Transcript SLWW_C60_b.ppt

A Search for the 8.5 mm 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
2Department
of Chemistry and Astronomy, University of Illinois at Urbana-Champaign
of Electrical Engineering, and the Princeton Institute for the Science and
Technology of Materials, Princeton University, Princeton, New Jersey 08544, USA
The discovery of C60
Laboratory experiments
designed to simulate
carbon star outflows
Kroto et al., Nature 318, 162 (1985)
C60 in space?
Di Brozolo et al.,
Nature 369, 37 (1994)
Becker et al.,
Science 291, 1530 (2001)
About C60
•
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
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
F1u(3)
13C12C ?
59
IR spectrum observed in p-H2 matrix
Sogoshi et al. J. Phys. Chem. 104, 2000
A rotationally cold, resolved, gas
phase C60 spectrum is needed to
guide observational searches!
What do we need?
• Gas phase C60
High temperature oven (>600 ºC)
• Vibrationally and rotationally cold C60
Supersonic expansion source
• Rotational resolution
Supersonic source
• Sensitivity
Continuous-wave cavity ringdown spectroscopy (cw-CRDS)
• Tunability at 1184 cm-1
Continuous-wave quantum cascade laser (cw-QCL)
Experimental Setup
Cryostat
with QCL
Aspheric
lens
Oven and
supersonic expansion
Mode-matching optics
AOM
Focusing optics
& detector
High finesse cavity
To Roots pump
Reference
cell
C60 Oven
Argon carrier gas
Strip heaters
C60
sample
C60
C60 + Ar
T > 600 ºC!
Aluminum radiation shield
Supersonic Expansion
Adiabatically cools the sample
gas by converting random
thermal motion into directed flow
CH2Br2
N2O
HITRAN
FWHM = 0.002 cm-1 (60 MHz)
0.7 mm pinhole source
P0/P1 ~ 1.7×104
N2+
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.
QCLs from the Gmachl Group
Pads for Bias Voltage
Common Ground Plate
Janis VPF-100
Wires
Individual Lasers
Laser Emission
Copper Ribbon for
Cold Plate
Thermal Conductivity
(77 K)
but Mechanical Isolation
Laser
Mount
“Sample
Mount”
Armature for
Mechanical
Rigidity
On Reverse:
Heater &
Temp. Sensor
QCL Scanning
N2O
HITRAN
Fine tuning with
current ~ 2 cm-1
Coarse tuning with Laser current (Amps)
temperature ~10 cm-1
What will the C60 band look like?
T = 20
10 K
50
Simulated observational spectrum
At T = 30 K and N = 1016 cm-2
Astronomical Search
“Blind” upper limit
• ~3×1015 cm-2
Data obtained June 2003
• < 0.6% of carbon
•
•
•
•
TEXES: Texas Echelon Cross Echelle Spectrograph
R Coronae Borealis
AFGL 2136
AFGL 2591
NGC 7538 IRS 1
NASA's 3-meter IRTF (InfraRed Telescope Facility), Mauna Kea, Hawaii
Lacy et al., PASP 114, 153 (2002)
Acknowledgments
Brett
McGuire
Brian
Brumfield
Brian Pohrte
(not pictured)
Matt Richter &
Dana Nuccitelli
(UC Davis)
NSF CHE
ACS
UIUC
The McCall Group
http://astrochemistry.uiuc.edu
Rich Saykally
(UC Berkeley)
Packard
Dreyfus
NASA
Laboratory
Astrophysics
QCL Scanning Difficulties
Some QCLs are
inherently multi-mode.
Electronic chopping and
back-reflection cause
mode hops.
N2O
HITRAN
Solutions:
• Single-mode laser
• Acousto-optical
modulator (AOM)
• Optical isolator
Laser current (Amps)