Observation Of Nuclear Spin Selection Rules In Supersonically Expanding Plasmas Containing H3

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Observation Of Nuclear Spin Selection Rules In
Supersonically Expanding Plasmas Containing H3
Brian Tom, Michael Wiczer, Andrew Mills, Kyle
Crabtree, and Ben McCall
Department of Chemistry
Department of Astronomy
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Motivations

• Source of para-H3
• Dissociative recombination studies

• H3 H2 ? H3 H2
• Simplest reaction of polyatomic

• Interstellar o/p H3
• To/p ? Tkin ??

Cygnus OB2 12
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Para-H2 Para-H2 ? Para-H3 H

• Produce para-H2

n-H2
p-H2
n-H2
p-H2
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Para-H2 Para-H2 ? Para-H3 H

• Produce para-H2
• Ionize para-H2
• React to para-H3

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H3 H2 ? (H5) ? H3 H2
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identity
H5
3
hop
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hop/exchange 0.5
exchange
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Dynamics of Reaction
C2v
3000 cm-1
1500 cm-1
50 cm-1
hop
exchange
Not obvious that statistical hop/exchange 0.5
is valid!
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Spin-Modification Probability
Products formed by Hop and Exchange
Reactants
Park Light JCP 126, 044305 (2007)
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Spin-Modification Probability
Products formed by Hop and Exchange
Reactants
1/2 ? 0 ? 3/2 ? 0
1/2 ? 0 ? 3/2 ? 1
Park Light JCP 126, 044305 (2007)
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Low Temperature Effects

• In pure p-H2
• p-H3 ? o-H3 requires p-H2 ? o-H2

ortho I 3/2
para I 1/2
ortho I 1
170 K
para I 0
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Oka Group Experiments
Cordonnier et al. JCP 113, 3181 (2000)
Pulsed Hollow Cathode
Positive Column Cell
reaction not yet measured at low T
p-H3
n-H2
p-H2
p-H2?o-H2
o-H3
p-H3
o-H3
p-H2
n-H2
_at_ 400 K
not 0.5!
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Supersonic Expansion Ion Source
Gas inlet 2 atm
H2
Solenoid valve

• Developed for DR experiments
• H3 formed near nozzle
• p-H2/H2 frozen
• p-H3/H3 evolves, but steady state
• Probed by spectroscopy

McCall et al. PRA 70, 052716 (2004)
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2.8 4.8 mm DFG System
NdYAG 1064 nm
532 nm pump laser
l/4
TiSapph 700 990 nm
l/2
AOM
25cm
20cm
reference cavity
Andrew Mills WG10
l/2
PPLN
InSb
mode- matching lenses
ringdown cavity
Glan prism
dichroic
20cm achromat
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Cavity Ringdown Spectra

• First results from our DFG laser!
• Clear enhancement of para-H3 in para-H2
• More enhanced in argon dilution
• Trot 80 K
• R(1,1)u vs R(2,2)l

ortho-H3
para-H3
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Theoretical Predictions
koppo ? k(o-H3 p-H2 ? p-H3 o-H2)

• Steady state
• High temp limit
• Low temp calculations of k by Park Light

Park Light JCP 126, 044305 (2007)
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Comparison with Experiment
Park Light
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H3 toward ? Persei
fp 0.62 0.09
McCall, et al. Nature 422, 500 (2003)
N(H2) from Copernicus
? 0.68 0.20
Savage et al. ApJ 216, 291 (1977)
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Diffuse Clouds Comparison
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Future Directions

• Experiment
• Hollow cathode _at_ 150 K
• Measure H3 and H2
• Expansion source
• Vary ?, T
• Theory
• Different values of a
• Observations
• More sources
• Improved error bars

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Acknowledgments
Brian Tom Michael Wiczer (?MIT) Andrew Mills Kyle
Crabtree Nick Indriolo
NSF Division of AMO Physics
Takeshi Oka (U. Chicago)
Tom Geballe (Gemini)
NASA Laboratory Astrophysics
http//astrochemistry.uiuc.edu