Transcript The ortho:para Ratio of H3+ in Laboratory and Astrophysical Plasmas Ben McCall Kyle Crabtree Dept.

The ortho:para Ratio of H3+ in
Laboratory and Astrophysical Plasmas
Ben McCall
Kyle
Crabtree
Dept. of Chemistry
Nick
Indriolo
Holger
Kreckel
Dept. of Astronomy
H3+: Cornerstone of Interstellar Chemistry
NH3
Si
C6H7+
H2
8
Proton Affinity
Affinity (eV)
Proton(eV)
7
6
5
4
C2H2
C4H3+
H
C4H2+
C3H2
C3H3+
e
H2
C3H+
C+
C2H2
C2H
e
C2H4
C2H3
C2H5+
e
+
e
C+
CH3+
CH4
e
CH3OCH3
CH5+
CH3OH
CH3NH2
H2
CH2+
CH
e
CH3CN
CH3+
CH2CO
H2O
C2H5CN
H2
HCN
H2
H3O+
H2
CH+
H2O+
H2
3
C3H
C
e
OH
N
C6H6
C6H5+
NH2
H2CO
CH
H 2O
C2
C
OH
CO
CH4
CO2
N2
O
H2
O2
e
C
OH+
O
H3
+
H2
H2+
HCO+
CO
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
R(1,1)u R(1,0)
1.02
R(1,1)l
AFGL 2136
1.00
0.98
AFGL 2591
0.96
0.94
36640
T. R. Geballe & T. Oka, Nature 384, 334 (1996)
+
36660
36680
36700
Wavelength (Å)
37150
37170
Surprise: H3+ in Diffuse Clouds!
Relative Intensity
1.01
Cygnus OB2 12
HD 183143
1.00
0.99
u
0.98
R(1,1)
R(1,0)
l
R(1,1)
0.97
3.667
3.668
3.669
3.715
Wav elength (µm)
HD 168625
HD 168607
X Per
ζ Per
HD 21389
HD 169454
BD -14 5037
3.716
3.717
W40 IRS 1a
HD 110432
HD 73882
HD 229059
Cyg OB2 5
WR 104
WR 121
HD 183143
HD 20041
HD 204827
HD 154368
HD 29647
Nick Indriolo
N. Indriolo, T. R. Geballe, T. Oka, &
B. J. McCall, ApJ 671, 1736 (2007)
= H3+ Detection
Chemistry → Cosmic Rays
cosmic-ray ionization rate
-]
[e
 = ke [H3+]
[H2]
electron recombination rate
ALS 8828
1 MeV
2 MeV
10 MeV
20 MeV
50 MeV
HD 254577
IC 443
N. Indriolo, et al., & B. J. McCall, Astrophys. J., in press
(diffuse)
(dense)
N. Indriolo, B. D. Fields & B. J. McCall, Astrophys. J., 694, 257 (2009)
A Puzzle: H3+ Ortho:Para Ratio
R(1,0)
R(1,1)
+
Cygnus OB2 12
+
para
I = 1/2
ortho
I = 3/2
No
Np
ΔE
=
go
gp
e
-ΔE/kTex
Tex ~ 27 K
but
Tkin ~ 60 K
What controls o:p ratio?
para-H3+ + e- vs. ortho-H3+ + eexperiment
TSR
para H2
para-H3+ fraction
normal H2
unknown (~0.55?)
para-H3+
theory
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)
Piezo Expansion Source
plunger
locking nut
discharge
electrode
skimmer
high pressure
H2 reservoir
low pressure
pinhole
6 cm
piezo actuator
CRDS Source Characterization
ortho-H3+
para-H3+
para-H3+
p-H2 → 71% para-H3+
n-H2
p-H2
n-H2 → 48% para-H3+
p-H2
p-H2
n-H2
n-H2
Recent TSR Results on H3+ + eBut interstellar H3+ is
para-enriched!!
1:5 p-H2:Ar
(71% p-H3+)
1:5 n-H2:Ar
(48% p-H3+)
Extrapolated:
100% p-H3+
100% o-H3+
H. Kreckel, et al., & B. J. McCall,
Physical Review A, in press
Imaging Measurements: Hot Ions?
H. Kreckel, et al., & B. J. McCall,
Physical Review A, in press
H3+ + H2 → (H5+)* → H2 + H3+
• simplest bimolecular reaction
involving a polyatomic
• most common bimolecular
reaction in the universe: ~1052 s-1
H5
1
“identity”
+
3
“hop”
what is branching ratio?
α = hop/exchange
3/6 ? T-dependent?
6
“exchange”
Nuclear Spin Selection Rules
para
+
para
+
1/2  0 = 1/2
ortho
→
+
para
+
3/2  0 = 3/2
How does α
vary with T?
hop
α = exch ~2.4 ≠ 0.5!
(@ 400 K)
Cordonnier et al., JCP 113, 3181 (2000)
Experimental Approach
ortho-H3+
para-H3+
measure steady state
p3[p-H3+]/[H3+] vs p2
Takayoshi
Amano
(p2) p-H2 +
(1-p2) o-H2
pump
Liquid-nitrogen cooled hollow cathode
Steady-State Model Predictions
“high T”
model
full thermal
equilibrium
n-H2
p-H2
o/p-H3+ vs. o/p-H2
o/p-H3+ vs. o/p-H2
o/p-H3+ vs. o/p-H2
o/p-H3+ vs. o/p-H2
T↓ → complex↑
 more scrambling
Recent Measurements
H3+
T ~ 130 K
Pulse off
Abs.
Pulse on
H5+
Tkin
Frequency
New Astronomical Observations
in diffuse clouds,
p2 is thermalized
K. Crabtree, N. Indriolo, H, Kreckel, B. A. Tom, & B. J. McCall,
Astrophys. J., submitted
“Low-Temperature” Model
f (T,Sid,)
Parameter
Value(s)
Trot
10 K
Tcoll
10-160 K
Sid
0.1-0.9
Shop
0-0.9
Sexch
0-0.9
koooo kooop koopo koopp
kopoo kopop koppo koppp
kpooo kpoop kpopo kpopp
kppoo kppop kpppo kpppp
“State-to-State” Rate Coefficients
Steady State of H3+ + H2 Reaction
• Assume rate of reaction with H2 is much
faster than formation or destruction rates:
• Solve for steady state para fraction:
K. Crabtree, N. Indriolo, H, Kreckel, B. A. Tom, & B. J. McCall,
Astrophys. J., submitted
H3+ + H2 Reaction Results
Sid=0.1
Sid=0.9
K. Crabtree, N. Indriolo, H, Kreckel, B. A. Tom, & B. J. McCall,
Astrophys. J., submitted
H3+ + H2 Reaction Results
=0
=
K. Crabtree, N. Indriolo, H, Kreckel, B. A. Tom, & B. J. McCall,
Astrophys. J., submitted
Steady State Model Revisited
• Include formation and destruction
reactions:
• Assume steady state, simplify:
K. Crabtree, N. Indriolo, H, Kreckel, B. A. Tom, & B. J. McCall,
Astrophys. J., submitted
Model Results for ke,p = ke,o
Rate Coefficients from McCall et al., PRA 2004, 70, 052716
Sid=0.9
Sid=0.1
K. Crabtree, N. Indriolo, H, Kreckel, B. A. Tom, & B. J. McCall,
Astrophys. J., submitted
Model Results for ke,p > ke,o
Rate Coefficients from dos Santos et al., JCP 2007, 127, 124309
K. Crabtree, N. Indriolo, H, Kreckel, B. A. Tom, & B. J. McCall,
Astrophys. J., submitted
Summary & Future Directions
H3+ + e- Recombination
•
H3+ + H2 Reaction
First high-resolution rate
coefficient measurements for
ortho and para H3+
•
Hop:exchange ratio ()
decreases with temperature
•
•
H5+ formation at low T, high P!
Surprise: Trot ~ 450 K!
•
•
Really need measurements
at low Trot!
Future measurements at lower
temperatures, using 22-pole
ion trap (Schlemmer)
Astronomical Observations and Modeling
• Observations in diffuse clouds indicate H3+ o:p is not thermalized
• o:p ratio results from competition between reactions with e- & H2
• Need experimental measurements at lower temperatures
• H3+ o:p could serve as thermometer in highly reddened sightlines
H3+
Acknowledgments
NSF
AMO Physics
Nick
Indriolo
Kyle
Crabtree
Holger
Kreckel
Carrie
Kauffman
Brian
Tom
Molecular Ion Spectroscopy
Dreyfus New
Faculty, TeacherScholar Awards
NASA Laboratory
Astrophysics
NSF Chemistry &
Astronomy
Andrew
Mills
Brian
Siller
Holger
Kreckel
Manori
Perera
Mike
Porambo
Solid
p-H2
C60
Jacob Susanna
Brian
Brumfield Stewart Weaver
Packard
Fellowship
Air Force Young
Investigator Award
Cottrell
Scholarship
Bill
Evans
Sloan
Fellowship
Para-H2 Production
• Helium cryostat
• Catalyst @ 15 K
• Up to 99.99% p-H2
n-H2
(25% para)
p-H2
NMR of
ortho-H2
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)