H3 : The Simplest Polyatomic Molecule in the Laboratory and the Interstellar Medium

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H3 The Simplest Polyatomic Moleculein the
Laboratory and the Interstellar Medium
Ben McCall
Dept. of Chemistry
Dept. of Astronomy
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Periodic Table
CCU
Chemists
Astronomers
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H3 Cornerstone of Interstellar Chemistry

• H3 O ? H2 OH
• OH H2 ? H H2O
• H2O H2 ? H H3O
• H3O e- ? H2O H

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Interstellar Cloud Classification

• Diffuse clouds
• H ? H2
• C ? C
• n(H2) 101103 cm-3
• 10-15 Torr
• T 50 K

• Dense molecular clouds
• H ? H2
• C ? CO
• n(H2) 104106 cm-3
• T 20 K

Snow McCall ARAA, 44, 367 (2006)
Pound ApJ 493, L113 (1998)
Photo Jose Fernandez Garcia
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Observing Interstellar H3

• Equilateral triangle
• No rotational spectrum
• No electronic spectrum
• Vibrational spectrum is only probe
• Absorption spectroscopy against background or
  embedded star
• Detected in 1996 in dense molecular clouds
  N(H3) 1014 cm-2

?1
?2
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Diffuse Cloud H3 Chemistry
Formation
? H2
cosmic ray
H2 H2 e- H2 H2 ? H3 H
Rate
Destruction
Rate ke H3 e-
H3 e- ? H H2 or 3H
dense cloud value

10-7 cm-3
L 3 pc 1019 cm N(H3) L H3 1012
cm-2
? ?I/I 0.01
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Lots of H3 in Diffuse Clouds!
Cygnus OB2 12
HD 183143
N(H3) 1014 cm-2 ?!?
McCall, et al. ApJ 567, 391 (2002)
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Big Problem with the Chemistry!
(order of magnitude)

Steady State

• To increase the value of H3, we need
• Smaller electron fraction e-/H2
• Smaller recombination rate constant ke
• Higher ionization rate ?

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H3 toward ? Persei
e-/H2 not to blame
N(C) from HST
McCall, et al. Nature 422, 500 (2003)
N(H2) from Copernicus
Cardelli et al. ApJ 467, 334 (1996)
Savage et al. ApJ 216, 291 (1977)
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Big Problem with the Chemistry!
Steady State

• To increase the value of H3, we need
• Smaller electron fraction e-/H2
• Smaller recombination rate constant ke
• Higher ionization rate ?

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Enigma of H3 Recombination

• Laboratory values of ke have varied by 4 orders
  of magnitude!
• Problem not measuring H3 in ground states

ke (cm3 s-1)
Larsson, McCall, Orel Chem. Phys. Lett., 462,
145 (2008)
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Storage Ring Measurements
CRYRING
900 keV
12.1 MeV
30 kV

• Very simple experiment
• Complete vibrational relaxation
• Control H3 e- impact energy
• Rotationally hot ions produced
• No rotational cooling in ring

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Supersonic Expansion Ion Source
Gas inlet 2 atm
H2

• Similar to sources used for laboratory
  spectroscopy
• Pulsed nozzle design
• Supersonic expansion leads to rapid cooling
• Discharge from ring electrode downstream
• Spectroscopy used to characterize ions

Solenoid valve
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Spectroscopy of H3 Source
Infrared Cavity Ringdown Laser Absorption
Spectroscopy

• Confirmed that H3 produced is rotationally cold,
  as in interstellar medium

McCall, et al. Nature 422, 500 (2003)
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CRYRING Results
not slow enough to explain interstellar H3

• Considerable amount of structure (resonances) in
  the cross-section
• ke 2.6 ? 10-7 cm3 s-1
• Factor of two smaller

McCall, et al. Phys. Rev. A 70, 052716 (2004)
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Other Recombination Studies

• Reasonable agreement between
• CRYRING
• Supersonic
• expansion
• TSR
• 22-pole trap
• Theory

S.F. dos Santos, V. Kokoouline and C. H. Greene,
J. Chem. Phys 127 (2007) 124309
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Big Problem with the Chemistry!
Astrophysics!!
Steady State

• To increase the value of H3, we need
• Smaller electron fraction e-/H2
• Smaller recombination rate constant ke
• Higher ionization rate ?

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Implications for ? Persei
N(H3)
?
N(H2)
H3


ke
N(e-)
L
N(e-)
ke
N(H3)
? L
(4.7?10-4)
(7?1013 cm-2)
(1.6?10-7 cm3 s-1)
N(H2)
(dense cloud value)

• L 5300 cm s-1

(firm)
Adopt ?3?10-17 s-1
Adopt ?n? 215 cm-3 ? L2.4 pc
?7.4?10-16 s-1 (25x higher!)
L 60 pc ?n? 9 cm-3
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Recent Astronomical Results

• Range of ? from 1.1-7.3 ? 10-16 s-1
• Biggest uncertainty is in adopted ?n?

N. Indriolo, T. R. Geballe, T. Oka, B. J.
McCall, ApJ 671, 1736 (2007)
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Surprise ? Conventional Wisdom

• Higher ? in diffuse (vs. dense) clouds initially
  greeted with skepticism
• Incorporated into models without incident

• Now generally accepted (but not understood!)

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Low Energy Cosmic Rays?

• Flux below lt1 GeV essentially unconstrained
• magnetic field due to solar wind
• Large low E flux can reproduce observations!

1 MeV
2 MeV
10 MeV
20 MeV
50 MeV
(diffuse)
(dense)
Photo M.D. Stage, G. E. Allen, J. C. Houck, J.
E. Davis, Nat. Phys. 2, 614 (2006)
N. Indriolo, B. D. Fields B. J. McCall,
Astrophys. J., 694, 257 (2009)
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Summary So Far

• Unexpectedly abundant H3
• Only in diffuse clouds not in dense clouds
• Rate constant for H3 e- pinned down
• H3 now a probe of cosmic-ray ionization rate
• Order of magnitude larger than thought
• Large, unrecognized flux of low-E cosmic rays?
• Local sources of cosmic rays?
• Future work
• Observations of ? versus column density, location
• Other cosmic ray probes?

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Another Puzzle H3 OrthoPara
Cygnus OB2 12
para I 1/2
ortho I 3/2
Tex 27 K but Tkin 60 K Why?
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para-H3 e- vs. ortho-H3 e-
TSR
para-H3 fraction unknown (0.55?)
theory
para-H3
ortho-H3
K
Theory S.F. dos Santos, V. Kokoouline, and C.
H. Greene, J. Chem. Phys. 127, 124309 (2007)
Experiment H. Kreckel, et al. Phys. Rev. Lett.
95, 263201 (2005)
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Recent CRYRING Results
But interstellar H3 is para-enriched!!
B. A. Tom, V. Zhaunerchyk, M. B.Wiczer, , M.
Larsson, R. D. Thomas, B. J. McCall, J. Chem.
Phys. 130, 031101 (2009)
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H3 H2 ? (H5) ? H3 H2

• Can exchange ortho ? para
• Most common bimolecular reaction in the Universe!
• Rate per H2 (kL 10-9 cm3 s-1) H3
• Dense clouds 10-13 s-1 per H2
• Diffuse clouds 10-14 s-1 per H2
• Milky Way has 1066 H2 molecules
• Total reaction rate 1052 s-1 !

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H3 H2 ? (H5) ? H3 H2
1
identity
H5
3
hop
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if purely statistical a 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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Nuclear Spin Selection Rules
?




3/2 ? 1 5/2 ? 3/2 ? 1/2
1/2 ? 0 1/2
3/2 ? 0 3/2
1/2 ? 0 1/2
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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
Park Light JCP 126, 044305 (2007)
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Spin-Modification Probability
Products formed by Hop and Exchange
Reactants
Park Light JCP 126, 044305 (2007)
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Steady State Analysis

• Assumptions
• Spin-modification probabilities map to
  kop,po ? k(o-H3 p-H2 ? p-H3 o-H2)
  etc.
• Gas has constant p2 ? p-H2/H2
• Constant a ? hop/exchange
• Steady state (reached in a few collisions)
• Results
• p3 ? p-H3/H3
• If p2 ¼, p3 ½ for all a

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Our Experimental Approach
Takayoshi Amano
H2
Hollow cathode plasma cell
pump
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Para-H2 Production

• Helium cryostat
• Catalyst _at_ low T
• Up to 99.99 p-H2

n-H2 p2¼
p-H2 p2(T)
n-H2 sample
p-H2 sample
Method B. A. Tom, S. Bhasker, Y. Miyamoto, T.
Momose, B. J. McCall, Rev. Sci. Instr. 80, 016108
(2009)
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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
l/2
PPLN
InSb
Hollow cathode
Glan prism
dichroic
20cm achromat
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Results _at_ 300 K
para-H3
ortho-H3
R(1,0)
R(1,1)u
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Oka Group Experiments
Cordonnier et al. JCP 113, 3181 (2000)
Pulsed Hollow Cathode
Positive Column Cell
p-H3
n-H2
p-H2
o-H3
p-H3
o-H3
p-H2
n-H2
How does a vary with T?
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o/p-H3 vs. o/p-H2
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o/p-H3 vs. o/p-H2
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Supersonic Expansion Ion Source
Gas inlet 2 atm
H2
Solenoid valve

• H3 formed near nozzle
• p-H2 / H2 fixed
• H2 / H3 gtgt Ncollisions
• p-H3 / H3 reaches steady state in few coll.
• p-H3 / H3 measured spectroscopically

McCall et al. PRA 70, 052716 (2004)
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Cavity Ringdown (cw)
for a given laser frequency
I
e-t/t
t

• Scan cw laser ? obtain "loss" spectrum
• High sensitivity DI/I lt 10-6

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Supersonic Expansion _at_ 80 K
ortho-H3
para-H3
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o/p-H3 vs. o/p-H2
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Diffuse Clouds Comparison
o/p H3 ratio not thermal, but steady state of
H3 H2
? Persei
Tkin60 K
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Future Work

• Experiments
• Hollow cathode H2
• cw supersonic source
• 22-pole ion trap
• Observations
• More sightlines
• Smaller error bars
• Theory
• Low temperature model
• Effect on interstellar H2

Stephan Schlemmer (Cologne)
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Astronomy
Chemistry
Astrochemistry
cold H3 e-
H3 Observations
too much?
o/p ratio?
ortho vs. para H3 e-
Understanding of Cosmic-Ray Flux
ortho vs. para H3 H2
Understanding orthopara
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Acknowledgments
http//bjm.scs.uiuc.edu
NSF Division of AMO Physics
Dreyfus New Faculty Award
NASA Laboratory Astrophysics
Andrew Mills
Kyle Crabtree
Nick Indriolo
NSF Divisions of Chemistry Astronomy
Brian Tom
Packard Fellowship
Air Force Young Investigator Award
Brett McGuire
Cottrell Scholarship
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Low Temperature Effects

• Angular momentum restrictions
• e.g. p-H3 p-H2 ? o-H3 p-H2
• At low T in pure p-H2, slow p-H3 ? o-H3

1/2 ? 0 ? 3/2 ? 0
ortho I 3/2
para I 1/2
ortho I 1
170 K
para I 0
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Theoretical Predictions

• Low temp calculations of k by Park Light

koppo(T) ? k(o-H3 p-H2 ? p-H3 o-H2)

• Steady state
• Requires a as input parameter

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o/p-H3 vs. o/p-H2
Expt vs. theory graph
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Diffuse Clouds Comparison
o/p H3 ratio not thermal, but steady state of
H3 H2
? Persei
Tkin60 K