Molecular%20Spectroscopy:%20The%20Key%20to%20Understanding%20the%20Interstellar%20Medium

1
Molecular Spectroscopy The Key to Understanding
the Interstellar Medium
Ben McCall
Dept. of Chemistry
Dept. of Astronomy
2
Dedication
1997
Richard Schorr, Director Hamburg
Observatory 1902-1941
1924
2009
from Boris Stoicheff, Gerhard Herzberg
2021
3
Molecular Spectroscopy The Key to Understanding
the Interstellar Medium
Ben McCall
Dept. of Chemistry
Dept. of Astronomy
4
History of Spectroscopy Newton
5
History of Spectroscopy Fraunhofer
Fraunhofer, 1817
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History of Spectroscopy Kirchhoff
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Interstellar Clouds
ApJ 19, 268 (1904)
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Interstellar Molecular Spectroscopy
Vibrational
Electronic
CH
C2H2
CN
Lacy et al., ApJ 342, L43 (1989)
Adams, ApJ 93, 11 (1941)
Inversion
Rotational
NH2CHO
NH3
Cheung et al., PRL 21, 1701 (1968)
Rubin et al., ApJ 169, L39 (1971)

• Now some 150 known interstellar molecules!

9
Diffuse clouds n 101103 cm-3 10-15 Torr T
60 K
Stellar Outflows n 108 ? cm-3 T 2000 ?
K dm/dt 2 tons/fs
Dense clouds n 104106 cm-3 T 20 K
Leão et al., AA 455, 187 (2006)
0.5 light years
Pound ApJ 493, L113 (1998)
Photo Jose Fernandez Garcia
10
Astronomers Periodic Table
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H3 Cornerstone of Interstellar Chemistry
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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
T. R. Geballe T. Oka, Nature 384, 334 (1996)
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Surprise H3 in Diffuse Clouds!
Cygnus OB2 12
HD 183143
HD 229059 Cyg OB2 5 WR 104 WR 121 HD 183143 HD
20041 HD 204827
HD 168624 HD 168607 X Per ? Per HD 21389 HD
169454 BD -14 5037 W40 IRS 1a
Nick Indriolo - WI09
N. Indriolo, T. R. Geballe, T. Oka, B. J.
McCall, ApJ 671, 1736 (2007)
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Chemistry ? Cosmic Rays
cosmic-ray ionization rate
electron recombination rate
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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Another Puzzle H3 OrthoPara
Cygnus OB2 12
para I 1/2
ortho I 3/2
Tex 27 K but Tkin 60 K What controls o/p
ratio?
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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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Supersonic Expansion Ion Source
Gas inlet 2 atm
H2
Solenoid valve

• Pulsed nozzle design
• Supersonic expansion leads to rapid cooling
• Discharge from ring electrode downstream
• Spectroscopy used to characterize ions

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Para-H2 Production

• Helium cryostat
• Catalyst _at_ low T
• 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)
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mid-IR cw Ringdown with DFG
ortho-H3
para-H3
NdYAG 1064 nm
532 nm pump laser
l/4
TiSapph 700 990 nm
l/2
AOM
reference cavity
l/2
PPLN
InSb
2.8 4.8 µm
ringdown cavity
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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) ? H2 H3

• simplest bimolecular reaction involving a
  polyatomic
• most common bimolecular reaction in the universe
  1052 s-1

1
identity
H5
3
hop

• not well understood
• branching ratio
• a hop/exchange
• quantum effects at low T

6
exchange
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Nuclear Spin Selection Rules
para
para
para
ortho
?




3/2 ? 0 3/2
1/2 ? 0 1/2
How does a vary with T?
p-H3
o-H3
a
? 0.5!
p-H2
n-H2
(_at_ 300 K)
Cordonnier et al., JCP 113, 3181 (2000)
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Our Experimental Approach
Brett McGuire WI11
Takayoshi Amano
H2
Hollow cathode plasma cell
pump
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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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o/p-H3 vs. o/p-H2
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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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Diffuse clouds n 101103 cm-3 10-15 Torr T
60 K
Stellar Outflows n 108 ? cm-3 T 2000 ?
K dm/dt 2 tons/fs
Dense clouds n 104106 cm-3 T 20 K
Leão et al., AA 455, 187 (2006)
0.5 light years
Pound ApJ 493, L113 (1998)
Photo Jose Fernandez Garcia
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Molecular Ions

• Key reactive intermediates
• low n, T
• Powerful probes of conditions
• Relatively little laboratory spectroscopy (vs.
  neutrals)

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Ion Spectroscopy Techniques
Sensitive Cooled Resolved Ion BEam Spectroscopy
Oka, Saykally
Maier, Nesbitt
Hirota, Amano
Supersonic Expansion
Velocity Modulation
Hollow Cathode
SCRIBES
?
?
?
?
High ion column density
?
?
?
?
Ion-neutral discrimination
?
?
?
?
?
?
Low rotational temperature
?
?
?
?
?
?
?
Narrow linewidth
?
?
?
?
Compatible with cavity-enhanced spectroscopy
?
Mass spectrometry of laser-probed ions
?
Spectral identification of ion mass
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SCRIBES Layout
Source Chamber
90 Bender
Beam Modulated Time of Flight Mass Spectrometer
Carrie Kauffman TE04 Kyle Crabtree TE05
Manori Perera TE06
Cavity-Enhanced Precision Spectroscopy
Andrew Mills WH02
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Current Status
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Progress to Date

• Uncooled system completely operational

1.5 µA of N2 overlapped with laser
N2
N2
O2
N
O
H2O
N

• Searching for first spectrum
• N2 A2?u-X2Sg _at_ 925 nm

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The Future of SCRIBES

• Move to DFG laser
• H3 ?2 ? 0 _at_ 3.67 µm
• Precision spectroscopy for Herschel SOFIA
• HCO, HOC, NH,
• Use supersonic ion source
• CH5, C3H3, C2H5,

Andrew Mills WH02
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Diffuse clouds n 101103 cm-3 10-15 Torr T
60 K
Stellar Outflows n 108 ? cm-3 T 2000 ?
K dm/dt 2 tons/fs
Dense clouds n 104106 cm-3 T 20 K
Leão et al., AA 455, 187 (2006)
0.5 light years
Pound ApJ 493, L113 (1998)
Photo Jose Fernandez Garcia
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Cyanopolyynes
Winnewisser Walmsley, AA 70, L37
HC7N
1977
1971
1978
HC3N
Kroto et al., ApJ 219, L133
Turner, ApJ 163, L35
HC5N
1975
HC9N
1977
Avery et al., ApJ 205, L173
Broten et al., ApJ 233, L105
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Carbon Vaporization Experiments

• Kroto, Smalley, Curl, Heath, OBrien
• How are HC2n1N formed in carbon stars?
• Laser vaporization ? TOF-MS
• Detected HC7N, HC9N !

C CH3CN
Heath et al., JACS 109, 359 (1987)
Kroto et al., Nature 318, 162 (1985)
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C60 !
Kroto et al., Nature 318, 162 (1985)
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Extraterrestrial C60?
Di Brozolo et al., Nature 369, 37 (1994)
Becker et al., Science 291, 1530 (2001)

• Really need spectroscopic search

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Spectroscopy of C60

• No rotational spectrum
• Inconvenient electronic spectra
• broad allowed or narrow forbidden bands
• interesting sources opaque in optical

Irshell _at_ NASA IRTF
Neugebauer et al., J Comput Chem 23, 895 (2002)
Clayton et al., AJ 109, 2096 (1995)
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Our Experimental Setup
Quantum Cascade Laser
QCLs from Prof. Claire Gmachl, Princeton
Acousto-Optic Modulator
High finesse cavity
Direct Absorption Cell
Mode-matching optics
Detector
Mid-IR cw Wavemeter
1
SO2
Oven Supersonic Expansion
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Preliminary Results N2 CH2Br2
Brian Brumfield FD06
qQ6 CH279Br81Br
qQ7 CH279Br81Br
qQ7 CH281Br2
qQ7 CH279Br2
qQ8 CH279Br2
qQ6 CH281Br2
??40 MHz
?8 CH2 wag 1197 cm-1
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Simulated Spectrum
T 10 K
T 20 K
T 50 K
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New Astronomical Search
TEXES Texas Echelon Cross Echelle Spectrograph

• Data obtained June 2003
• R Coronae Borealis
• AFGL 2136
• AFGL 2591
• NGC 7538 IRS 1
• Blind upper limit 31015 cm-2
• lt 0.6 of carbon

Lacy et al., PASP 114, 153 (2002)
R Coronae Borealis
NASA's 3-meter IRTF (InfraRed Telescope
Facility), Mauna Kea, Hawaii
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Diffuse clouds n 101103 cm-3 10-15 Torr T
60 K
Stellar Outflows n 108 ? cm-3 T 2000 ?
K dm/dt 2 tons/fs
Dense clouds n 104106 cm-3 T 20 K
Leão et al., AA 455, 187 (2006)
0.5 light years
Pound ApJ 493, L113 (1998)
Photo Jose Fernandez Garcia
47
Acknowledgments
Visit us at http//bjm.scs.uiuc.edu
NSF Division of AMO Physics
Dreyfus New Faculty Award
NASA Laboratory Astrophysics
NSF Divisions of Chemistry Astronomy
Packard Fellowship
Air Force Young Investigator Award
Cottrell Scholarship