A Search for the 8.5  m Vibrational Spectrum of C 60 in the Laboratory and Space Susanna L. Widicus Weaver 1, Brian E. Brumfield 1, Andrew A. Mills 1, Scott Howard 2, Claire Gmachl 2, and Benjamin J. McCall 1 1 Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign 2 Department 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 C 60 Laboratory experiments designed to simulate carbon star outflows

Di Brozolo et al., Nature 369, 37 (1994) Becker et al., Science 291, 1530 (2001) C 60 in space?

3(60)-6 = 174 vibrational degrees of freedom Sixty quantum-mechanically indistinguishable (spin 0) bosons Icosahedral (I h ) Symmetry: 6 five-fold axes, 10 three-fold axes, 15 two-fold axes Symmetry restrictions on total wavefunction 4 F 1u IR active modes About C 60

Previous laboratory studies of C 60 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-H 2 matrix Sogoshi et al. J. Phys. Chem. 104, 2000 F 1u (3) 13 C 12 C 59 ? A rotationally cold, resolved, gas phase C 60 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 C 60 Rotational resolution Tunability at 1184 cm -1 Sensitivity Vibrationally and rotationally cold C 60 Gas phase C 60

Experimental Setup Cryostat with QCL Aspheric lens Mode-matching optics Focusing optics & detector AOM Reference cell High finesse cavity Oven and supersonic expansion To Roots pump

C 60 Argon carrier gas Strip heaters C 60 + Ar C 60 Oven C 60 sample Aluminum radiation shield T > 600 ºC!

Supersonic Expansion Adiabatically cools the sample gas by converting random thermal motion into directed flow 0.7 mm pinhole source P 0 /P 1 ~ 1.7 × 10 4 CH 2 Br 2 N2+N2+ N 2 O HITRAN FWHM = 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 for Thermal Conductivity but Mechanical Isolation “Sample Mount” Armature for Mechanical Rigidity On Reverse: Heater & Temp. Sensor Laser Mount Janis VPF-100 QCLs from the Gmachl Group Common Ground Plate Individual Lasers Wires Pads for Bias Voltage Laser Emission

Fine tuning with current ~ 2 cm -1 Laser current (Amps) Coarse tuning with temperature ~10 cm -1 N 2 O HITRAN QCL Scanning

What will the C 60 band look like? T = 10 KT = 20 K T = 50 K Simulated observational spectrum At T = 30 K and N = 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 ~3×10 15 cm -2 < 0.6% of carbon

Acknowledgments NSF CHE ACS UIUC Brian Brumfield Matt Richter & Dana Nuccitelli (UC Davis) Rich Saykally (UC Berkeley) NASA Laboratory Astrophysics The McCall Group Brett McGuire Brian Pohrte (not pictured) Packard Dreyfus

Laser current (Amps) N 2 O HITRAN 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