a search for the 8.5 m vibrational spectrum of c 60 in the laboratory and space susanna l. widicus...
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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
Kroto et al., Nature 318, 162 (1985)
The discovery of C60
Laboratory experiments designed to simulate carbon star outflows
Di Brozolo et al., Nature 369, 37 (1994)
Becker et al., Science 291, 1530 (2001)
C60 in space?
• 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
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!
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
Experimental Setup
Cryostat with QCL
Asphericlens
Mode-matching opticsFocusing optics
& detector
AOM
Reference cell
High finesse cavity
Oven and supersonic expansion
To Roots pump
C60
Argon carrier gas
Strip heaters
C60 + Ar
C60 Oven
C60
sample
Aluminum radiation shieldT > 600 ºC!
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)
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.
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
Fine tuning with current ~ 2 cm-1
Laser current (Amps)Coarse tuning with temperature ~10 cm-1
N2OHITRAN
QCL Scanning
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
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
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
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