CCH in Orion

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CCH in Orion
The CCH radical in the Orion Nebula (1974)
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Old spectrometer
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Filtered baseline
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Old discoveries
Some of the discoveries with free space
spectrometer (2-3 per year before 1995) C3H2
A free carbene, the first astronomical organic
ring SiCC Ionic ring, abundant in
IRC10216 SiC Defied detection for over 40
years Carbon chain radicals C2H, C3H, C4H, C5H,
C6H C3N HCCCO Carbon chain carbenes H2CCC H2CCCC

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Principles of FTM spectroscopy
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FTM spectrometer
FTM Spectrometer
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FTM lineshape Si3
I (29Si) 1/2
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New lab mols stick diags
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Summary of new mols
122 New Lab Molecules
No misassignments (abundant checks isotope
shifts, elemental composition, Zeeman effect,
hfs, etc.) Lab astrophysics complete (entire
radio spectrum known to 1 km/sec or better)
Lab detection now generally well ahead of the
astronomical (e.g., the cyanopolyynes HCnN)
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New lab mols
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New lab mols
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New lab mols
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Known interstellar mols
Cumulative total
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Radio identifications secure
Radio identifications are remarkably secure in
both the lab and space Specific, not generic.
Rotational lines in cold molecular gas a
very sharp fingerprint ?/?? 100,000 (vs lt 100
for UIR bands) Identifications therefore
possible with only a few astronomical lines.
Dozens typically observed. Rotational
isotope shifts large (typically 2-10) and highly
specific hfs and Zeeman effect for open shell
species Molecules extremely stiff, so to
first order rotate as rigid bodies. Rotational
spectra therefore determined by at most three
parameters the three principal moments of
inertia (or equivalently their reciprocals, the
three rotational constants A, B, C). For planar
molecules, the three rotation constants are
constrained by the Pythagorean Theorem to only
two, and for linear molecules to only one B.
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S3 and S4
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S3 predicted lines
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Io
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Whistlers mother
James McNeill Whistler (1834-1903) Arrangement
in Grey and Black
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Astron. Si mols
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SiC3 isomers
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Silicon Hydrides the Si2H2 Isomers
One of the simplest Si-H systems amenable to
spectroscopic study 3 low-lying singlet isomers
predicted
0 kcal/mol 8.7 kcal/mol 11.6
kcal/mol ??ma 0 D ma 0.85 D ma
0.1 D (Grev Schaefer 1992)
Two bridged forms detected by Bogey et al. in
early 1990s partial structure derived for
Si(H)SiH
30Si(H)30SiH
1 ??0
10 min
Relative Intensity
Frequency (MHz)
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Other Silicon Hydrides

• Little known about hydrides beyond Si2H2
• More than 40 unidentified lines detected in a
• wide survey (10.9-15.2 GHz) of dilute silane
  in Ne
• Carriers are likely molecules of the form Si2Hm
  mgt2
• The carrier of about 10 closely-spaced lines
  near    12.1 GHz is the nearly planar radical
  Si2H3

High-level calculations underway by Schaefer group
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Benzene derivatives
phenyl
o-benzyne
1.264(3) Å
ethylene CC 1.339 Å acetylene CºC 1.207 Å
1.390 Å
1.403 Å
Length characteristic of weak triple bond
1.404 Å
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GBT
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Any astron. molecule
Any astronomical molecule in the radio band can
probably be detected in the laboratory A
reasonable projection based on recent discoveries
and the very sensitive laboratory techniques now
available--NOT the statement that everything is
understood. There are still many hundreds of
weak unidentified lines, a few fairly strong
ones, and at least one harmonic progression from
a new molecule that defies laboratory detection
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Any astron. molecule
Any astronomical molecule in the radio band can
probably be detected in the laboratory Not at
all true in the infrared and optical
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UIR bands
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DIBs
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Recent collaborators
Recent Collaborators Michael McCarthy Carl
Gottlieb Maria Sanz Elizabeth Nathan Shazrene
Mohamed Sandra Bruenken Robert McMahon John
Stanton Sven Thorwirth
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s the
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?4428 in an O5If star (Schulte 9) in the Cyg OB2
association at 1700 pc, V 10.96 Massey
Thompson, AJ 101, 1408 (1991)
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Summary Any radio astronomical molecule can
now be detected in the laboratory. In space
as on Earth, when nature assembles large
structures, she makes use of the remarkable
properties of the carbon bond. The 135
molecules so far identified in molecular clouds
may be the smaller members of a very large
population of specific structures many of the
interstellar dust grains may be large
molecules. The so far unidentified optical
diffuse interstellar bands (DIBs), and the
unidentified infrared (UIR) bands at 3-14 microns
(some of which are plausibly assigned to
polycyclic aromatic hydrocarbon molecules, PAHs)
are the best current evidence for this
population.
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Total number as of xxxx xxx Laboratory
astrophysics complete in radio band
in sense that entire radio spectrum now known to
1 km/sec or better in equivalent radial
velocity Most have unexplored infrared and
optical spectra which should now be detectable.
Many are plausible DIB carriers
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Silicon Chemistry

• Si is heavily depleted onto grains, but owing
  to shock waves some is returned to gas-phase
• 10 silicon-bearing molecules detected
• Si is the fourth most cosmically abundant element
  after H and He
• in space
• Many ring and chain isomers are predicted to have
  comparable stability

HSiCN HSiNC Maria Sanz TJ06 H2CnSi
chains TJ07
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Rhomboidal SiC3
1.435 Å
1.832 Å
1.490 Å
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Advantages of FTM
ADVANTAGES OF FTM SPECTROSCOPY Improved
sensitivity--(by 1-2 orders of magnitude) High
spectral resolution characteristic of molecular
beam spectroscopy Much lower gas flow (at least
50-fold--important for nasty precursor molecules
like silane and diacetylene DISADVANTAGES Limi
ted spectral range 3-40 GHz (vs. 50-850 GHz
with free space spectrometer) Double-horned
line shape (with axial beam)