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

Containing HContaining H33++

Brian Tom, Michael Wiczer,Brian Tom, Michael Wiczer,Andrew Mills, Kyle Crabtree,Andrew Mills, Kyle Crabtree,

and Ben McCalland Ben McCall

Department of Chemistry Department of Astronomy

MotivationsMotivations

• 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

• Produce para-H2

Para-HPara-H22 + Para-H + Para-H22+ + →→ Para-H Para-H33

++ + H + H

n-H2

p-H2

n-H2

p-H2

Para-HPara-H22 + Para-H + Para-H22+ + →→ Para-H Para-H33

++ + H + H

• Produce para-H2

• Ionize para-H2

• React to para-H3+ + → +

+

+

+ → +e- 2e-

+

HH33++ + H + H22 → (H→ (H55

++)* → H)* → H33++ + H + H22

“identity”

“hop”

“exchange”

H5+

1

3

6hop/exchange = 0.5

Dynamics of ReactionDynamics of Reaction

C2v

D2dC2v

“hop”

“exchange”

Not obvious that “statistical” hop/exchange = 0.5 is valid!

~30

00 c

m-1

~50 cm-1

~1500 cm-1

Spin-Modification ProbabilitySpin-Modification Probability

Total I o-H3+ + o-H2 o-H3

+ + p-H2 p-H3+ + o-H2 p-H3

+ + p-H2

o-H3+ + o-H2

5/2 1 1 0 0 0 0 0 0

3/2 4/9 1/36 0 5/12 5/9 5/9 0 0

1/2 1/9 1/9 0 0 8/9 2/9 0 2/3

o-H3+ + p-H2 3/2 0 5/12 0 1/4 1 1/3 0 0

p-H3+ + o-H2

3/2 5/9 5/9 1 1/3 4/9 10/9 0 0

1/2 8/9 2/9 0 0 1/9 17/18 1 5/6

p-H3+ + p-H2 1/2 0 2/3 0 0 1 5/6 1 1/2

Rea

ctan

ts

Products formed by Hop and Exchange

Park & Light JCP 126, 044305 (2007)

Spin-Modification ProbabilitySpin-Modification Probability

Total I o-H3+ + o-H2 o-H3

+ + p-H2 p-H3+ + o-H2 p-H3

+ + p-H2

o-H3+ + o-H2

5/2 1 1 0 0 0 0 0 0

3/2 4/9 1/36 0 5/12 5/9 5/9 0 0

1/2 1/9 1/9 0 0 8/9 2/9 0 2/3

o-H3+ + p-H2 3/2 0 5/12 0 1/4 1 1/3 0 0

p-H3+ + o-H2

3/2 5/9 5/9 1 1/3 4/9 10/9 0 0

1/2 8/9 2/9 0 0 1/9 17/18 1 5/6

p-H3+ + p-H2 1/2 0 2/3 0 0 1 5/6 1 1/2

Rea

ctan

ts

Products formed by Hop and Exchange

1/2 0 ↔ 3/2 0 1/2 0 ↔ 3/2 1Park & Light JCP 126, 044305 (2007)

Low Temperature EffectsLow Temperature Effects

• In pure p-H2

– p-H3+ → o-H3

+ requires p-H2 → o-H2

orthoI = 3/2

paraI = 1/2

paraI = 0

orthoI = 1

170 K

Oka Group ExperimentsOka Group Experiments

o-H3+ p-H3

+

Pulsed Hollow CathodePositive Column Cell

Cordonnier et al. JCP 113, 3181 (2000)

p-H2→o-H2

hopexch ~2.4

@ 400 K

[not 0.5!]

p-H2

n-H2

o-H3+

p-H3+

n-H2 p-H2

reaction not yetmeasured at low T

Supersonic Expansion Ion SourceSupersonic Expansion Ion Source

• Developed for DR experiments

• H3+ formed near nozzle

• [p-H2]/[H2] “frozen”• [p-H3

+]/[H3+] evolves, but

~ steady state• Probed by spectroscopy

H2Gas inlet

2 atm

Solenoid valve

-450 Vring

electrode

Pinhole flange/ground

electrode

H3+

McCall et al. PRA 70, 052716 (2004)

2.8 – 4.8 2.8 – 4.8 m DFG Systemm DFG System

Ti:Sapph700 – 990 nm

532 nmpump laser

reference cavity

dichroic

/2

Nd:YAG1064 nm

AOM

PPLN

25cm20cm

/2/4

Glanprism

20cmachromat

InSb mode-matching

lenses

ringdowncavity

Andrew MillsWG10

Cavity Ringdown SpectraCavity 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

+

Theoretical PredictionsTheoretical Predictions

• Steady state:

• High temp limit:

• Low temp calculations of k by Park & Light

fp = koppo γ + (koopp+koopo)(1-γ)

(koppo+kppoo)γ + (koopp+koopo+kpoop+kpooo)(1-γ)

fp = α+1+2αγ

3α+2

α hop

exchange

γ [p-H2]

[p-H2]+[o-H2]fp [p-H3

+]

[p-H3+]+[o-H3

+]

koppo k(o-H3+ + p-H2 → p-H3

+ + o-H2)

Park & Light JCP 126, 044305 (2007)

Comparison with ExperimentComparison with Experiment

Park & Light

HH33++ toward toward Persei Persei

Savage et al. ApJ 216, 291 (1977)

N(H2) from Copernicus

McCall, et al. Nature 422, 500 (2003)

fp = 0.62 ± 0.09

γ = 0.68 ± 0.20

Diffuse Clouds ComparisonDiffuse Clouds Comparison

Future DirectionsFuture Directions

• Experiment– Hollow cathode @ 150 K

• Measure H3+ and H2

– Expansion source• Vary γ, T

• Theory– Different values of α

• Observations– More sources– Improved error bars

AcknowledgmentsAcknowledgments

http://astrochemistry.uiuc.edu

Tom Geballe(Gemini)

Takeshi Oka(U. Chicago)

NASA LaboratoryAstrophysics

NSF Division of AMO Physics

Brian Tom

Michael Wiczer (→MIT)

Andrew Mills

Kyle Crabtree

Nick Indriolo