Dissociative Electron Attachment
to Bromotriuoromethane
Stefán Þór Kristinsson
Faculty
Faculty of
of Physical
Physical Sciences
Sciences
University
University of
of Iceland
Iceland
2014
2014
Dissociative Electron Attachment
to Bromotriuoromethane
Stefán Þór Kristinsson
15 ECTS thesis submitted in partial fulllment of a
Baccalaureus Scientiarum degree in Chemistry
Advisor
Oddur Ingólfsson
Faculty of Physical Sciences
School of Engineering and Natural Sciences
University of Iceland
Reykjavik, May 2014
Dissociative Electron Attachment
to Bromotriuoromethane
15 ECTS thesis submitted in partial fulllment of a B.Sc. degree in Chemistry
c 2014 Stefán Þór Kristinsson
Copyright All rights reserved
Faculty of Physical Sciences
School of Engineering and Natural Sciences
University of Iceland
VRII, Hjarðarhagi 2-6
107, Reykjavik
Iceland
Telephone: 525 4000
Bibliographic information:
Stefán Þór Kristinsson, 2014, Dissociative Electron Attachment
to Bromotriuoromethane, B.Sc. thesis, Faculty of Physical Sciences, University of Iceland.
Printing: Háskólaprent, Fálkagata 2, 107 Reykjavík
Reykjavík, Iceland, May 2014
Abstract
The interaction between molecules and low energy electrons often leads to the production of
transient negative ions (TNIs) through electron capture. These ions are formed in an excited
state and are therefore bound to relax. They do so mostly through two competing channels.
The molecule can reemit the trapped electron, through autodetachment (AD), or bonds can
be ruptured to form a negatively charged molecular fragment and its neutral counterparts.
The latter process is called dissociative electron attachment (DEA).
DEA behavior, that takes place when bromotrifluoromethane, CF3 Br, interacts with electrons of energy below the molecule’s ionization limit, was studied. The fragment formation
from CF3 Br was examined using a crossed molecular and electron beam apparatus termed
SIGMA, set up at the University of Iceland. The dissociation behavior of the molecule was
compared and contrasted to calculated thermochemical threshold values as well as the behavior of congeners, CF3 I and CF3 Cl, that have been previously studied.
A total of four negative fragments, Br– , F– , FBr– and CF2 Br– , were detected, proceeding
through three different resonances. Peaks in the negative ion yields were observed around
0 eV, 4 eV and 8 eV. The resonance close to 0 eV yields exclusively negative bromine ions
and that peak is the most intense by far, measured at a count rate of more than three orders
of magnitude larger than the next. Peaks close to 0 eV can also be observed in negative
ion yields for the other three fragments, but they were attributed to other causes than the
formation of the anion from primary dissociation of the molecule. A resonance at 4 eV
yields all four fragments with different intensity peaks. A peak at 8 eV was also detected
with low intensity in the ion yields for Br– and F– .
iv
Útdráttur
Þegar sameindir víxlverka við lágorkurafeindir myndast oft tímabundið neikvætt hlaðnar
jónir í gegn um föngun rafeinda. Þessar jónir myndast í örvuðu ástandi og þurfa að losa
orku. Þær gera það aðallega með tveim leiðum sem eru í samkeppni hvor við aðra. Annars
vegar getur sameindin losað sig við rafeindina beint (e. autodetahment; AD) eða tengi geta
rofnað til að mynda neikvætt hlaðið sameindabrot og andstæð óhlaðin sameindabrot. Það
ferli kallast rjúfandi rafeinda álagning(e. dissociative electron attachment; DEA)
DEA ferlið, sem á sér stað þegar brómotríflúorómetan, CF3 Br, víxlverkar við lágorku rafeindir, var rannsakað. Myndun á neikvætt hlöðnum sameindabrotum var skoðuð mælingartæki með skerandi rafeinda og sameindageisla. Tækið er sett upp á Raunvísindastofnun
við Háskóla Íslands og kallast SIGMA. Hegðun niðurbrotsins var borið saman við reiknað
varmafræðilegt lágorkugildi sem og við niðurbrotshegðanir líkra sameinda, CF3 I og CF3 Cl
sem hafa verið rannsakaðar áður.
Samtals voru fjögur neikvætt hlaðin sameindabrot greind, Br– , F– , FBr– and CF2 Br– , mynduð í gegn um þrjú mismunandi tímabundin ástönd. Toppar mynduðust við 0 eV, nálægt
4 eV og við 8 eV. Jónin sem myndast við 0 eV brotnar aðeins niður til að mynda neikvæðar
brómjónir, Br– , en sá toppur er þrem stærðargráðum sterkari í talningahraða en sá næststærsti. Toppar í heimtum hinna sameindabrotanna mynduðus einnig við 0 eV, en eru af
öðrum ástæðum en beint niðurbrot CF3 Br til myndunar sameindabrotanna. Merki fékkst
fyrir öll fjögur sameindabrotin í kringum 4 eV, með mis miklum styrkleika. Einnig sást
toppur við 8 eV með lágum styrkleika í heimtum fyrir Br– and F– .
v
Contents
List of Figures
2
List of Tables
2
1
Introduction
3
2
Theory
6
2.1
The electron attachment process . . . . . . . . . . . . . . . . . . . . . . .
6
2.2
Dissociative electron attachment . . . . . . . . . . . . . . . . . . . . . . .
7
2.3
Formation paths of resonances . . . . . . . . . . . . . . . . . . . . . . . .
8
2.3.1
Core-excited resonances . . . . . . . . . . . . . . . . . . . . . . .
9
2.3.2
Single particle shape resonances . . . . . . . . . . . . . . . . . . .
9
3
4
Methods
11
3.1
Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
11
3.2
Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
12
Results and Discussion
14
4.1
Mass Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
4.2
Negative ion yield . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
4.2.1
Formation of Br– . . . . . . . . . . . . . . . . . . . . . . . . . . .
17
4.2.2
F– . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
18
4.2.3
FBr–
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
4.2.4
CF2 Br– . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
19
Other possible fragments . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
4.3.1
Parent ion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
4.3.2
CF–3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
21
4.3.3
F–2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
22
4.3
5
Summary
22
References
24
1
List of Figures
1
Halon 1301 used in a firefighting system . . . . . . . . . . . . . . . . . . .
3
2
Electron attachment processes . . . . . . . . . . . . . . . . . . . . . . . .
6
3
Relative energy levels of resonances . . . . . . . . . . . . . . . . . . . . .
8
4
The interaction potential for different angular momentums of an electron . .
10
5
SIGMA, electron attachment apparatus . . . . . . . . . . . . . . . . . . . .
11
6
Positive ion mass spectrum of CF3 Br from m/z 0 to 160 . . . . . . . . . . .
15
7
Ion yield curves observed for electron attachment to CF3 Br . . . . . . . . .
17
8
UV spectra of trifluorohalomethanes . . . . . . . . . . . . . . . . . . . . .
23
List of Tables
1
Computed thermochemical thresholds . . . . . . . . . . . . . . . . . . . .
2
Relative intensity for peaks resulting from contribution from the resonance
around 4 eV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2
16
22
1
Introduction
This study was conducted as a part of a larger project with the objective of better understanding dissociation dynamics as a result of the interaction between halomethane molecules and
low energy electrons.
Bromotrifluoromethane, or Halon 1301, like
many other halomethanes, has had several important industrial applications over the last few
decades. Halon 1301 was used extensively as a
refrigerant gas in many industrial chillers and as
a fire extinguishing agent. Usage of CF3 Br for
the suppression of fire started in the 1960s, when
its remarkable efficiency in the field of firefighting was discovered. Even in low concentrations,
CF3 Br is a highly efficient flame inhibitor as it
starts a chain of events wherein oxygen is contained, ignition is stunted and thus, heat is re-
Figure 1: Halon 1301 used in a total
flooding firefighting system. Figure from1
c Guangdong Ping’an Co Ltd
duced.2 As a gaseous fire suppressor, it is also a clean agent, meaning that it leaves little, or
no trace after usage. Therefore, it is still used around valuable objects such as in computer
labs and in data processing centers, where it is used in total flooding systems.
Halon 1301 was also used as a fuel inerting agent in various military vehicles3 as well as
commercial aircrafts. The substance was pumped into the fuel tank, where its high concentration in the vapor phase above the fuel reduced the risk of explosive reactions between the
fuel and the atmosphere. This served the purpose of increased flight safety as well as making
military vehicles less vulnerable during combat operations.
However, the usage of bromotrifluoromethane has decreased significantly over the last 25
years. The reason for this lies in the harm that it can do to our environment, both as a
greenhouse gas and in its potential to destroy the stratospheric ozone layer. Over the last
few decades, awareness of ozone depletion and global warming and their impact on the
environment has increased considerably. The calculated relative potentials, called global
warming potential (GWP) and ozone depletion potential (ODP) are used to describe the
effects of various substances.
3
Global warming potential is measured by the Intergovernmental Panel on Climate Change,
both in the time horizons of 20 years and 100 years to describe the relative capability of a
substance to trap heat within the atmosphere. The calculated GWP of Carbon dioxide at its
current level of 380 ppm, is used as a reference,4 and its GWP for both time horizons is set
to the value of 1. Similarly, ozone depletion potential is used to describe the degradation of
the ozone layer of a chemical substance. It is measured relative to trichlorofluoromethane
(CFC-11), which has a fixed ODP of 1.0.
Bromotrifluoromethane is both highly effective as a greenhouse gas as well as being termed
a class one ozone depleting substance. It has a GWP20 of 7800 and a GWP100 of 6290.5
The latter is lower, as the gas has an average atmospheric lifetime of 65 years. The carbonbromine bond is strong enough to survive its journey to the stratosphere and act as a greenhouse gas there. The bond is, however, weak enough to break through a photochemical
reaction in the stratosphere. When the substance breaks down, ozone destruction takes over,
as the bromine radical is an aggressive reactant with ozone. Halon 1301 has an ODP of 10,6
meaning that it is ten times more potent as an ozone depleting substance than the scale’s
reference substance, CFC-11. The bromine radical reacts more effectively with ozone than
both the chlorine and the fluorine radicals.
The Montreal Protocol on Substances that Deplete the Ozone Layer7 was signed into law
on the first of January, 1989 and is currently ratified by 197 nations8 including all members
of the United Nations as well as the European Union. In accordance with this agreement,
all production of ozone depleting substances is currently in the process of being phased
out.9 These substances include, but are not limited to, various halocarbons such as halons,
chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). Halon 1301 is still
commercially used, but its manufacture has been terminated so the usage is now mainly
limited to the properly recycled substance.
Because of its limited availability and impacts of its use, replacements for CF3 Br are needed
for many of its former applications. The iodinated analogue, CF3 I has been suggested as a
viable, more environmentally friendly alternative.10 Even though the iodine radical is even
more effective at destroying ozone than bromine, CF3 I has an ODP in the range of 0.070.25, which is only a fraction of the ODP of CF3 Br. This is because the carbon-iodine bond
is weaker and can easily be cleaved by sunlight radiation in the troposphere, which results
in a much shorter atmospheric lifetime of the molecule. Dissociative electron attachment
to CF3 I (see for example11, 12 ) as well as CF3 Cl (see for example13 ) has been studied fairly
4
extensively with crossed beam instruments. The literature on CF3 I has been thoroughly
reviewed by Christopherou et al.14 More recently, these two chemicals have been studied
further using velocity slice imaging.15, 16
Studies in the field of electron attachment to CF3 Br are scarce. They were, however, started
in 1963 by T.G. Lee,17 in an attempt to better understand the compound’s flame inhibiting
potential. There, the electron attachment coefficient of CF3 Br was measured using electron swarming spectroscopy. The dissociative attachment cross section of CF3 Br along
with several other halogenated compounds was first measured in 1968 by Blaunstein and
Christophorou18 in the energy range of 0 to 0.7 eV. In 1994, Underwood-Lemons et al.19
measured the total electron scattering cross section from 0 to 12 eV for many halofluoromethanes (CFn X4-n ), including CF3 Br. A year later, the same group20 studied the effect
of temperature on the total DEA cross section of some halofluoromethanes, again including
CF3 Br.
The most extensive DEA study on CF3 Br to date was conducted in 2006 by Marienfeld et
al.21 They used photoelectron attachment to study the dissociation below 2 eV with target
gas temperature fixed at 300K. In this energy range, results are constrained only to the process leading to the formation of the bromine anion, a dominating reaction in DEA to CF3 Br,
observed close to 0 eV. This process has been studied substantially using various methods,
as is summarized by Marienfeld.21 Additionally, the absolute dissociative electron attachment coefficient was studied by Marienfeld et al. in the same energy range with target gas
temperatures ranging from 173K to 600K.
In this study the fragment formation from CF3 Br as it interacts with low energy electrons is
explored. Negative ion yield curves through dissociative electron attachment, in an energy
range of about 0 to 10 eV are studied and discussed. Furthermore, the dissociation behavior of CF3 Br is compared to that of its congeners, CF3 Cl and CF3 I as well as calculated
thermochemical threshold energies. The structure of this thesis is as follows. In section 2
a theoretical overview of the electron attachment process is provided followed by section 3
wherein the apparatus and the computational calculations are described. In section 4, the
results of measurements are presented. The fragmentation channels are discussed in the context of computed thermochemical thresholds and the behavior of similar substances. Finally,
in section 5, a short overview is provided to summarize the results.
5
2
2.1
Theory
The electron attachment process
When molecules in the gas phase interact with low energy electrons, they may be capable
of capturing an incident electron, which leads to the formation of a temporary negative ion
(TNI), also called a resonance. This is usually observed below the ionization potential of the
respective molecule. Figure 2 shows a two dimensional Born-Oppenheimer potential energy
diagram representing electron attachment to a diatomic molecule MX.
Figure 2: Two dimensional Born-Oppenheimer potential energy curves describing the process of
electron attachment to a diatomic molecule (MX). More information regarding individual processes
c Benedikt Ómarsson
are in the text. Figure originally from.22 As shown in figure 2, the attachment of an electron above 0 eV can be considered a vertical
transition within the Franck-Condon region (shaded with diagonal lines in figure 2), that
describes the transition of a molecule from its neutral ground state to a given anionic state.
In the attachment process, the electron brings its kinetic energy into the system.
6
The transient negative ions generally form in an excited state and are therefore bound to
relax. They do so, mainly through two competing pathways, autodetachment (AD) and the
main focus of this thesis; dissociative electron attachment (DEA). Equation 1 shows how the
two relaxation processes differ for a TNI, CX3 Y–* .
CX3 Y + e − (ε 0) −−→ CX3 Y−∗
AD
−−→
CX3 Y + e − (ε ≤ ε 0)
DEA
−−→ CX3 + Y− .
(1)
The most common means of relaxation for the resonances, is simply to reemit the trapped
electron through AD. By releasing the electron, the molecule relaxes down to its neutral
parent state as is shown by vertical arrows in figure 2. In the case where the electron is
emitted with less energy than it initially brought in, the molecule relaxes, not to the ground
state of the neutral, but to various vibrationally excited states (ν). This process is called
vibrationally inelastic scattering. The electron can also be emitted with energy equal to its
initial energy, in which case the molecule would decay into its original the vibrational state.
This is called vibrationally elastic scattering.
2.2
Dissociative electron attachment
The process of a resonance relaxing through bond rupture and thus the formation of smaller
fragments, is termed dissociative electron attachment. In DEA, the transient negative ion is
generally formed in a repulsive state or above a given dissociative asymptote. As the bond
length increases, the resonance gets closer to a crossing point of the repulsive curve with the
neutral ground state potential. The intramolecular distance at that point is called the critical
bond length (rc ). The relaxation of the molecule is a competition between DEA and AD.
The results of this competition depend on the survival probability, P(surv), which describes
the relationship between the autodetachment lifetime, τAD and the time, t D , required for the
system to regress past rc :
P(survival) = e−t
D /τ
AD
.
(2)
If the resonance lives long enough to exceed the critical bond length , the fragmentation of
the molecule is almost inevitable.
7
The resonance width, Γ, is proportional to the resonance’s autodetachment lifetime, τAD , in
accordance to Heisenberg’s uncertainty law.
Γ=
h̄
τAD
.
(3)
In electron attachment, the actual width is determined by the overlap of the ground vibrational state with the respective anionic state, i.e. Franck-Condon factors, resulting in an
attachment profile, σ EA , shown schematically in figure 2. In dissociative electron attachment, on the other hand, the energy dependence of the ion yield reflects a convolution of
σ EA and P(surv):
σ DEA = σ EA · P(surv),
2.3
(4)
Formation paths of resonances
In general, transient negative ions can be categorized into two groups, single particle resonances and two particle, one hole resonances, also termed core excited resonances. Figure 3
shows schematically the energy of two types of core excited resonances and a single particle
shape resonance, both in relation to each other as well as their corresponding neutral parent
state.
Figure 3: A representation of the relative energy levels at which different resonances form
compared to their parent state. The core-excited resonances form through having a core electron
excited by the incoming incident electron, both of whom occupy previously unoccupied,
c Frímann Ómarsson
energetically higher orbitals. Figure taken from23 8
2.3.1
Core-excited resonances
Core-excited resonances are formed when a captured electron concomitantly excites a core
electron from within the molecule. In the TNI, both electrons are housed by previously
unoccupied orbitals. This means that if the incident electron were removed it would leave
the neutral parent in an electronically excited state.
There are two distinct types of core-excited resonances that differ mainly in their energy
relative to that of their neutral state and thus, in their trapping mechanism. Core excited
shape resonances forms energetically above the parent state and can therefore relax, by an
open pathway through AD, to its parent state. Because of this, the lifetime of core-excited
shape resonances is usually short and peaks therefore broad, as explained by equation 3.
On the other hand, core-excited Feshbach resonances form just below their neutral parent
state. This makes it much more difficult for the resonance to relax through autodetachment,
although it can be achieved if the electron manages to absorb sufficient energy from the
system, or alternatively, from electron reconfiguration. This, however, occurs on a relatively
long time scale so it becomes favorable for the molecule to relax through other means, such as
through dissociation. The longer lifetime of these resonances results in sharper peaks.
2.3.2
Single particle shape resonances
Single particle shape resonances, just as their core-excited counterparts, form energetically
above their neutral parent state. However, single particle resonances form by their ground
parent state. The excitation of the molecule is limited to the trapping of an incident electron
into a previously unoccupied orbital with symmetry such that it coheres with the electrons
angular momentum, l.
The trapping mechanism of shape resonances is often described with a relatively simple
model, where the total effective interaction potential between the incoming electron and the
molecule is given by:
Veff (r) = Vl +Vα =
l(l + l)
α
− 4
2
2r
2r
(5)
where l is the angular momentum quantum number of the incident electron, α is a polarizability factor of the molecule and r is the distance between the electron and the molecule.
9
As the electron gets increasingly close to the molecule, the system experiences a highly repulsive potential due to the presence of valence electrons, as explained by Pauli’s exclusion
principle.
Figure 4: A semi-classical figurative representation of the total effective molecule-electron
interaction potential. It shows a description of the centrifugal barrier at different angular momentum
quantum numbers for the electron. If the incident electron has energy ε, it cannot be trapped if the
angular momentum quantum number of the electron is l < 2. If l = 2, ε represents the energy of a
hypothetical shape resonance when an electron is captured within the potential. The figure was
adapted from Bald et al.24
For all angular momentum quantum numbers larger than, l = 0, a repulsive centrifugal potential barrier is formed as is shown in figure 4. Electrons can tunnel through the barrier
and be temporarily trapped there within. If l = 0, the term, Vl , is zero and thus, the barrier
does not form. In this case, an electron will not be trapped and a shape resonance cannot be
formed. The latter part of equation 5, describes an attractive polarization potential that results
from the incoming electron inducing a dipole within the molecule, which further attracts the
electron.
10
3
Methods
3.1
Experimental
All measurements of negative ion formation from CF3 Br reported in this thesis were conducted on SIGMA (Simply a Gas-phase MAchine). SIGMA is a high vacuum crossed-beam
electron attachment apparatus with a base pressure in the order of 10−8 mbar. The instrument
was recently set up at the University of Iceland as a part of Elías H. Bjarnason’s PhD. work
and its means of operation have been described in detail.25 Figure 5 shows a schematic of
the apparatus.
Figure 5: SIGMA, the electron attachment apparatus used for measurements of negative ion
c
formation from CF3 Br. Figure was taken from,22 Benedikt
Ómarsson
SIGMA consists of a Trochoidal Electron Monochromator (TEM) that generates a fairly
monochromatic beam of electrons. This beam interacts with an effusive gas beam from
which the generated ions are detected and analyzed by HIDEN EPIC 1000 quadrupole mass
spectrometer.
Electrons generated from a tungsten filament are collimated by a magnetic field controlled
by two large coils outside the apparatus. The electrons are guided into a region of crossed
electric and magnetic fields that drive the electrons in the direction orthogonal to both fields.
The exit slit of this region is not completely collinear to the entrance slit, which allows for
11
the selection of electrons with a certain velocity in that direction, forming an electron beam
with the corresponding energy.
When the sample gas is introduced, the electron beam crosses an effusive molecular beam.
The interaction between the two beams in the reaction region results in electron attachment
reactions, forming ions. These ions are then guided by a weak electric field toward the
quadrupole mass spectrometer, where they are analyzed. The instrument can in principle be
operated in two different modes, (i) scanning over a mass range at fixed electron energy and
(ii) over the electron energy at a fixed mass.
Bromotrifluoromethane was introduced into the vacuum chamber through an effusive gas
inlet system. The working pressure was kept steady at 1.2 · 10−6 mbar during measurements
to establish single collision conditions. The temperature of the inlet system was maintained
at 60◦ C. The monochromator was kept at a constant temperature of 120◦ C to avoid surface
absorption. The target gas was assumed to be at thermal equilibrium with the inlet system,
thus at 60◦ C.
The electron energy scale was calibrated with respect to SF–6 formation from SF6 , observed
in electron attachment at 0 eV. The full width at half maximum (FWHM) of the SF–6 signal was used to estimate the electron energy resolution as the natural width of SF–6 has
been determined to be less than 1 meV.26 The FWHM measured was in the range of 90 −
100 meV.
3.2
Calculations
The thermochemical threshold for a dissociative electron attachment reaction is the absolute
minimum energy at which the fragmentation of the molecule can take place. This means that
for a given DEA reaction, the onset of the ion yield curve for the respective fragment cannot
be lower than the thermochemical threshold. For the dissociation of a diatomic molecule,
MX:
MX− −−→ M + X− ,
the threshold can be viewed as the difference between the electron affinity (EA) of the neutral
12
precursor of the negatively charged fragment and the bond dissociation energy (BDE) of the
bond that is broken in the reaction:
Eth (X− ) = BDE(M−X) − EA(X).
(6)
In reactions involving electron attachment to many polyatomic molecules, however, DEA
channels are observed where the reaction is not limited to a single bond rupture. In such
cases, multiple bonds can be broken and new bonds can even form. The threshold for these
reactions is the sum of the BDE of all bonds broken, less that of all bonds formed over the
course of the reaction as well as the EA of the neutral precursor of the negatively charged
fragment. For the fragmentation and formation of the negative ion X– , where N bonds are
broken and M bonds are formed, the thermochemical threshold is:
−
N
M
Eth (X ) = ∑ BDE(educt) − ∑ BDE(product) − EA(X).
(7)
Of course, this only gives the absolute minimum energy required for the respective reaction.
The peaks in the ion yield spectra are more often than not observed well above their thermochemical threshold. In many cases, this is simply due to the resonant nature of the electron
attachment process, where the additional energy can manifest in translational, vibrational,
rotational or electronic form. Furthermore, the threshold represents the enthalpy of the reaction, but as many chemical reactions are associated with considerable reaction barriers, the
optimal path could be inaccessible by the system.
Single point energy (SPE) was computed for all negative ion fragments along with their neutral counterparts formed through the dissociation, as well as for the neutral parent molecule.
Vibrational frequency calculations were performed with the optimal geometry of each species
for the following reasons. The vibrational modes show whether the geometry actually represent an energetical minimum. The calculations also yield the zero temperature vibrational
energy (ZPE) as well as the thermal corrections needed.
A thermal correction has to be accounted for, as the fact is that the parent molecule is not
close to absolute zero and neither are the fragments when formed. The parent molecule is
assumed to be at 60◦ C, as the target gas is at thermochemical equilibrium with the inlet
system, which is maintained at that temperature. However, little is known about the temperature of various fragments at their formation, so they are assumed to form at absolute zero
13
temperature to yield the lowest threshold possible.
The thermally corrected threshold can now be expressed as the difference between the sum
of the total energy of the two fragments formed in the reaction process and the total energy
of the parent molecule:
N SPE
ZPE
0
+ Eparent
+ Hcorr ,
Eth
= ∑ EiSPE + EiZPE − Eparent
(8)
i=1
where Hcorr is the parent molecule’s thermal correction at 60◦ C, which is calculated as the
sum of the translational, rotational, vibrational and electronic energy corrections along with
a thermal enthalpy correction, which is the product of the temperature and Boltzmann’s
constant, kB :
Hcorr = Etrans + Erot + Evib + Eel + kB T.
(9)
The geometry optimization and harmonic vibrational frequency calculations were conducted
using the hybrid functional, B3LYP and a large minimally augmented Karlsruhe basis set maTZVP.27, 28 The single point energy was calculated using the same level of theory in addition
to a double hybrid generalized gradient functional approximation B2PLYP29 as well as a
newer, double hybrid meta-generalized gradient density functional PWPB95.30 Calculated
thresholds using all three levels of theory are reported in this thesis, but the values from the
B2PLYP level of theory will be used when comparing energy values to the ion yield signal
onset energies.
All computations were performed using ma-TZVP.27, 28 The basis set has augmented diffusion functions suitable for calculations with thermochemically stable anions. Computations
were performed using the quantum chemistry program, ORCA 3.0.31
14
4
4.1
Results and Discussion
Mass Spectroscopy
Figure 6 shows a positive ion mass spectrum for CF3 Br from mass per charge (m/z) of about
0 to 160 recorded at ∼70 eV. Bromine has two, almost equally abundant, stable isotopes, 79Br
(50.69%) and 81Br (49.31%). As a result, all bromine containing fragments are immediately
recognizable on a mass spectrum by the isotope ratio.
Figure 6: Positive ion mass spectrum of CF3 Br from m/z 0 to 160, measured at 70 eV. Energy this
high is considerably above the ionization energy and electron impact causes fragmentation of the
molecule. This is well suited to test the purity of the compound and its presence in the chamber.
15
The atomic mass of CF3 Br is either 148
g
mol
and 150
g
mol
depending on which bromine
isotope the molecule has. The twin peaks representing the cationic state of the parent can
be seen around m/z ratio of 150 on the spectrum. The peak with the highest intensity is at
m/z 69, which corresponds to CF+3 , i.e. the rupture of the C−Br bond. The intensity of this
signal indicates the relative weakness of that bond compared to a C−F bond. The measured
spectra matches well to that reported in NIST.32 No impurities are detected except for the
residual gas evident through oxygen and nitrogen peaks. It should be noted that the nitrogen
to oxygen ratio is very close to that of the atmosphere, indicating that these contributions are
from the inlet system, rather than the chamber itself.
4.2
Negative ion yield
To record the negative ion yield, the quadrupole mass spectrometer was set at a fixed mass
per charge ratio and the electron energy was scanned over a specific range. Negative ion yield
scans were measured at various fixed m/z ratios and four separate negatively charged fragments were detected. To ensure maximum transmission through the quadrupole, the mass
resolution was reduced considerably. A negative ion mass scan was measured around the
mass of bromine and a maximum of the peaks was observed at a m/z 79.5. A corresponding
mass per charge value was used when examining negative ion yield curves for all bromine
containing fragments.
Figure 7 shows the negative ion yield curves for the formation of (a) Br– , (b) F– , (c) FBr–
and (d) CF2 Br– in DEA to CF3 Br measured from about 0 to 10 eV. Contributions are formed
through resonances close to 0 eV, 4 eV and 8 eV. Table 1 shows the computationally calculated thermochemical thresholds, calculated through three different levels of theory.
Table 1: Calculated values for thermochemical threshold energies,
0 ) and not (E )
both thermally corrected (Eth
th
Fragment
Br–
F–
FBr–
CF2 Br–
B3LYP
−0.63
1.49
1.28
2.61
Eth [eV]
B2PLYP PWPB95
−0.46
−0.32
1.67
1.87
1.64
1.83
2.84
2.96
16
B3LYP
−0.81
1.32
1.10
2.43
E0th [eV]
B2PLYP PWPB95
−0.64
−0.50
1.49
1.69
1.47
1.65
2.67
2.79
Figure 7: Negative ion yield curves for fragments formed through dissociative electron attachment
to CF3 Br. The spectra were recorded in the energy range from about 0 to 10 eV
The signals observed for the four fragments in the negative ion yield spectra are examined
individually and compared to their computed thresholds as well as previous measurements
of related compounds.
4.2.1
Formation of Br–
Figure 7a shows the negative ion yield for the formation of Br– . A negative bromine ion can
form through three separate resonances, with peaks observed in the negative ion yield curve
close to 0 eV, 4.3 eV and 8 eV.
The most intense signal observed was in the negative ion yield for the bromine anion close to
0 eV. It has the count rate by three orders of magnitude higher than any other peak. Bromine
has a similar electron affinity (3.36 eV) to fluorine (3.40 eV), so any difference in dissociative
electron attachment behavior would be as a result of the strength of the bond between the
halogen and carbon. At 0K, the bond dissociation energy of the C−Br bond in CF3 Br is
reported to be 2.92 eV, and that of the C−F bond is 5.05 eV.33 As the C−Br bond is the
weaker one in the molecule, it is the easiest bond to cleave to release the bromine anion.
17
The dissociation happens through the following simple DEA process, that has been studied
extensively with various methods, as summarized by Marienfeld et al.21
e− + CF3 Br −−→ CF3 Br− −−→ CF3 + Br−
In 1984, Alge et al.34 measured the energy release of the attachment reaction to be 0.41 eV,
which corresponds well to the observed peak position as well as the calculated threshold.
The threshold energy value is negative for the Br– fragment, so the reaction is predicted to
be thermochemically possible at 0 eV.
In comparison, contribution from Cl– in DEA to the chlorinated analogue, CF3 Cl, results
in two peaks in the negative ion yield curve, at 1.3 eV and around 5 eV.16, 35 The bond
dissociation energy of the carbon-chlorine bond is reported to be 3.77 eV,33 which would
make the threshold of the dissociation and formation of a chlorine anion slightly positive as
opposed to the negative threshold for bromine.
Anion formation of I– from CF3 I has been observed close to 0 eV similar to DEA to CF3 Br.12
There are, however, no signs of a contribution from a resonance forming I– around 4-5 eV.
Furthermore, the bromine fragment also shows a smaller contribution from a resonance at
8 eV, which neither of its compatriots do.
4.2.2
F–
Figure 7b shows the ion yield for the formation of F– . The negative fluorine fragment yield
curve shows a peak at 4 eV and a smaller resonant contribution at 8 eV, but it also has a small
peak close to 0 eV.
The thermochemical threshold for this reaction was calculated to be 1.49 eV. This means that
it would be impossible for F– to form through a resonance at 0 eV. However, at that energy
there is an abundance of residual CF3 neutral fragments as a result of the dissociation of Br– .
It is possible for that fragment to undergo further electron induced dissociation through the
following process.
e− + CF3 −−→ CF2 + F−
The thermochemical threshold for this process was calculated to be 0.139 eV at 60◦ C with
B2PLYP, meaning that it could happen close to 0 eV. If extra energy is needed to overcome
18
the barrier, it may be attributed in part to the excess energy released in the earlier dissociation.
It is conceivable that a small portion of CF3 fragments are still within reach of the electron
beam for further interaction after its formation. There is also the possibility that the CF3
fragment was formed from contact with the filament and eventually drifts back to the reaction
region where further dissociation occurs. It is however noted that the intensity is very low
and we thus attribute the contribution observed close to 0 eV to be the result of an artifact,
not resulting from DEA of CF3 Br.
The largest peak in the ion yield of F– is at ∼ 4 eV. A resonance has been detected both from
CF3 Cl16 and CF3 I.12 at a similar energy. A small contribution from a resonance at around
8 eV can also be seen.
4.2.3
FBr–
Figure 7c show the negative ion yield for the formation of FBr– . Only a single resonant
contribution was detected in the ion yield and the peak is around 4 eV. It clearly results
from the same resonance detected for previous fragments. The threshold was calculated to
be 1.49 eV so this process is thermochemically possible. A resonance has been observed
for the formation of FI– from CF3 I at around 4 eV12 as well as the formation of FCl– from
CF3 Cl.35 A small peak at around 0 eV can also be seen, but measured at a count rate of less
than 0.5 s−1 , it is the peak lowest in intensity to form close to 0 eV, compared to the negative
ion yield curves of each of the other fragments observed. The signal could be a result of
formation through similar mechanisms as the 0 eV contribution observed for F– .
4.2.4
CF2 Br–
Figure 7d shows the negative ion yield curve for CF2 Br– . A low intensity contribution is
observed close to 4 eV and a more intense contribution is observed around 0 eV. There are
two signals noticeable in the ion yield curve, close to 4 eV and a more intense contribution
around 0 eV, but only one of them can be attributed to a contribution from a resonance that
leads to the formation of the CF2 Br– anion.
19
The contribution close to 0 eV in the negative ion yield curve, can be attributed to the formation of SF–5 from residual SF6 gas still in the vacuum chamber. The negative ion yield was
measured at a fixed mass per charge ratio of 129.5. The preponderant isotope of sulfur is
32S,
but 4.21% of the isotopes are 34S. The latter makes for SF–5 ions with a m/z 129, which
overlaps the fixed m/z ratio of the measurements at the resolution used.
SF6 has a huge electron capture cross section near 0 eV, that is the same for all of its initial
vibrational states.36 This results in peaks close to 0 eV for at least three separate anion
yields, SF–6 , SF–5 and F– , depending on temperature. The DEA cross section near zero for
SF–5 is substantial and increases greatly with rising target gas temperatures. The dissociation
limit is only 0.1 eV above the vibrational ground state (ν = 0) of the neutral SF6 .36 From
Boltzmann distribution, about 3% of all SF6 have an internal energy equal to or larger than
0.1 eV at 60◦ C. This means that only about 0.1% of all SF6 is likely to dissociate to form a
SF–5 anion containing a
34S
isotope at 0 eV. A further dissociative attachment peak for SF–5
has been observed at 0.38 eV, whose intensity is insensitive to the target gas temperature.37
It is therefore conceivable that the peak observed in the negative ion yield for CF2 Br– is a
blend of these two peaks from SF–5 . It should also be noted that the dissociation limit for
F– is about 0.25 eV above the vibrational ground state of the neutral SF6 and therefore it
cannot be excluded that a portion of the 0 eV contribution observed in the negative ion yield
for F– stems from this process as well. This is however unlikely, as the peak does not form
substantially, until around 600 K.37
The resonance at 4 eV can be seen clearly although the intensity is very low. The corresponding fragment from DEA to CF3 I, has not been detected. The dissociation channel has,
however, been observed with low relative intensity from DEA to CF3 Cl.
20
4.3
4.3.1
Other possible fragments
Parent ion
The parent ion does form temporarily, otherwise there would be no fragments forming from
dissociative electron attachment. Its observance is therefore not a matter of if it forms, but
rather if its lifetime is long enough to survive through the mass spectrometer and be detected
there. When conducting negative ion yield scans over various masses, a distinct peak started
to form for CF3 Br– close to 0 eV. This was though, considered to be an unlikely source of this
peak. The scan was measured at m/z of 148.5. At the resolution measured, this corresponds
well to the mass per charge of SF–6 , with the
34S
isotope, which is 148. A 0 eV negative ion
mass scan was performed from m/z of 142 to 156, and as was predicted, the bromine isotope
pattern was not detected. A big peak was observed at m/z of 146 and a much smaller one at
m/z of 148, which fits the isotope distribution of sulfur. This meant that the peak measured
close to 0 eV did not belong to the parent ion, but rather, to SF–6 from residual SF6 gas.
4.3.2
CF–3
The bond that is easiest to break in the molecule is between the carbon and the bromine.
The positive ion mass scan shows a huge peak for CF+3 and there was a clear peak for the
Br– fragment in the negative ion yield scan at around 0 eV. The assumption would be that
at least some of the negative charge would set on the CF3 fragment rather than the Br when
the molecule breaks up in that fashion. A scan was taken of the CF3 fragment and no peak
seemed to be starting to form. It could be happening very minimally. A peak was observed in
the negative ion yield for CF–3 , forming through DEA to CF3 Cl around 4 eV, but its intensity
was a twentieth of that of the CF2 Cl– peak.35 If the peaks held the same ratio for CF3 Br, the
peak would be measurable at an intensity of about 0.01 counts per second. That is too low to
detect over noise. The signal has been observed for the fragment acquired from dissociation
of CF3 I, also at a relatively low intensity.12
21
4.3.3
F–2
A quick scan looked like it was showing a peak starting to form around 4 eV. It was perhaps
possible for this fragment to form in the same way as the FBr– . As the bromine breaks away
from the molecule very easily, it was very unlikely that two fluorine atoms would break
away from the molecule without a bromine. A longer scan resulted in no peaks forming.
A negative ion mass scan was measured around m/z = 35 − 42 and displayed no signs of
formation of F–2 at all.
5
Summary
Dissociative electron attachment to bromotrifluoromethane was studied in a crossed molecular and electron beam apparatus. A total of four negatively charged fragments were detected,
formed through three distinct resonances, close to 0 eV, 4 eV and 8 eV.
The dominating contribution was observed close to 0 eV. The contribution exclusively yields
Br– and is the most intense peak by three orders of magnitude in counts per second measured in the negative ion yield curve. Much less intense peaks can observed at around 0 eV
in the negative ion yields for all three of the other fragments. The thermochemical calculations show that these fragments cannot form through a resonance close to 0 eV in DEA to
CF3 Br. These signals have therefore been attributed to other causes than the formation of
the fragments through DEA to CF3 Br.
The situation for the second resonance may be more complicated. All four fragments observed show a peak in the anion yield curve close to 4 eV and can thus form through this
resonance. Table 2 shows their estimated peak positions and intensity.
Table 2: Relative intensity for peaks resulting from contribution from the resonance around 4 eV
Fragment
Br–
F–
FBr–
CF2 Br–
Peak position [eV]
4.4 ± 0.1
3.9 ± 0.1
4.0 ± 0.1
3.8 ± 0.1
Intensity (s−1 )
17.0 ± 0.5
26.0 ± 0.5
2.5 ± 0.5
0.25 ± 0.05
22
Relative intensity
65
100
10
1
A resonance detected in studies for
CF3 Cl35 and CF3 I12 at a similar
energy was concluded to be a coreexcited resonance, although there
was not enough information to determine which kind. However, a
UV spectra of CF3 Br shown in figure 8, only shows an electronic
excitation, only starting to form
around 5 eV.
It is highly unlikely that an electronic excitation contributes to a
resonance at a much lower energy
Figure 8: UV spectra of two trifluorohalomethanes, CF3 Br
and CF3 I. Figure created with figures from38
value, where the negative ion yield signal onset is. Comparatively, CF3 I shows an electronic
excitation peak at a lower energy value that may well coincide with an electronic excitation
leading to the formation of a core-excited resonance in that energy range.
It is, however, possible that this excitation, observed for CF3 Br, contributes to resonance that
results in a signal in the negative ion yield curve at 8 eV as the decline of the UV signal
overlaps the onset energy of the signal in the negative ion yield. The relatively high energy
also indicates that this is a core-excited resonance. This resonance is low in intensity and can
lead to the formation of bromine and fluorine anions.
23
References
[1] ECPlaza.
Fighting
Halon 1301 firefighting system,
Equipment
Co.,
Guangdong Ping’An Fire-
http://chinapingan.en.ecplaza.net/
Ltd.
halon-1301-firefighting-system--88317-242967.html. Accessed: 2014-0419.
[2] H3 R Clean Agents, Basic facts about Halon. http://www.h3rcleanagents.com/
support_faq_2.htm. Accessed: 2014-05-10.
[3] J. K. Klein. The F-16 Halon Tank Inerting System. Technical report, American Institute of Aeronautics and Astronautics, Aeronautical Systems Division, Wright-Patterson
AFB, Ohio, 1981.
[4] I.P.C.C.
Direct Global Warming Potentials, International Panel on Cli-
mate Change.
http://www.ipcc.ch/publications_and_data/ar4/wg1/en/
ch2s2-10-2.html. Accessed: 2014-04-21.
[5] G. Myhre, D. Shindell, F. Breon, W. Collins, J. Fuglestvedt, J. Huang, D. Koch,
J. Lamarque, D. Lee, B. Mendoza, T. Nakajima, A. Robock, G. Stephens, T. Takemura and H. Zhang. Anthropogenic and Natural Radiative Forcing. In: Climate
Change 2013: The Physical Science Basis. Contribution of Working Group I to the
Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Technical
report, Intergovernmental Panel on Climate Change[Stocker, T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley
(eds.)].Cambridge University Press, Cambridge, United Kingdom and New York, NY,
USA., 2013.
[6] E.P.A. Class I Ozone-depleting Substances, U.S. Environmental Protection Agency.
http://www.epa.gov/Ozone/science/ods/classone.html. Accessed: 2014-0415.
[7] U.N.E.P. Handbook for the Montreal Protocol on Substances that Deplete the Ozone
Layer. Secretariat for The Vienna Convention for the Protection of the Ozone Layer &
The Montreal Protocol on Substances that Deplete the Ozone Layer, 9th edition, 2012.
[8] U.N.E.P.
The Montreal Protocol on Substances that Deplete the Ozone Layer,
United Nations Environment Programme. http://ozone.unep.org/new_site/en/
24
montreal_protocol.php. Accessed: 2014-04-21.
[9] E.P.A. Clean Air Act, U.S. Environmental Protection Agency. http://www.epa.gov/
air/caa/challenges.html. Accessed: 2014-04-18.
[10] Y. Li, K. Patten, D. Youn and D. Wuebbles. Potential impacts of CF3 I on ozone as
a replacement for CF3 Br in aircraft applications. Atmos Chem Phys, 6 (12), (2006),
4559.
[11] M. Heni and E. Illenberger. Dissociative electron attachment to CF3 I: an example of
a completely unbalanced excess energy distribution. Chem Phys Lett, 131 (4), (1986),
314.
[12] T. Oster, O. Ingólfsson, M. Meinke, T. Jaffke and E. Illenberger. Anion formation
from gaseous and condensed CF3 I on low energy electron impact. J Chem Phys, 99
(7), (1993), 5141.
[13] E. Illenberger. Energetics of Negative Ion Formation in Dissociative Electron Attachment to CCl4 , CFCl3 , CF2 Cl2 , and CF3 Cl. Berich Bunsen Gesell, 86 (3), (1982), 252.
[14] L. G. Christophorou and J. K. Olthoff. Electron interactions with CF3I. J Phys Chem
Ref Data, 29 (4), (2000), 553.
[15] F. Ómarsson, N. Mason, E. Krishnakumar and O. Ingólfsson. Product state selectivity in Dissociative Electron Attachment to CF3 I revealed through Velocity Slice
Imaging.
[16] F. Ómarsson, O. Ingólfsson, N. Mason and E. Krishnakumar. Dissociative electron
attachment to CF3 Cl. Eur Phys J D, 66 (2), (2012), 1.
[17] T. Lee. Electron attachment coefficients of some hydrocarbon flame inhibitors. J Phys
Chem, 67 (2), (1963), 360.
[18] R. Blaunstein and L. Christophorou. Electron attachment to halogenated aliphatic
hydrocarbons. J Chem Phys, 49 (4), (1968), 1526.
[19] T. Underwood-Lemons, D. C. Winkler, J. A. Tossell and J. H. Moore. Low-energy
electron scattering cross sections of halofluorocarbons. J Chem Phys, 100 (12), (1994),
9117.
[20] T. Underwood-Lemons, T. J. Gergel and J. H. Moore. Dissociative electron attachment cross sections for halofluoromethanes. J Chem Phys, 102 (1), (1995), 119.
25
[21] S. Marienfeld, T. Sunagawa, I. Fabrikant, M. Braun, M.-W. Ruf and H. Hotop. The
dependence of low-energy electron attachment to CF3 Br on electron and vibrational
energy. J Chem Phys, 124 (15), (2006), 154316.
[22] B. Ómarsson.
Promoting reaction channels in dissociative electron attachment
through bond formation and rearrangement. Ph.D. thesis, University of Iceland, 2013.
[23] F. Ómarsson. Symmetries, dynamics and energetics in dissociative electron attachment
to selected group IV halides - Velocity slice imaging and mass spectrometric study.
Ph.D. thesis, University of Iceland, 2014.
[24] I. Bald, J. Langer, P. Tegeder and O. Ingólfsson. From isolated molecules through
clusters and condensates to the building blocks of life. Int J Mass Spectrom, 277 (1),
(2008), 4.
[25] E. Bjarnason, F. Ómarsson, M. Hoshino, H. Tanaka, M. Brunger, P. Limão-Vieira
and O. Ingólfsson. Negative ion formation through dissociative electron attachment to
the group IV tetrafluorides: Carbon tetrafluoride, silicon tetrafluoride and germanium
tetrafluoride. Int J Mass Spectrom, 339, (2013), 45.
[26] D. Klar, M.-W. Ruf and H. Hotop. Attachment of electrons to molecules at submillielectronvolt resolution. Chem Phys Lett, 189 (4), (1992), 448.
[27] F. Weigend and R. Ahlrichs. Balanced basis sets of split valence, triple zeta valence
and quadruple zeta valence quality for H to Rn: design and assessment of accuracy.
Phys. Chem. Chem. Phys., 7 (18), (2005), 3297.
[28] J. Zheng, X. Xu and D. G. Truhlar. Minimally augmented Karlsruhe basis sets. Theor
Chem Acc, 128 (3), (2011), 295.
[29] S. Grimme. Semiempirical hybrid density functional with perturbative second-order
correlation. J Chem Phys, 124 (3), (2006), 034108.
[30] L. Goerigk and S. Grimme. Efficient and Accurate Double-Hybrid-Meta-GGA Density Functionals Evaluation with the Extended GMTKN30 Database for General Main
Group Thermochemistry, Kinetics, and Noncovalent Interactions. J Chem Theory
Comput, 7 (2), (2010), 291.
[31] F. Neese. The ORCA program system. Wiley Interdisciplinary Reviews: Computational
Molecular Science, 2 (1), (2012), 73.
26
[32] NIST.
NIST Mass Spectrometry Data Center, Bromotrifluoromethane.
//webbook.nist.gov/cgi/cbook.cgi?ID=C75638&Mask=200, 2011.
http:
Accessed:
2014-04-10.
[33] J. A. Dean. Lange’s Handbook of Chemistry. McGraw-Hill, Inc., 15 edition, 1999.
[34] N. A. Alge, E. and D. Smith. Rate coefficients for the attachment reactions of electrons
with c-C7 F14 , CH3 Br, CF3 Br, CH2 Br2 and CH3 I determined between 200 and 600K
using the FALP technique. J Phys B: At , Mol Opt Phys.
[35] F. Weik, E. Illenberger, K. Nagesha and L. Sanche. Dissociative electron attachment
to gas-and condensed-phase CF3 Cl: Anion desorption and trapping. J Phys Chem B,
102 (5), (1998), 824.
[36] D. Spence and G. Schulz. Temperature dependence of electron attachment at low
energies for polyatomic molecules. J Chem Phys, 58 (5), (2003), 1800.
[37] C. Chen and P. Chantry. Temperature Dependence of SF–6 ,SF–5 , and F– Production
from SF6 . In Bulletin of the American Physical Society, volume 15, p. 418. Amer Inst
Physics Circulation Fulfillment Div, 500 Sunnyside Blvd, Woodbury, NY 11797-2999,
1970.
[38] S. Sander, J. Abbatt, J. R. Barker, J. B. Burkholder, R. R. Friedl, D. M. Golden,
R. E. Huie, C. E. Kolb, M. J. Kurylo, G. K. Moortgat, V. L. Orkin and P. H. Wine.
Chemical Kinetics and Photochemical Data for Use in Atmospheric Studies, Evaluation
Number 17. http://jpldataeval.jpl.nasa.gov. Accessed: 2014-04-23.
27