
174425.
5. CONCLUSION
In conclusion, we have reported experimental results of positive
ion fractions of C−, O−, and F− impinging on a HOPG surface
in the 8.5−22.5 keV energy range performed at a scattering
angle of 8°. We observed that positive-ion fractions decrease
18545
DOI: 10.1021/acs.jpcc.6b03680
J. Phys. Chem. C 2016, 120, 18538−18547
Article
The Journal of Physical Chemistry C
(8) Wang, Y.; Pochet, P.; Jenkins, C. A.; Arenholz, E.; Bukalis, G.;
Gemming, S.; Helm, M.; Zhou, S. Defect-induced Magnetism in
Graphite through Neutron Irradiation. Phys. Rev. B: Condens. Matter
Mater. Phys. 2014, 90, 214435.
(9) Moaied, M.; Alvarez, J. V.; Palacios, J. J. Hydrogenation-induced
Ferromagnetism on Graphite Surfaces. Phys. Rev. B: Condens. Matter
Mater. Phys. 2014, 90, 115441.
(10) Brongersma, H. H.; Draxler, M.; de Ridder, M.; Bauer, P.
Surface Composition Analysis by Low-energy Ion Scattering. Surf. Sci.
Rep. 2007, 62, 63−109.
(11) Qian, G.; Li, Y.; Gerson, A. R. Applications of Surface Analytical
Techniques in Earth Sciences. Surf. Sci. Rep. 2015, 70, 86−133.
(12) Kurnaev, V. A.; Matveev, D. I.; Trifonov, N. N. Simulation of
Charge Exchange Neutrals Interactions with Gaps in First Wall
Cladding. J. Nucl. Mater. 2007, 363-365, 797.
(13) Hanley, L.; Sinnott, S. B. The Growth and Modification of
Materials via Ion−Surface Processing. Surf. Sci. 2002, 500, 500−522.
(14) Scheer, J. A.; Wieser, M.; Wurz, P.; Bochsler, P.; Hertzberg, E.;
Fuselier, S. A.; Koeck, F. A.; Nemanich, R. J.; Schleberger, M. High
Negative Ion Yield from Light Molecule Scattering. Nucl. Instrum.
Methods Phys. Res., Sect. B 2005, 230, 330−339.
(15) Aanesland, A.; Meige, A.; Chabert, P. Electric Propulsion Using
Ion-Ion Plasmas. J. Phys: Conf. Ser. 2009, 162, 012009.
(16) Popelier, L.; Aanesland, A.; Chabert, P. Response of an Ion−Ion
Plasma to Dc Biased Electrodes. J. Phys. D: Appl. Phys. 2011, 44,
315203.
(17) Ahmad, A.; Pardanaud, C.; Carrère, M.; Layet, J.-M.; Gicquel,
A.; Kumar, P.; Eon, D.; Jaoul, C.; Engeln, R.; Cartry, G. Negative-Ion
Production on Carbon Materials in Hydrogen Plasma: Influence of the
Carbon Hybridization State and the Hydrogen Content on H− Yield. J.
Phys. D: Appl. Phys. 2014, 47, 085201.
(18) Chen, L.; Shen, J.; Jia, J. J.; Kandasamy, T.; Bobrov, K.;
Guillemot, L.; Fuhr, J. D.; Martiarena, M. L.; Esaulov, V. A. Li+-Ion
Neutralization on Metal Surfaces and Thin Films. Phys. Rev. A: At.,
Mol., Opt. Phys. 2011, 84, 052901.
(19) Chen, L.; Guo, Y. L.; Jia, J. J.; Zhang, H. Q.; Cui, Y.; Shao, J. X.;
Yin, Y. Z.; Qiu, X. Y.; Lv, X. Y.; Sun, G. Z.; et al. Absence of a Guiding
Effect and Charge Transfer in the Interaction of keV-Energy Negative
Ions with Al2O3 Nanocapillaries. Phys. Rev. A: At., Mol., Opt. Phys.
2011, 84, 032901.
(20) Chen, L.; Lv, X. Y.; Jia, J. J.; Ji, M. C.; Zhou, P.; Sun, G. Z.;
Wang, J.; Chen, Y. F.; Xi, F. Y.; Cui, Y.; et al. Charge Exchange of keV
O− Ions Transmitted Through Al2O3 Nano-capillaries. J. Phys. B: At.,
Mol. Opt. Phys. 2011, 44, 045203.
(21) Zhou, H.; Chen, L.; Feng, D.; Guo, Y. L.; Ji, M. C.; Wang, G. Y.;
Zhou, W.; Li, Y.; Zhao, L.; Chen, X. M. Formation of Negative Ions in
Grazing Scattering of Neutral Atoms from Alkali-Metal Halide (100)
Surfaces. Phys. Rev. A: At., Mol., Opt. Phys. 2012, 85, 014901.
(22) Bonetto, F. J.; Romero, M. A.; Iglesias-García, A.; Vidal, R. A.;
Goldberg, E. C. Time−Energy Uncertainty and Electronic Correlation
in H+−Graphite Collisions. J. Phys. Chem. C 2015, 119, 3124−3131.
(23) Vidal, R. A.; Bonetto, F.; Ferrón, J.; Romero, M. A.; García, E.
A.; Goldberg, E. C. Electron Capture and Loss in the Scattering of H+
from HOPG Graphite. Surf. Sci. 2011, 605, 18−23.
(24) Datta, D.; Shen, J.; Esaulov, V. A. Hydrogen Negative Ion
Formation on a Graphite HOPG Surface. Nucl. Instrum. Methods Phys.
Res., Sect. B 2013, 315, 42−44.
(25) Bonetto, F. J.; Romero, M. A.; García, E. A.; Vidal, R. A.; Ferrón,
J.; Goldberg, E. C. Relevant Effects of Localized Atomic Interactions
and Surface Density of States on Charge Transfer in Ion-Surface
Collisions. Europhys. Lett. 2007, 80, 53002.
(26) Souda, R.; Asari, E.; Kawanowa, H.; Suzuki, T.; Otani, S.
Capture and Loss of Valence Electrons during Low Energy H+ and H−
Scattering from LaB6(100), Cs /Si(100), Graphite and LiCl. Surf. Sci.
1999, 421, 89−99.
(27) Wethekam, S.; Mertens, A.; Winter, H. Survival of He+ Ions
during Grazing Scattering f rom a Ag(111) Surface. Phys. Rev. Lett. 2003,
90, 037602.
(28) Iglesias-García, A.; García, E. A.; Goldberg, E. C. Role of He
Excited Configurations in the Neutralization of He+ Ions Colliding
with a HOPG Surface. Phys. Rev. B: Condens. Matter Mater. Phys. 2013,
87, 075434.
(29) Iglesias-García, A.; Bonetto, F.; Vidal, R.; Ferrón, J.; Goldberg,
E. C. Ion Neutralization and High-Energy Electron Emission in He+
Scattering by a Highly Oriented Pyrolytic Graphite Surface. Phys. Rev.
A: At., Mol., Opt. Phys. 2014, 89, 042702.
(30) Luna, N. B.; Bonetto, F. J.; Vidal, R. A.; Goldberg, E. C.; Ferrón,
J. Low Energy Ion Scattering in He/HOPG System. J. Mol. Catal. A:
Chem. 2008, 281, 237−240.
(31) Bajales, N.; Cristina, L.; Mendoza, S.; Baragiola, R. A.; Goldberg,
E. C.; Ferrón, J. Exciton Autoionization in Ion-Induced Electron
Emission. Phys. Rev. Lett. 2008, 100, 227604.
(32) Romero, M. A.; Iglesias-García, A.; Goldberg, E. C. Localized
Description of Band Structure Effects on Li atom Interaction with
Graphene. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 83, 125411.
(33) Bonetto, F.; Romero, M. A.; García, E. A.; Vidal, R. A.; Ferrón,
J.; Goldberg, E. C. Large Neutral Fractions in Collisions of Li+ with a
Highly Oriented Pyrolytic Graphite Surface: Resonant and Auger
Mechanisms. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78,
075422.
(34) Chen, L.; Ding, B.; Li, Y.; Qiu, S. L.; Xiong, F. F.; Zhou, H.;
Guo, Y. L.; Chen, X. M. Evidence for the Incoming-Velocity Effect on
Negative Fluorine Ions Scattering From a Highly-Oriented-PyrolyticGraphite Surface. Phys. Rev. A: At., Mol., Opt. Phys. 2013, 88, 044901.
(35) Wang, Q. J.; Qiu, S. L.; Xiong, F. F.; Li, Y.; Ding, B.; Guo, Y. L.;
Chen, X. M.; Chen, L. Charge Exchange of O− Scattering on a Si(111)
Surface. Eur. Phys. J. D 2015, 69, 210.
(36) Chen, L.; Qiu, S. L.; Xiong, F. F.; Lu, J. J.; Liu, P. Y.; Ding, B.;
Li, Y.; Cui, Y.; Guo, Y. L.; Chen, X. M. Nonadiabatic Dynamics In
Energetic Negative Fluorine Ions Scattering from a Si(100) Surface. J.
Chem. Phys. 2015, 143, 114703.
(37) Rabalais, J. W.; Chen, J.-N.; Kumar, R.; Narayana, M. Inelastic
Processes in Ion/Surface Collisions: Scattered Ion Fractions and VUV
Photon Emission for Ne+ and Ar+ Collisions with Mg and Y Surfaces.
J. Chem. Phys. 1985, 83, 6489.
(38) Guillemot, L.; Lacombe, S.; Huels, M.; Tuan, V. N.; Esaulov, V.
A. Inelastic Processes in Ne+ Collisions with a Mg Surfaces; Charge
Fractions and Surface Roughness. Nucl. Instrum. Methods Phys. Res.,
Sect. B 1994, 90, 270−273.
(39) Lavery, A. C.; Sosolik, C. E.; Keller, C. A.; Cooper, B. H. Charge
Transfer and Memory Loss in keV Oxygen-Ion Scattering from
Cu(001). Phys. Rev. B: Condens. Matter Mater. Phys. 2000, 61, 2291.
(40) Winter, H.; Siekmann, G.; Ortjohann, H. W.; Poizat, J. C.;
Remillieux, J. Charge State Distributions after the Scattering of Fast
He+-Ions from an Al (111)-Surface under a Grazing Angle of
Incidence. Nucl. Instrum. Methods Phys. Res., Sect. B 1998, 135, 372−
376.
(41) Wethekam, S.; Valdés, D.; Monreal, R. C.; Winter, H.
Dynamical Auger Charge Transfer of Noble Gas Atoms and Metal
Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 033105.
(42) Mouhammad, S.; Benoit-Cattin, P.; Benazeth, C.; Cafarelli, P.;
Reynes, P.; Richard-Viard, M.; Ziesel, J. P. Charge Exchange and
Excitation in Ne+/a:Si Collision at Medium Energy (2−30 keV).
Experiment and Simulation. J. Phys.: Condens. Matter 1998, 10, 8629−
8641.
(43) Gao, R. S.; Gibner, P. S.; Newman, J. H.; Smith, K. A.;
Stebbings, R. F. Absolute and Angular Efficiencies of a MicrochannelPlate Position-Sensitive Detector. Rev. Sci. Instrum. 1984, 55, 1756.
(44) Straub, H. C.; Mangan, M. A.; Lindsay, B. G.; Smith, K. A.;
Stebbings, R. F. Absolute Detection Efficiency of a Microchannel Plate
Detector for Kilo-Electron Volt Energy Ions. Rev. Sci. Instrum. 1999,
70, 4238.
(45) Takahashi, N.; Adachi, Y.; Saito, M.; Haruyama, Y. Absolute
Detection Efficiencies for keV Energy Atoms Incident on a
Microchannel Plate Detector. Nucl. Instrum. Methods Phys. Res., Sect.
B 2013, 315, 51−54.
18546
DOI: 10.1021/acs.jpcc.6b03680
J. Phys. Chem. C 2016, 120, 18538−18547
Article
The Journal of Physical Chemistry C
Atoms and Ions on Mg, Al and Ag Surfaces. Nucl. Instrum. Methods
Phys. Res., Sect. B 1997, 125, 283−287.
(68) Maazouz, M.; Guillemot, L.; Esaulov, V. A.; O’Connor, D. J.
Electron Capture and Loss in the Scattering of Hydrogen and Oxygen
Ions on a Si Surface. Surf. Sci. 1998, 398, 49−59.
(69) Gordon, M. J.; Mace, J.; Giapis, K. P. Charge-Exchange
Mechanisms at the Threshold for Inelasticity in Ne+ Collisions with
Surfaces. Phys. Rev. A: At., Mol., Opt. Phys. 2005, 72, 012904.
(70) Goldberg, E. C.; Monreal, R.; Flores, F.; Brongersma, H. H.;
Bauer, P. New Model for Ion Neutralization at Surfaces. Surf. Sci.
1999, 440, L875.
(71) Esaulov, V. A. Inelastic Ion Collisions Involving Metal Surfaces
Exposed to Oxygen and Electron Transfer Processes. Surf. Sci. 1998,
415, 95−105.
(72) Boumsellek, S.; Tuan, V. N.; Esaulov, V. A. Two-Electron
Detachment and Excitation Processes in S− Collisions with Atoms and
Molecules. Phys. Rev. A: At., Mol., Opt. Phys. 1990, 41, 2515.
(73) Boumsellek, S.; Esaulov, V. A. Ionisation Processes in Collisions
of Open-Shell Atoms: I. Fluorine Atom Collisions with Inert Gases
and Molecules. J. Phys. B: At., Mol. Opt. Phys. 1990, 23, 279−292.
(74) Boumsellek, S.; Esaulov, V. A. Ionisation Processes in Collisions
of Open Shell Atoms: II. Oxygen and Sulphur Atom Collisions with
Inert Gases. J. Phys. B: At., Mol. Opt. Phys. 1990, 23, 1303−1313.
(75) Boumsellek, S.; Esaulov, V. A. Ionization Processes in Collisions
of Open-Shell Atoms: III. The Autoionizing States of Nitrogen. J. Phys.
B: At., Mol. Opt. Phys. 1990, 23, L605−L610.
(76) Carmina Monreal, R. Auger Neutralization and Ionization
Processes for Charge Exchange Between Slow Noble Gas Atoms and
Solid Surfaces. Prog. Surf. Sci. 2014, 89, 80−125.
(77) Morgenstern, R.; Niehaus, A.; Zimmermann, G. Autoionising
States Formed by Electron Capture in Collisions of Multiply Charged
Ne ions with He, H2 and Xe. J. Phys. B: At. Mol. Phys. 1980, 13, 4811.
(78) Fano, U.; Lichten, W. Interpretation of Ar-Ar Collisions at 50
keV. Phys. Rev. Lett. 1965, 14, 627.
(79) Zwally, H. J.; Koopman, D. W. Single-Electron Capture by C4+
in Helium, Neon, and Argon below 40 keV. Phys. Rev. A: At., Mol., Opt.
Phys. 1970, 2, 1851.
(80) Esaulov, V. A.; Guillemot, L.; Grizzi, O.; Sánchez, E. A. SurfaceTopography Dependence of Line Shapes in Electron Spectra due to
Decay of Autoionizing States Produced in Inelastic Ion-Surface
Collisions. Phys. Rev. A: At., Mol., Opt. Phys. 2002, 65, 052901.
(46) Winecki, S.; Cocke, C. L.; Fry, D.; Stöckli, M. P. Neutralization
and Equilibration of Highly Charged Argon Ions at Grazing Incidence
on a Graphite Surface. Phys. Rev. A: At., Mol., Opt. Phys. 1996, 53,
4228.
(47) Maazouz, M.; Ustaze, S.; Guillemot, L.; Esaulov, V. A. Electron
Capture and Loss in the Scattering of Flourine and Chlorine Atoms
and Ions on Metal Surfaces. Surf. Sci. 1998, 409, 189−198.
(48) Hagstrum, H. D. Theory of Auger Ejection of Electrons from
Metals by Ions. Phys. Rev. 1954, 96, 336.
(49) Hagstrum, H. D.; Petrie, P.; Chaban, E. E. Interaction of He+
and Ne+ Ions with Ni(100)-K and Cu(100)-K Surfaces Having
Variable Potassium Coverage. Phys. Rev. B: Condens. Matter Mater.
Phys. 1988, 38, 10264.
(50) Winter, H. Collisions of Atoms and Ions with Surfaces under
Grazing Incidence. Phys. Rep. 2002, 367, 387.
(51) Richard-Viard, M.; Bénazeth, C.; Benoit-Cattin, P.; Cafarelli, P.;
Nieuwjaer, N. Role of the Image Charge and Electronic Corrugation
on Charge-Fraction Azimuthal Variations for keV Li and Na Ions
Interacting with Cu(110) and Ag(110) under Grazing Incidence. Phys.
Rev. B: Condens. Matter Mater. Phys. 2007, 76, 045432.
(52) Fang, Z. L.; Lau, W. M.; Rabalais, J. W. Computational Study of
Low Energy Ion Surface Hyperchanneling. Surf. Sci. 2005, 581, 1−8.
(53) Cafarelli, P.; Richard-Viard, M.; Bénazeth, C.; Nieuwjaer, N.;
Lorente, N. Simulations of the Azimuthal Distribution of Low-Energy
H Atoms Scattered off Ag(110) at Grazing Incidence: DFT ManyBody Versus Model Pair Potentials. Nucl. Instrum. Methods Phys. Res.,
Sect. B 2003, 203, 211−217.
(54) Lui, K. M.; Fang, Z. L.; Lau, W. M.; Rabalais, J. W. Computer
Simulations of Hyperchanneling of Low Energy Ions in Semichannels
of Pt(111)-(1 × 1) Surface. Nucl. Instrum. Methods Phys. Res., Sect. B
2001, 182, 200−206.
(55) Kolasinski, K. W. Surface Science: Foundations of Catalysis and
Nanoscience, 2nd ed.; John Wiley & Sons Ltd.: England, 2008.
(56) Karolewski, M. A. Kalypso: a Software Package for Molecular
Dynamics Simulation of Atomic Collisions at Surfaces. Nucl. Instrum.
Methods Phys. Res., Sect. B 2005, 230, 402.
(57) Ziegler, J. F.; Biersack, J. P.; Littmark, U. The Stopping and Range
of Ions in Matter; Pergamon: New York, 1985; Vol. 1.
(58) Foster, C.; Hoogkamer, T. P.; Woerlee, P.; Saris, F. W. An
Estimate of Direct Coulomb K-Shell Vacancy Production in Heavy
Ion-Atom Collisions. J. Phys. B: At. Mol. Phys. 1976, 9, 1943.
(59) Meyerhof, W. E. Ionization of the 3dσ Molecular Orbital in
Heavy-Ion Collisions. Phys. Rev. A: At., Mol., Opt. Phys. 1978, 18, 414.
(60) Magno, C.; Milazzo, M.; Pizzi, C.; Porro, F.; Rota, A.;
Riccobono, G. Effects of Coulomb Repulsion in the Inner-Shell
Ionization Cross-Section by Protons, Deuterons and Alpha-Particles.
Nuovo Cimento A 1979, 54, 277.
(61) Barat, M.; Lichten, W. Extension of the Electron-Promotion
Model to Asymmetric Atomic Collisions. Phys. Rev. A: At., Mol., Opt.
Phys. 1972, 6, 211.
(62) Lörinčík, J.; Šroubek, Z.; Cernusca, S.; Diem, A.; Winter, H.;
Aumayr, F. Ion Induced Kinetic Electron Emission from Highly
Oriented Pyrolytic Graphite by Impact of H+, C+, N+ and O+. Surf. Sci.
2002, 504, 59−65.
(63) Rabalais, J. W. Principles and Applications of Ion Scattering
Spectrometry: Surface Chemical and Structural Analysis; Wiley: New
York, 2003.
(64) Li, T.; MacDonald, R. J. Inelastic Energy Loss and Charge
Exchange in Low-Energy Heavy Ions Scattered from Cu and Cu-alloy
Surfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 1995, 51, 17876.
(65) Xu, F.; Manicò, G.; Ascione, F.; Bonanno, A.; Oliva, A.;
Baragiola, R. A. Inelastic Energy Loss in Low-Energy Ne+ Scattering
from a Si Surface. Phys. Rev. A: At., Mol., Opt. Phys. 1998, 57, 1096.
(66) Mace, J.; Gordon, M. J.; Giapis, K. P. Evidence of Simultaneous
Double-Electron Promotion in F+ Collisions with Surfaces. Phys. Rev.
Lett. 2006, 97, 257603.
(67) Maazouz, M.; Guillemot, L.; Schlatholter, T.; Ustaze, S.;
Esaulov, V. A. Electron Capture and Loss in the Scattering of Oxygen
18547
DOI: 10.1021/acs.jpcc.6b03680
J. Phys. Chem. C 2016, 120, 18538−18547