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Integrity Service Excellence
Ion chemistry of metals in the
gas phase: From catalysis to
chemi-ionization
Boston College Fall Lecture Series
October 17, 2016
Shaun Ard
Research Physicist
Boston College ISR
Air Force Research Laboratory
Space Vehicles Directorate
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Acknowledgments
AFRL Plasma Chemistry Group
– Al Viggiano
– Nick Shuman
– Josh Melko (now UNF)
– Tom Miller
– Jenny Sanchez
– Justin Wiens
– Oscar Martinez
– John Williamson
The outside world
– Hua Guo (UNM)
– Jürgen Troe (Göttingen)
– Peter Armentrout (U. of Utah)
– Michael Heaven (Emory)
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1) Fundamental studies of metal ion kinetics
Experiment
T dependences as probe of PES
Ab initio Calculations
Calculation of stationary points Statistical Modeling
Energetics and mechanistic insight
2) Chemical releases in the ionosphere
- Analysis of samarium releases
- Efforts to guide future releases
Overview
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AFRL Plasma Chemistry Laboratory
Reentry Plasma Scenario
GPS Blackout
Comm Blackout
Sensor Blackout
Antenna
Reflected energy
Propagating wave
Plasma
ic
n
The Plasma Chemistry group studies the kinetics of charged species
Why does the AF care about ion chemistry?
The AF cares about communications
Communications greatly impacted by Total Electron Content (TEC)
TEC highly dependent on ion chemistry
The AF cares about plasma chemistry!
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D
E
F
ther
mos
pher
eex
osph
ere
mesosphere
troposphere
100
200
300
400
500
Altit
ude
(km
)
Cation density (cm-3) Anion/e- density (cm-3) T(K)
102 103 104 105
O+
H+
He+
102 103 104 105
e-
N+
O2+ NO+
N2+
MSP-
molecular-
102 103 104
Te
ions
neutrals
Ti=Tn
clusters
Neutral density (cm-3)
104 108 1012 1016
H
He
NO
N2O2
Ti=Tn=Te
N
OISS
Hubble Satellite measurements
Rocket-borne MS
Balloon-borne MS
a) b) c) d)
stratosphere
Charged Species in the Atmosphere
Ambient and Modified Atmospheric Ion Chemistry: From Top to Bottom Chem. Rev. 2015, 115, 4542.
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New Directions:
Metal Ion Chemistry
Ionospheric Chemistry • 2000 MT of meteoric
material deposited daily, much of it metallic.
• Metal chemistry can have an outsized effect on the ionosphere.
Fuel Production • 7.5 Billion dollars spent annually
on jet fuel. • AF goal of 50/50 blend of
alternative jet fuel. • Metal chemistry figures
prominently in solar and biofuel catalysts. Cluster Chemistry
• Clusters display many properties unique from atomic or bulk phases.
• Kinetics will help characterize and utilize unique characteristics.
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Reactant Gas Inlets
Venturi Inlet
Source Gas
Variable Temperature Selected Ion
Flow Tube VT- SIFT
Fe+
FeO+
FeO+
Fe+ FeO+
He
He
He
He
He
He
He He
He
He
He
He
He He
He
He
R He
He
R
R R
R
P+ P+ P+ He
He
Electron
Impact
Source
He
He
He He
He
He
He
He He
He
He
He
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Reaction Barriers
A + BC
A B+C
positive T dependence
Fra
ctio
n a
bo
ve
thre
sh
old
Temperature
e-Ea/kT
• Arrhenius behavior (Positive T dep)
• Can fit T dep to find barrier height)
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Arrhenius Behavior
Ar+
N2+
+ ClF
ICl
Not usually observed in SIFT
• Temperature range 100-700K
• Only reactions with Ea≤ 10 kJ/mol will react enough to be seen, k>1e-12 cm3s-1.
Ea 1 kJ/mol
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x x
?
Submerged Barrier
Rate
co
nsta
nt (s
-1)
Energy
At higher T, reverse reaction wins
out Negative T dependence
Loose Entrance Channel
(lots of available states)
Tight Transition State
(only specific states available)
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0
2
4
6
8
10
0 5 10 15 20
Oxidation Efficiency
[C2H
xO
] /
[FeO
+] 0
Reaction Cycles
163 K
600 K
163 K
600 K
+ CH4
+ C2H2
+ C2H4
+ C2H6
163 K
600 K
Mo
lecu
les o
f C
xH
yO
FeO+
163 K
600 K
Alcohol Product Efficiency
What Do We Learn?
• Temperature dependence of catalytic cycles towards improved effectiveness and efficiency.
• Rate data gives information on the rate limiting step, branching data give relative product info.
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“A physical system remains in its instantaneous eigenstate if a given perturbation is acting on it slowly enough and if there is a gap between the eigenvalue and the rest of the Hamiltonian's spectrum.”
Role of Spin in Kinetics
Adiabatic Principle
Born-Oppenheimer
Spin is Conserved in Complex-Forming Reactions
Or Is It?
𝜳𝐭𝐨𝐭𝐚𝐥 = 𝝍𝐞𝐥𝐞𝐜𝐭𝐫𝐨𝐧𝐢𝐜 x 𝝍𝐧𝐮𝐜𝐥𝐞𝐚𝐫
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Spin Forbidden Reactions
• Many metal ion reaction involve reactants and
products of differing ground spin states.
• Spin appears conserved in some cases resulting
in low reactivity, while in other cases it is not,
resulting highly efficient reactivity.
𝑷 = 𝟏 + 𝒆−
𝝅𝜺
𝟒 𝟏 − 𝒆−𝝅𝜺
𝟒 ,
𝜺 = 𝟖𝚿𝑯𝑺 𝑯 𝑺𝑶 𝚿𝑳𝑺
𝟐
𝒉 𝒈𝒂∗ 𝒗𝒂𝜶
• Theoretical treatments such as the
Landau-Zener formulism are not
currently tractable as calculations
remain challenging for metal
systems.
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Spin Catalyzed Reactions
• Formally spin-allowed, these
reactions have excited states
which allow a lower energy
path to products.
• Two-State Reactivity, as it
has been dubbed, has been
found to play a role in a wide
range of reactions, such as
C-H and C-C activation by
atomic metal ions
hydroxylation and
epoxidation by the protein
cytochrome P-450.
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Calculate ab
initio surface
CCSD(T)-
F12/AVTZ
Modeling Approach
Statistical modeling
Vary energies of
rate limiting TS
Exp. rate
constants
and
branching
Vibrational and
rotational frequencies
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• Model captures the T-dependence over a wide energy range including
the isotopic effects.
• Initial spin crossing effective: ie. takes place on a faster time scale than
the rate limiting quartet TS.
FeO+ + H2
Further Insight into the Reaction FeO+ + H2 → Fe+ + H2O: Temperature Dependent Kinetics, Isotope Effects, and Statistical Modeling J. Phys. Chem. A, 2014, 118 (34), pp 6789–6797 DOI: 10.1021/jp5055815
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Excited State Production
• While the reactant well crossing was effective,
the product well crosses with near zero
effectiveness producing excited state
products.
• Recent trajectory calculations by Jeremy
Harvey confirm these results.
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To Cross Or Not to Cross
Effective crossing
𝝉𝒄𝒓𝒐𝒔𝒔𝒊𝒏𝒈 < 𝝉𝒂𝒅𝒊𝒂𝒃𝒂𝒕𝒊𝒄 Ineffective crossing
𝝉𝒄𝒓𝒐𝒔𝒔𝒊𝒏𝒈 > 𝝉𝒂𝒅𝒊𝒂𝒃𝒂𝒕𝒊𝒄 Competitive crossing
𝝉𝒄𝒓𝒐𝒔𝒔𝒊𝒏𝒈~ 𝝉𝒂𝒅𝒊𝒂𝒃𝒂𝒕𝒊𝒄
6FeO+ + H2 (Ent well) 6FeO+ + CH4 (Ent well)
6FeO+ + CO (Ent well)
6Fe+ + CH3I (Ent & Prod well)
6Fe+ + CH3Cl (Ent & Prod well)
6Fe+ + CH3Br (Ent & Prod well)
6Fe+ + CH3OCH3
Spin Allowed
Spin Forbidden
4NiO+ + CH4 4Ti+ + N2O
6FeO+ + H2 (Prod well)
6FeO+ + CH4 (Prod well)
6Fe+ + N2O (Ent well)
4Ti+ + CO2
4Ti+ + CH3OH
Analysis of the Pressure and Temperature Dependence of the Complex-Forming Bimolecular Reaction CH3OCH3 +
Fe+. J. Phys. Chem. A (2016) 120(27): 5264-5273.
Spin-inversion and spin-selection in the reactions FeO+ + H-2 and Fe+ + N2O. Phys. Chem. Chem. Phys. (2015)
17(30): 19709-19717.
Reactions of Fe+ and FeO+ with C2H2, C2H4, and C2H6: Temperature-Dependent Kinetics J. Phys. Chem. A (2013)
117(40): 10178-10185.
Further Insight into the Reaction FeO+ + H-2 -> Fe+ + H2O: Temperature Dependent Kinetics, Isotope Effects, and
Statistical Modeling. J. Phys. Chem. A (2014) 118(34): 6789-6797.
Activation of Methane by FeO+: Determining Reaction Pathways through Temperature-Dependent Kinetics and
Statistical Modeling. J. Phys. Chem. A (2014) 118(11): 2029-2039.
Reactivity from excited state (FeO+)-Fe-4 + CO sampled through reaction of ground state (FeCO+)-Fe-4 + N2O.
J. Chem. Phys. (2016) 144(23).
Iron cation catalyzed reduction of N2O by CO: gas-phase temperature dependent kinetics . Phys. Chem. Chem.
Phys (2013) 15(27): 11257-11267.
Statistical modeling of the reactions Fe+ + N2O -> FeO+ + N-2 and FeO+ + CO -> Fe+ + CO2 . Phys. Chem. Chem.
Phys (2016) 17(30): 19700-19708.
4Ti+ + O2
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Conclusions
• Combined experimental, computational,
and statistical approach offers unique
insight into reaction dynamics • Derived energetics of transition states and intermediates
offer computational benchmarks.
• Gives mechanistic insight, such as the adiabatic or non-
adiabatic nature of spin crossings in TSR.
• TSR is Complex! • Data on a wider variety of systems is needed to build
upon the simple current models to a quantified
understanding.
• Detailed Calculations of crossing seams
and SOC are greatly needed!
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Active Ionospheric Modification
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Artificial Ionosphere Efforts
Photo-ionization • Low IP atomic species (Ba,Cs,Sr,Li,)
produce an ion cloud with clear optical signature
• Limited to daytime use
HAARP • High Frequency Active Auroral
Research Program • Employed high power high frequency
waves to produce enhanced densities from excited electron impact
• Requires large non-mobile facility
Chemi-Ionization • Also called associative ionization • M + O → MO+ + e- (M= most
lanthanides and a few transition metals)
• Poorly characterized chemistry
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Samarium Release Experiments
Discharge
Ports
Sm
Sm
Sm
Sm
Sm
- -
- -
-
- -
-
Sm
Sm
Sm
-
-
Sm SmO+
SmO+
SmO+
SmO+
SmO+ SmO+
SmO+
SmO+
Expelled
Metal
Vapor
Quickly Reacts
with Ambient
Oxygen
And
Spontaneously
Ionizes
To form dense
long-lived
plasma
Terrier-Improved
Orion Sounding
Rocket
Thermite
Release
Canisters Actual burst-disc
release canister
A few kg of metal, vaporized, ionized, and dispersed, predicted to
create “enhanced” ionosphere over areas up to 100 km
O O
O O
O O
O
O
O
O
O
O
O
O
O
O
O
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Overview of Field Experiments
• Two sounding rocket experiments: MOSC (Kwajalein, March 2013); AIC (White Sands, Feb 2015); Future launch (PRECISE) in planning stages
• Both campaigns successful! Ionization unambiguously observed, but only ~10% of predicted electron density
MOSC Kwajalein, 2013 AIC White Sands, 2015
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Sm+ and SmO+ chemistry
SmI2
2 kV
+ + +
+
+
Mass filter
Sm+
or
SmO+ Helium Buffer flow
Electrospray ionization
Mass filter Electron
multiplier
reacta
nt
Product
ions
Lo
g (
Co
un
ts/s
)
[reactant]
Slope = k
SmO2+ + e would be an electron sink
Measured ( T ≥ 300k):
• Sm+ + O2 → SmO+ + O k(300) = 2.8 x 10-10 cm3s-1 k T-.1
• Sm+ + N2O → SmO+ + N2 k(300) = 1.3 x 10-10 cm3s-1 k T-.4
• Sm+ + NO → No Rxn
• Sm+ + C2H4 → No Rxn
• Sm+ + CO2 → No Rxn
• Sm+ + SO2 → SmO+ + SO k(300) = 4 x 10-10 cm3s-1 k T-.6
• Sm+ + NO2 → SmO+ + NO k(300) = 6 x 10-10 cm3s-1 k T-.6
• SmO+ + O2 No Rxn
• SmO+ + N2O + He → SmO(N2O)+ + He kter = (1.2 x 10-28 cm6s-1)
• SmO+ + SO2 + He → SmO(SO2)+ + He kter = (1.2 x 10-27 cm6s-1)
• SmO+ + CO2 + He → SmO(CO2)+ + He kter = (1.1 x 10-28 cm6s-1)
• SmO+ + NO2 + He → SmO(NO2)+ + He kter = (2.1 x 10-27 cm6s-1)
Implies BDE(SmO+) > BDE(SO2) = 5.71 eV
Selected Ion Flow Tube Apparatus (AFRL)
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Sm + O
Sm+ + O
Sm IE 5.6437 eV
SmO
SmO+
SmO IE 5.5±0.1 eV (1976)
SmO BDE 5.83 eV±0.07 eV (1977)
SmO+ BDE
Ener
gy
Peter Amentrout
U. Utah 5.725 ± 0.07
eV
Mike Heaven Emory
5.7427 ± 0.0006 eV
Sm + O → SmO+ + e- -ΔHRXN= -0.08 ± 0.07 eV Old = -0.35 eV
Thermochemistry in Doubt
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SmO+ Dissociative Recombination
0.0 0.2 0.4 0.6 0.8
Electron energy (eV)
Pro
bab
ility
Electron Etrans distribution at 200 km (1000 K)
Lit. value
Corrected Value
Could SmO+ + e Sm + O be limiting electron density?
• Dissociative recombination is typically a very fast process (k > 10-7 cm3 s-1)
• Endothermic DR has never been studied
• Estimate of equilibrium constant suggests kDR ~103 x kchemi-ionization at 1000 K; resulting equilibrium correct order of magnitude to limit e-density to that observed.
• Can also explain observed red/blue separation in cloud
• Experiments on endothermic DR planned (Cryogenic Storage Ring in Heidelberg) within the year
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Other Release Candidates
Most of the rare earth metals have energetics analogous to Sm, but
even more favorable!
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
101
102
103
Va
po
r P
ressu
re (
To
rr)
200018001600140012001000800600
Temperature (K)
Sm
Ho
Nd
Pr
La
However, vapor pressure is anti-correlated with chemi-ionization
exothermicity
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Chemi-ionization Kinetics
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4
6
810
-11
2
4
6
810
-10
2
4
6
810
-9
k (
cm
3 s
-1)
500400300200
T (K)
Sm
La
Nd
Ho
Pr
Tb
Kinetics Data and Results
Chemi-ionization rate constants w/ O
• 5 of 6 systems studied proceed at or near the hard sphere collisional value, increasing as T1/2 (as does the collisional rate)
• Sm is an outlier • Is Sm reactivity state dependent?
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P Dependence
• Sm chemi-ionization shows a large P dependence. • Consistent with excited states being primarily
responsible for observed reactivity.
SmO
SmO+
r (Sm-O) En
ergy
7D
9P
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Alternative Release Mechanisms
Burst canister used in MOSC experiment
• Vaporizing pure metal requires a lot of energy
• Mass of thermite required ~2x mass of active material
• Cylinder must withstand high pressure; adds weight
• Ground tests indicate incomplete vaporization
Ln
R
R
R
Instead of pure metal, use a compound containing ligands weakly bound to a metal core
atom
500 K Ln + 3R
Decomposition to the bare metal can occur at ambient
thermosphere temperatures
• Reduces weight of inactive material • No high energy material to be transported • No pressure rating for the vessel required • Allows for metered release (important for cloud shaping to
optimize efficiency of active material) • But need to ensure carrier material does not inadvertently
destroy desired plasma • Compounds with similar properties are developed for
chemical vapor deposition established community exists that can tailor compounds to meet these requirements
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Conclusions
• Progress made towards improving modeling of chemical release experiments and towards identifying ideal candidates for future releases
• Samarium reactivity is unique among studied species; appears to be electronic state-dependent
• Further studies of kinetics and determination of emission spectra for candidate species will be beneficial
• Alternative oxidizers are unlikely to be practical
• Efficiency of the release mechanism may be significantly improved
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Questions?