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