a search for the 8.5 m vibrational spectrum of c 60 in the laboratory and space susanna l. widicus...
TRANSCRIPT
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A Search for the 8.5 m Vibrational Spectrum of C60 in the Laboratory and Space
Susanna L. Widicus Weaver1, Brian E. Brumfield1, Andrew A. Mills1, Scott Howard2, Claire Gmachl2,
and Benjamin J. McCall1
1 Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign
2Department of Electrical Engineering, and the Princeton Institute for the Science and
Technology of Materials, Princeton University, Princeton, New Jersey 08544, USA
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Kroto et al., Nature 318, 162 (1985)
The discovery of C60
Laboratory experiments designed to simulate carbon star outflows
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Di Brozolo et al., Nature 369, 37 (1994)
Becker et al., Science 291, 1530 (2001)
C60 in space?
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• 3(60)-6 = 174 vibrational degrees of freedom
• Sixty quantum-mechanically indistinguishable (spin 0) bosons
• Icosahedral (Ih) Symmetry: 6 five-fold axes, 10 three-fold axes, 15 two-fold axes
• Symmetry restrictions on total wavefunction
• 4 F1u IR active modes
About C60
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Previous laboratory studies of C60
Gas phase IR emission spectrum observed at 1065 K; no rotational structure resolved
Frum et al. Chem. Phys. Lett. 176, 1991
IR spectrum observed in p-H2 matrix
Sogoshi et al. J. Phys. Chem. 104, 2000
F1u(3)
13C12C59?
A rotationally cold, resolved, gas phase C60 spectrum is needed to guide observational searches!
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What do we need?
Supersonic expansion source
High temperature oven (>600 ºC)
Supersonic source
Continuous-wave cavity ringdown spectroscopy (cw-CRDS)
Continuous-wave quantum cascade laser (cw-QCL)
• Gas phase C60
• Rotational resolution
• Tunability at 1184 cm-1
• Sensitivity
• Vibrationally and rotationally cold C60
• Gas phase C60
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Experimental Setup
Cryostat with QCL
Asphericlens
Mode-matching opticsFocusing optics
& detector
AOM
Reference cell
High finesse cavity
Oven and supersonic expansion
To Roots pump
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C60
Argon carrier gas
Strip heaters
C60 + Ar
C60 Oven
C60
sample
Aluminum radiation shieldT > 600 ºC!
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Supersonic Expansion
Adiabatically cools the sample gas by converting random thermal
motion into directed flow
0.7 mm pinhole sourceP0/P1 ~ 1.7×104
CH2Br2
N2+
N2OHITRAN
FWHM = 0.002 cm-1 (60 MHz)
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CW Cavity Ringdown Spectroscopy (cw-CRDS)
• A high finesse cavity is placed around the supersonic expansion.
• Laser light is coupled into the cavity, which is cycled in and out of resonance.
• When the cavity is on resonance the laser light is diverted or switched off.
• The exponential decay rate is a direct measurement of absorption.
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Cold Plate(77 K)
Copper Ribbon forThermal Conductivity
but Mechanical Isolation
“SampleMount”
Armature forMechanical
Rigidity
On Reverse:Heater &
Temp. Sensor
LaserMount
Janis VPF-100
QCLs from the Gmachl GroupCommon Ground Plate
Individual LasersWires
Pads for Bias Voltage
Laser Emission
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Fine tuning with current ~ 2 cm-1
Laser current (Amps)Coarse tuning with temperature ~10 cm-1
N2OHITRAN
QCL Scanning
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What will the C60 band look like?
T = 10 KT = 20 KT = 50 K
Simulated observational spectrumAt T = 30 K and N = 1016 cm-2
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Astronomical Search
Data obtained June 2003• R Coronae Borealis• AFGL 2136• AFGL 2591• NGC 7538 IRS 1
TEXES: Texas Echelon Cross Echelle Spectrograph
NASA's 3-meter IRTF (InfraRed Telescope Facility), Mauna Kea, Hawaii
Lacy et al., PASP 114, 153 (2002)
“Blind” upper limit
• ~3×1015 cm-2
• < 0.6% of carbon
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Acknowledgments
NSF CHE
ACS
UIUC
BrianBrumfield
Matt Richter &Dana Nuccitelli
(UC Davis)
Rich Saykally(UC Berkeley)
NASA Laboratory
AstrophysicsThe McCall Group
http://astrochemistry.uiuc.edu
BrettMcGuire
Brian Pohrte(not pictured)
Packard
Dreyfus
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Laser current (Amps)
N2OHITRAN
QCL Scanning Difficulties
Some QCLs are
inherently multi-mode.
Electronic chopping and back-reflection cause mode hops.
Solutions:• Single-mode laser• Acousto-optical modulator (AOM)• Optical isolator