New Laboratory and Theoretical Studies of Astrophysically Important Reactions of H 3 +
description
Transcript of New Laboratory and Theoretical Studies of Astrophysically Important Reactions of H 3 +
New Laboratory and Theoretical New Laboratory and Theoretical Studies of Astrophysically Studies of Astrophysically Important Reactions of HImportant Reactions of H33
++
Ben McCallBen McCall
Dept. of Chemistry Dept. of Astronomy
OutlineOutline
• Background– Importance of H3
+
– Interstellar Clouds• H3
+ in Diffuse Clouds– Abundance: H3
+ + e- → H + H + H– Ortho/Para: p-H3
+ + e- → H + H + H H3
+ + H2 → H3+ + H2
• H3+ in Dense Clouds
– Abundance: H3+ + O → OH+ + H2
– Puzzle: H3+ + O2 ↔ HO2
+ + H2
Astronomer's Periodic TableAstronomer's Periodic Table
H He
C N O Ne
Mg
Fe
Si S Ar
HH33++: Cornerstone of Interstellar Chemistry: Cornerstone of Interstellar Chemistry
H 2
H 2+
C H 3+
C H 5+
C H 4
C H2 3+
C H2 2
C H3+
C H3 3+
C Hm n
C H +
C H 2+
N H2+
H C O+O H +
H O2+
H O3+
H O2
O H
C H C N H2 5+
C H C N H3+
C H N H3 2+
H C N2+
H C N
C H N H3 2
C H N H2
C H C N3
C H C N2 5
C H
C H C O3+
C H O H3 2+
C H C O2
C H O H3
H C OC H O H
C H O C H
2
2 5
3 3
C H2 5+
C H2 4
H C O2 3+
C O3
C H2
H C N3 3+
H C N3
H C N5
H C N7
H C N9
H C N11
C H3
C 4+
C H4+
C H4 2+
C H4 3+
C H4
C H3 2
cosm ic ray
H 2N 2
C OO
H 2
H 2
e
e
e
ee
e
e
N
N H 3
H C N
C H C N3
e
C OH O2
C H O H , e3
C
H 2
H 2
H 2
e
e
C H 3+
e
e eH C N
C+
e
C+
H 2
e eC
+
H 2H 2
e
C O
C
H
+
e
e
8
7
6
5
4
3
2
Pro
ton
Affin
ity (e
V)
N
O2
H2
ON2
CO2
CH4
OHCC2
H2O
H2COCH
NH2
SiNH3
CO
Pro
ton
Affi
nity
(eV
)
H3+ + O H2 + OH+
OH+ + H2 H + H2O+
H2O+ + H2 H + H3O+
H3O+ + e- H2O + H
Interstellar Cloud ClassificationInterstellar Cloud ClassificationDiffuse clouds:
• H ↔ H2
• C C+
• n(H2) ~ 101–103 cm-3
– [~10-15 Torr]• T ~ 50 K
PerseiPersei
Photo: Jose Fernandez Garcia
Snow & McCallARAA, 44, 367 (2006)
Dense molecular clouds:
• H H2
• C CO• n(H2) ~ 104–106 cm-3
• T ~ 20 K
PoundApJ 493, L113 (1998)
OutlineOutline
• Background– Importance of H3
+
– Interstellar Clouds• H3
+ in Diffuse Clouds– Abundance: H3
+ + e- → H + H + H– Ortho/Para: p-H3
+ + e- → H + H + H H3
+ + H2 → H3+ + H2
• H3+ in Dense Clouds
– Abundance: H3+ + O → OH+ + H2
– Puzzle: H3+ + O2 ↔ HO2
+ + H2
Rate = ke [H3+] [e-]
[H2]
Diffuse Cloud HDiffuse Cloud H33++ Chemistry Chemistry
H2 H2+ + e-
H2 + H2+ H3
+ + H
cosmic ray
H3+ + e- H + H2 or 3H
Rate =
Formation
Destruction
[H3+]
=
ke[e-]
Steady State[H2] =
(310-17 s-1)
(510-7 cm3 s-1) (2400) = 10-7 cm-3
L ~ 3 pc ~ 1019 cm
N(H3+) ≡ L × [H3
+] ~ 1012 cm-2
dense cloud value
ΔI/I ~ 0.01%
Lots of HLots of H33++ in Diffuse Clouds! in Diffuse Clouds!
8
6
4
2
0
H3+ C
olum
n D
ensi
ty (1
014cm
-2)
6543210
E(B-V) (mag)
OphP Cygni
HD 183143
WR 118
Cyg OB2 12
WR 104
Cyg OB2 5
WR 121
HD 168607
HD 194279
GC IRS 3
2 Ori
HD 20041
1.01
1.00
0.99
0.98
0.97
Rel
ativ
e In
tens
ity
3.7173.7163.7153.6693.6683.667Wavelength (µm)
R(1,1)u
R(1,1)l
R(1,0)
HD 183143
McCall, et al.ApJ 567, 391 (2002)
Cygnus OB2 12
N(H3+) ~ 1014 cm-2 ?!?
Big Problem with the Chemistry!Big Problem with the Chemistry!
Steady State: [H3+]
= ke [e-]
[H2]
To increase the value of [H3+], we need:
• Smaller electron fraction [e-]/[H2]
• Smaller recombination rate constant ke
• Higher ionization rate
(order of magnitude)
^
ruled out byobservations
Enigma of HEnigma of H33++ Recombination Recombination
• Laboratory values of ke have varied by 4 orders of magnitude!
• Problem: not measuring H3
+ in ground states
k e (c
m3 s
-1)
Larsson, McCall, & OrelChem. Phys. Lett., in press
Ion Storage Ring MeasurementsIon Storage Ring Measurements
20 ns 45 ns
electron beam
H3+
H, H2
+ Very simple experiment+ Complete vibrational relaxation
+ Control H3+ – e- impact energy
+ Rotationally cold ions from supersonic expansion source
CRYRING
30 kV30 kV
900 keV900 keV
12.1 MeV12.1 MeV
CRYRING ResultsCRYRING Results
• Considerable amount of structure (resonances) in the cross-section
• ke = 2.6 10-7 cm3 s-1
• Factor of two smallerMcCall et al.Nature 422, 500 (2003)
Agreement with Other WorkAgreement with Other Work• 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
Big Problem with the Chemistry!Big Problem with the Chemistry!
Steady State: [H3+]
= ke [e-]
[H2]
To increase the value of [H3+], we need:
• Smaller electron fraction [e-]/[H2]
• Smaller recombination rate constant ke
• Higher ionization rate =7.410-16 s-1
(25× higher thandense clouds!)N. Indriolo, T. R. Geballe, T. Oka, &
B. J. McCall, ApJ 671, 1736 (2007)
Astrophysics!!
Low Energy Cosmic Rays?Low Energy Cosmic Rays?• Flux below <1 GeV essentially unconstrained
– magnetic field due to solar wind• Large low E flux can reproduce observations!
Photo: M.D. Stage, G. E. Allen, J. C. Houck, J. E. Davis, Nat. Phys. 2, 614 (2006)
1 MeV
2 MeV
10 MeV20 MeV
50 MeV(diffuse) (dense)
N. Indriolo, B. D. Fields & B. J. McCall, in preparation
OutlineOutline
• Background– Importance of H3
+
– Interstellar Clouds• H3
+ in Diffuse Clouds– Abundance: H3
+ + e- → H + H + H– Ortho/Para: p-H3
+ + e- → H + H + H H3
+ + H2 → H3+ + H2
• H3+ in Dense Clouds
– Abundance: H3+ + O → OH+ + H2
– Puzzle: H3+ + O2 ↔ HO2
+ + H2
HH33++ Ortho/Para Ratio Ortho/Para Ratio
+
orthoI = 3/2
paraI = 1/2
+
Cygnus OB2 12
No
Np
go
gp
e-ΔE/kTex=
ΔE
R(1,0) R(1,1)
Tex ~ 27 Kbut
Tkin ~ 60 K
Why?
para-Hpara-H33++ + e + e-- vs. ortho-H vs. ortho-H33
++ + e + e--
Theory: S.F. dos Santos, V. Kokoouline, and C. H. Greene, J. Chem. Phys. 127, 124309 (2007)
normal H2
para H2
experiment
para-H3+
ortho-H3+
theory
Experiment: H. Kreckel, et al.Phys. Rev. Lett. 95, 263201 (2005)
TSR
K
para-H3+ fraction
unknown (~0.55?)
Recent CRYRING ResultsRecent CRYRING Results85% p-H3
+
[100% p-H3+]
50% p-H3+
[100% o-H3+]
×2!
B. Tom et al.,in preparation
• Big ortho-para difference
• But ortho/para H3+ may be
equilibrated by H3+ + H2 collisions
OutlineOutline
• Background– Importance of H3
+
– Interstellar Clouds• H3
+ in Diffuse Clouds– Abundance: H3
+ + e- → H + H + H– Ortho/Para: p-H3
+ + e- → H + H + H H3
+ + H2 → H3+ + H2
• H3+ in Dense Clouds
– Abundance: H3+ + O → OH+ + H2
– Puzzle: H3+ + O2 ↔ HO2
+ + H2
HH33++ + H + H22 → (H→ (H55
++)* → H)* → H33++ + H + H22
“identity”
“hop”
“exchange”
H5+
1
3
6if purely statistical:α = hop/exchange = 0.5
Dynamical EffectsDynamical Effects
C2v
D2dC2v
“hop”
“exchange”
Not obvious that “statistical” α = hop/exchange = 0.5 is valid!
~300
0 cm
-1
~50 cm-1~1500 cm-1
Energetic EffectsEnergetic 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
+
orthoI = 3/2
paraI = 1/2
paraI = 0
orthoI = 1
170 K
1/2 0 ↔ 3/2 0
Oka Group ExperimentsOka Group Experiments
o-H3+ p-H3
+
Pulsed Hollow CathodePositive Column Cell
Cordonnier et al. JCP 113, 3181 (2000)
p-H2
n-H2
o-H3+
p-H3+
n-H2 p-H2
hopexch ~2.4
T ~ 400 K
α = ≠ 0.5!
How does α vary with T?
Supersonic Expansion Ion SourceSupersonic Expansion Ion Source
• H3+ formed near nozzle
• [p-H2] / [H2] fixed– [H2] / [H3
+] >> Ncollisions
• [p-H3+] / [H3
+] reaches steady state in few coll.
• [p-H3+] / [H3
+] measured spectroscopically
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
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
+
HH33++ + H + H22 Results Results
α=2.5α=1.0α=0.5
80 KPark & Light
JCP 126, 044305 (2007)
ζ Persei
Tex o/p H3+ ratio not
thermal, but steady state of H3
+ + H2
(Oka)
Tkin~60 K
OutlineOutline
• Background– Importance of H3
+
– Interstellar Clouds• H3
+ in Diffuse Clouds– Abundance: H3
+ + e- → H + H + H– Ortho/Para: p-H3
+ + e- → H + H + H H3
+ + H2 → H3+ + H2
• H3+ in Dense Clouds
– Abundance: H3+ + O → OH+ + H2
– Puzzle: H3+ + O2 ↔ HO2
+ + H2
NO
Ne
HH33++ in Dense Clouds in Dense Clouds
CCO
8
7
6
5
4
3
2
Pro
ton
Affin
ity (e
V)
N
O2
H2
ON2
CO2
CH4
OHCC2
H2OCH
CO
Pro
ton
Affi
nity
(eV
)
H
NeHe
• Relatively few electrons• C → CO• H3
+ destroyed by proton transfer– CO– O, O2?
?
ζ [H2]
Dense Cloud HDense Cloud H33++ Chemistry Chemistry
H2 H2+ + e-
H2 + H2+ H3
+ + H
cosmic ray
H3+ + CO HCO+ + H2
Rate =
Formation
DestructionRate = kCO [H3
+] [CO]
ζ [H3
+] =kCO [CO]
Steady State
=(310-17 s-1)
(210-9 cm3 s-1)
[H2] (6700)
= 10-4 cm-3
(fast)
McCall, Geballe, Hinkle, & OkaApJ 522, 338 (1999) L ~ 1 pc ~ 3×1018
cm → N(H3+) ~ 3×1014 cm-2
H3+ + O OH+ + H2 Rate = kO [H3
+] [O]
210-9 cm3 s-1
0.810-9 cm3 s-1
=
??
HH33++ + O + O → OH→ OH++ + H + H22
Stephen Klippenstein (2008)
Ryan Bettens (1999)
• At T<50, kO kCO ζ or L ↑ by factor of ~2
Tcloud
OutlineOutline
• Background– Importance of H3
+
– Interstellar Clouds• H3
+ in Diffuse Clouds– Abundance: H3
+ + e- → H + H + H– Ortho/Para: p-H3
+ + e- → H + H + H H3
+ + H2 → H3+ + H2
• H3+ in Dense Clouds
– Abundance: H3+ + O → OH+ + H2
– Puzzle: H3+ + O2 ↔ HO2
+ + H2
HH33++ + O + O22 ↔ HO↔ HO22
++ + H + H22
• HO2+ is last simple protonated species yet
to be observed spectroscopically• O2 difficult to observe in dense clouds;
HO2+ may be a useful tracer?
• Nearly thermoneutral formation reaction• Our work:
– Re-examine thermochemistry– Calculate spectroscopic constants
S. L. Widicus Weaver, D. E. Woon, B. Ruscic,and B. J. McCall, in preparation
Thermochemical CalculationsThermochemical Calculations• Active Thermochemical Tables (ATcT)
– PA0 K(O2) = 417.18 ± 0.11 kJ/mol
– PA0 K(H2) = 417.78 ± 0.01 kJ/mol
– ΔrE0 = 0.60 ± 0.11 kJ/mol = 50 ± 9 cm-1
• Ab initio calculations– ΔEe valence complete basis set (CBS) limit: +222.1 cm-1
– ΔEe core-valence contribution +28.3– harmonic vibrational ZPE correction -199.5– anharmonic vibrational ZPE correction +76.4– rotational ZPE correction -63.0
– ΔE0 net +64.3 cm-1
Branko Ruscic(Argonne)
Dave Woon(Illinois)
S. L. Widicus Weaver, D. E. Woon, B. Ruscic,and B. J. McCall, in preparation
Interstellar Abundance of HOInterstellar Abundance of HO22++
rH°298 = 1.31 ± 0.11 kJ/mol
rG°298 = -1.75 ± 0.11 kJ/mol
)H()O()H(
2
23 nnnKT
)HO( 2n
= 2 × (10-4 cm-3) × (10-4)
N(HO2+) = n(HO2
+) L ~ (2×10-8 cm-3)(3×1018 cm) ~ 6×1010 cm-2
(likely undetectable)S. L. Widicus Weaver, D. E. Woon, B. Ruscic,and B. J. McCall, in preparation
HOHO22++ Spectroscopic Constants Spectroscopic Constants
S. L. Widicus Weaver, D. E. Woon, B. Ruscic,and B. J. McCall, in preparation
AcknowledgmentsAcknowledgmentshttp://bjm.scs.uiuc.edu
NASA LaboratoryAstrophysics
NSF Chemistry, AMO Physics
H3+ Observations:
Takeshi Oka (U. Chicago)Tom Geballe (Gemini)
Storage Ring Measurements:Mats Larsson (Stockholm)Richard Thomas (Stockholm)
Cosmic Ray Theory:Brian Fields (Illinois)
H3+ + H2:
Kisam Park (U. Chicago → TTU)
H3+ + O:
Stephen Klippenstein (Argonne)
H3+ + O2:
Susanna Widicus Weaver (Illinois → Emory)Dave Woon (Illinois)Branko Ruscic (Argonne)
BrianTom
NickIndriolo
KyleCrabtree
MichaelWiczer
AndrewMills
[and manyothers]
Critical Research Initiative
http://astrochemistry.uiuc.edu
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)