modeling the influence of nuclear spin in the reaction of h 3 + with h 2
DESCRIPTION
Modeling the influence of nuclear spin in the reaction of H 3 + with H 2. Kyle N. Crabtree , Brian A. Tom, and Benjamin J. McCall University of Illinois. Overview. This talk : Modeling p-H 3 + fraction vs p-H 2 fraction to assess nuclear spin dependence of H 3 + + H 2. - PowerPoint PPT PresentationTRANSCRIPT
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Modeling the influence of nuclear spin in the reaction of H3
+ with H2
Modeling the influence of nuclear spin in the reaction of H3
+ with H2
Kyle N. Crabtree, Brian A. Tom, and Benjamin J. McCall
University of Illinois
Kyle N. Crabtree, Brian A. Tom, and Benjamin J. McCall
University of Illinois
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20 June 2011 66th OSU International Symposium on Molecular Spectroscopy
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OverviewOverview
This talk: Modeling p-H3
+ fraction vs p-H2 fraction to assess
nuclear spin dependence of H3
+ + H2
Next talk: Spectroscopic studies at 350 K and 135
K
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20 June 2011 66th OSU International Symposium on Molecular Spectroscopy
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MotivationMotivation• Hydrogen- most abundant
element in the Universe
• H3+ is readily formed from
H2
H2 + CR H2+ + CR¶ + e-
H2 + H2+ H3
+ + H
• Spectroscopic observations of H3
+ in astronomy have been used to measure:– Temperature– Density– Ionization rate– Object size– Ionosphere gas velocity
• Hydrogen- most abundant element in the Universe
• H3+ is readily formed from
H2
H2 + CR H2+ + CR¶ + e-
H2 + H2+ H3
+ + H
• Spectroscopic observations of H3
+ in astronomy have been used to measure:– Temperature– Density– Ionization rate– Object size– Ionosphere gas velocity
Astronomer’s periodic table
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20 June 2011 66th OSU International Symposium on Molecular Spectroscopy
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Nuclear spinNuclear spin
• Observed lines: R(1,0), R(1,1)u, and R(1,1)l
• Nonthermal H3+ in space
• To understand the ortho:para H3
+ ratio,the reaction
H3+ + H2 H2 + H3
+, which interconverts the nuclear spin configurations of H3
+ and H2, must be understood
para-H3+ ortho-H3
+
1/2 3/2
For more on astronomy, see TF09, 160 MA, 3:52 pm
For more on astronomy, see TF09, 160 MA, 3:52 pm
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20 June 2011 66th OSU International Symposium on Molecular Spectroscopy
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H3+ + H2 H2 + H3
+H3+ + H2 H2 + H3
+
“identity”
“hop”
H5+
“exchange”
3
1111
6
Branching fractions: Sid, Shop, Sexch
≡ Shop/Sexch (3/6 = 0.5?)Conservation of nuclear spinangular momentum
Strategy: ensure [H2]>>[H3+],
ortho:para H2 ratio constant
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20 June 2011 66th OSU International Symposium on Molecular Spectroscopy
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Mechanism-specific branching fractionsMechanism-specific branching fractions
id
Reactants o-H3+ p-H3
+
o-H3+ + o-H2 1 0
o-H3+ + p-H2 1 0
p-H3+ + o-H2 0 1
p-H3+ + p-H2 0 1
o-H3+ + o-H2 2/3 1/3
o-H3+ + p-H2 0 1
p-H3+ + o-H2 2/3 1/3
p-H3+ + p-H2 0 1
o-H3+ + o-H2 2/3 1/3
o-H3+ + p-H2 2/3 1/3
p-H3+ + o-H2 1/3 2/3
p-H3+ + p-H2 1/3 2/3
id
exch
hop
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Steady state p-H3+ fractionSteady state p-H3+ fraction
• Write rate equation for p-H3+, assume ONLY H3
+ + H2 rxn:
• Steady state; p3 ≡ [p-H3+]/[H3
+]; p2 ≡ [p-H2]/[H2]
• Divide through by [H3+][H2]:
• Solve; =kH/kE
• Write rate equation for p-H3+, assume ONLY H3
+ + H2 rxn:
• Steady state; p3 ≡ [p-H3+]/[H3
+]; p2 ≡ [p-H2]/[H2]
• Divide through by [H3+][H2]:
• Solve; =kH/kE
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“High temperature” model“High temperature” model
Spectroscopically measurable
Experimentally controllable
(n-H2,n-H3+)
(p2=0.25, p3=0.5)
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Three-body reactionsThree-body reactions
• High densities can lead to formation of larger H2n+1
+ clusters
• At lower pressures, can (H5
+)* clusters undergo proton scrambling with H2?
• High densities can lead to formation of larger H2n+1
+ clusters
• At lower pressures, can (H5
+)* clusters undergo proton scrambling with H2?
H3+ + H2 (H5
+)* H3+ + H2
H5+ + H2
+ H2
H3+ + 2H2 (H7
+)*
?
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20 June 2011 66th OSU International Symposium on Molecular Spectroscopy
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H5+ + H2 H2 + H5
+H5+ + H2 H2 + H5
+
“identity”
1111
“hop”
10
“exchange”
10
H7+
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“3-body high temperature model”“3-body high temperature model”
• 2 = 2-body branching fraction
• 2, 3 = 2, 3-body hop:exchange ratio
• 2 = 2-body branching fraction
• 2, 3 = 2, 3-body hop:exchange ratio
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Low temperature modelLow temperature model
• High temperature models based on assumption that nuclear spin statistics exclusively determine reaction outcomes
• At low temperatures, some reaction channels are inhibited due to energetic considerations
• High temperature models based on assumption that nuclear spin statistics exclusively determine reaction outcomes
• At low temperatures, some reaction channels are inhibited due to energetic considerations
p-H2; J = 0
o-H2; J = 1
E = 170 K
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Low temperature modelLow temperature model
Parameter Value(s)
Trot 10-160 K
Tcoll 10-160 K
Sid 0.1-0.9
Shop 0-1
Sexch 0-1
koooo kooop koopo koopp
kopoo kopop koppo koppp
kpooo kpoop kpopo kpopp
kppoo kppop kpppo kpppp
f (T,Sid,)
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20 June 2011 66th OSU International Symposium on Molecular Spectroscopy
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T
160 K
140 K
120 K
100 K
80 K
60 K
40 K
20 K
Low temperature modelLow temperature model
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SummarySummary
• Measure p-H3+ fraction in known p-H2 fraction plasma
• High temperature models– Reaction outcomes governed by nuclear spin statistics
– 2-body model: only H3+ + H2 reaction
– 3-body model: additional scrambling via (H7+)*
• Low temperature model– Only H3
+ + H2 reaction
– Conservation of nuclear spin, energetic considerations– Dependent on T and Sid
• Next talk, application of models to experimental data• More info:
K. N. Crabtree, B. A. Tom, B. J. McCall, J. Chem. Phys. (2011), 134, 194310.
• Measure p-H3+ fraction in known p-H2 fraction plasma
• High temperature models– Reaction outcomes governed by nuclear spin statistics
– 2-body model: only H3+ + H2 reaction
– 3-body model: additional scrambling via (H7+)*
• Low temperature model– Only H3
+ + H2 reaction
– Conservation of nuclear spin, energetic considerations– Dependent on T and Sid
• Next talk, application of models to experimental data• More info:
K. N. Crabtree, B. A. Tom, B. J. McCall, J. Chem. Phys. (2011), 134, 194310.