spectroscopic studies of the h 3 + + h 2 reaction at astrophysically relevant temperatures brian a....
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University of Illinois at Urbana-Champaign 1
Spectroscopic Studies of the H3
+ + H2 Reaction at Astrophysically Relevant Temperatures
Brian A. Tom, Brett A. McGuire, Lauren E. Moore, Thomas J. Wood, Benjamin J. McCall
June 24, 2009
University of Illinois at Urbana-Champaign 2
Astrophysical Motivation• H3
+ + H2 → H2 + H3+ most common bimolecular reaction in
the universe
• H2 and H3+ used as probes of ISM
June 24, 2009
ortho-H3+para-H3
+
Indriolo et al., ApJ., 671, 1736, (2007)
University of Illinois at Urbana-Champaign 3
H3+ + H2 → H2 + H3
+
• Is this really a reaction?– YES!
June 24, 2009
½ + ½ + ½ = 3/2ortho-H3
+
½ + ½ - ½ = 1/2para-H3
+
½ + ½ = 1ortho-H2
½ - ½ = 0para-H2
86.96 cm-1
64.121 cm-1
R(1,0) (ortho)
2725.898 cm-1
R(1,1)u (para)
2726.219 cm-1
Difference of ~0.32 cm-1
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Reaction Dynamics
June 24, 2009
H3+ + H2 → (H5
+)* → H3+ + H2
“identity”
“hop”
“exchange”
H5+
1
3
6if purely statistical:
α = khop/kexchange = 0.5
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Nuclear Spin Considerations
June 24, 2009
o-H3+ + p-H2 → p-H3
+ + p-H2
3/2 0 1/2 0≠
p2 p3
[para-H2][para-H2] + [ortho-H2]
[para-H3+]
[para-H3+] + [ortho-H3
+]
University of Illinois at Urbana-Champaign 6
High Temperature Model
• Based on work by Cordonnier and Oka• Assumes all reactions are possible • High energy
• Assume steady state for p3
• [H2] >> [H3+]
June 24, 2009
α+1+2α p2
3α+2p3 ≡ [p-H3
+]/[H3+] =
p2
p3
¼
½
M. Cordonnier et al., J. Chem. Phys. 113, 3181 (2000)
University of Illinois at Urbana-Champaign 7
Low Temperature Model
• Based on work done by Park and Light of Chicago• Accounts for the energetic considerations of the
reactions
June 24, 2009
•Predicts a non-linear relationship between p2 and p3
•Plug in α and T, model gives a curve which is compared to the data
K. Park and J. C. Light, J. Chem. Phys., 126, 044305 (2007)
p2
p3
¼
½
University of Illinois at Urbana-Champaign 8
How do we get para-H2 enrichment (p2)?
June 24, 2009
Method: B. A. Tom, S. Bhasker, Y. Miyamoto, T. Momose, B. J. McCall, Rev. Sci. Instr. 80, 016108 (2009)
•Helium Cryostat
•Enrichment controlled by T or mixing
•Catalyst @ ~14 K
• > 99.9% p-H2
•Purity monitored by NMR and thermal conductance
University of Illinois at Urbana-Champaign 9
How do we measure the para-H3
+ fraction (p3)?
June 24, 2009
Takayoshi Amano
H2
pumpHollow cathode plasma cell
~310 K
~130 K~180 K
University of Illinois at Urbana-Champaign 10
Data Analysis
June 24, 2009
α νL
α νL
It
Io
= e-ανL
ανL
|μ2| nL
p3 = npara
npara + northo
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Results – High Temperature Model
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310 K
June 24, 2009
University of Illinois at Urbana-Champaign 13
180 K
June 24, 2009
University of Illinois at Urbana-Champaign 14
130 K
June 24, 2009
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Experimental Conclusions
• α is dependant on T– khop and kexchange not constant
– Lower T → Lower α → exchange dominates
• Neither models work well– High temperature
• Linear relationship• Ignores energetics
– Low temperature • Must use high temperatures to replicate data
• p3 does not converge to 0.5
June 24, 2009
University of Illinois at Urbana-Champaign 16
Astronomical Observations
June 24, 2009
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Future Work
• More low temperature measurements
• Refine our models for α• More astronomical
observations
June 24, 2009
University of Illinois at Urbana-Champaign 18June 24, 2009
Acknowledgments
Lauren Moore
Brian Tom
Tom Wood
Team Hydrogen
McCall Group
University of Illinois at Urbana-Champaign 19June 24, 2009
Acknowledgments
Lauren Moore
Brian Tom
Tom Wood
Team Hydrogen
McCall Group
University of Illinois at Urbana-Champaign 20
Temperature
June 24, 2009
Tk
E
beg
g
n
n
)1,1(
)0,1(
)1,1(
)0,1(