the Vibrational Spectrum of C 60

Transcription

1 A Laboratory and Observational Search for the Vibrational Spectrum of C 60 Susanna L. Widicus Weaver 1, Brian E. Brumfield 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

2 Kroto et al., Nature 318, 162 (1985) The discovery of C 60 Laboratory experiments designed to simulate carbon star outflows

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

4 About C 60 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

5 IR Spectroscopy of C 60 4 F 1u IR active modes at 1432, 1183, 577, 528 cm -1 Simulated Laboratory Spectrum at 10 K Simulated Interstellar Spectrum at 30 K and N = cm -2

6 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 F 1u (3) 13 C 12 C59? IR spectrum observed in p-h 2 matrix Sogoshi et al. J. Phys. Chem. 104, 2000 A rotationally cold, resolved, gas phase C 60 spectrum is needed to guide observational searches!

7 What do we need? Gas phase C 60 High temperature oven (>600 ºC) Vibrationally and rotationally cold C 60 Supersonic source Sensitivity Continuous-wave cavity ringdown spectroscopy (cw-crds) Rotational resolution Supersonic source, high-resolution cw-crds Tunability at 1184 cm -1 Continuous-wave quantum cascade laser (cw-qcl)

8 Cryostat with QCL Aspheric lens Experimental Setup Oven and supersonic expansion Mode-matching optics AOM High finesse cavity Focusing optics and detector

9 To Roots pump Experimental Setup

10 Vapor Pressure of C 60 Need high T to vaporize But large partition function! Piacente et al., JPC 99, (1995) ~750 ºC

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

12 Supersonic Expansion Adiabatically cools the sample gas by converting random thermal motion into directed flow CH 2 Br mm pinhole source P 0 /P 1 ~ N 2 +

13 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.

14 cw-crds with a Quantum Cascade Laser (QCL) Invented at Bell Labs (1994) Multiple quantum wells made using molecular beam epitaxy Ordinary Diode Laser Conduction Band Energy Valence Band hν= E gap Emission depends on well depth & width: Particle in a box Energy Fabricate to desired λ Quantum Cascade Laser Position

15 QCLs from the Gmachl Group Pads for Bias Voltage Common Ground Plane Wires Individual Lasers Laser Emission

16 Janis VPF-100 Liquid Nitrogen Cryostat Copper Ribbon for Thermal Conductivity but Mechanical Isolation Laser Mount Sample Mount Cold Plate (77 K) Armature for Mechanical Rigidity On Reverse: Heater & Temp. Sensor

17 Resolution N 2 O HITRAN FWHM = cm -1 (60 MHz) Laser current (Amps)

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

19 What do we have? Gas phase C 60 Vibrationally and rotationally cold C 60 Sensitivity Rotational resolution Tunability at 1184 cm -1? Not mode hop free!

20 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) Polarization optics N 2 O HITRAN Laser current (Amps)

21 What will the C 60 band look like? T = K Simulated observational spectrum At T = 30 K and N = cm -2

22 Astronomical Search Data obtained June 2003 R Coronae Borealis AFGL 2136 AFGL 2591 NGC 7538 IRS 1 TEXES: Texas Echelon Cross Echelle Spectrograph Lacy et al., PASP 114, 153 (2002) NASA's 3-meter IRTF (InfraRed Telescope Facility), Mauna Kea, Hawaii

23 Blind upper limit ~ cm -2 Astronomical Search < 0.6% of carbon R Coronae Borealis

24 Implications? Where is the C 60? What comprises the other 99.4% of the carbon budget in the ISM?

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