Vibrational Spectroscopy of C₆₀

The Fabry-Perot quantum cascade laser used to probe the spectrum of C₆₀ at 8.4 microns.

Spectroscopic Interest

C₆₀ is a member of the I_h point group. Due to its high symmetry only 12 out of 174 vibrational modes (the 4 triply degenerate F_1u representations) are IR active. Additional symmetry consequences arise given an isotopically pure molecule of C₆₀ composed of spin-0 (¹²C) bosons. Only certain rotational levels in the ground state are allowed to exist based upon symmetry constraints laid out by the Pauli principle. As such we anticipate missing transitions in the rovibrational band due to these missing rotational levels in the ground state with forbidden symmetry.

The high-temperature oven used to create vapor-phase C₆₀, along with the supersonic expansion nozzle that rotationally cools the gas.

Astronomical Interest

C₆₀ was discovered during experiments trying to understand the chemistry that could lead to long carbon chain molecules in carbon-rich stellar outflows. This lends support to the idea of carbon rich stars providing a source of C₆₀. Considering the stability of C₆₀ against photofragmentation it is possible that a detectable column density of C₆₀ is available for ground based infrared telescope searches. A high-resolution gas-phase spectrum will provide an essential roadmap for identifying spectral features belonging to C₆₀ in such an astronomical search.

A rotationally-resolved cavity ringdown spectrum of methylene bromide acquired with a FP-QCL at ~8.5 micron.

Experimental Details

The experimental setup couples a high temperature oven with a supersonic pinhole source. Argon carrier gas is forced through the oven at high pressures through a crushed sample of solid C₆₀. The oven reaches a temperature of ~600 Celsius in order to produce an adequate amount of C₆₀ vapor. The argon carrier gas and C₆₀ vapor are then passed through a pinhole opening and supersonically expanded into a chamber under vacuum. This provides necessary rotational and vibrational cooling for the thermally hot C₆₀ molecules exiting the pinhole. The supersonic expansion is then probed with the highly sensitive continuous wave cavity ringdown spectroscopic technique. Our tunable light source is a Fabry-Perot quantum cascade laser which provides mid-IR tuning coverage around the suspected rovibrational band center of interest in C₆₀ (~1184 cm^-1). As a test molecule for the spectrometer and probe of expansion conditions, we have acquired spectra of methylene bromide (CH₂Br₂) at ~1197 cm^-1.

The C₆₀ project is currently being worked on by Brian Brumfield, Jacob Stewart, and Brian Siller.

Related Content

Proceedings

13

S. L. Widicus Weaver, B. E. Brumfield, S. Howard, C. Gmachl and B. J. McCall "A Laboratory and Observational Search for the Vibrational Spectrum of C60" Proceedings of International Conference on Molecules in Space and Laboratory, Eds. J. L. Lemaire and F. Combes, (2007), 1-4.

Talks

69	B. E. Brumfield, J. T. Stewart, M. D. Escarra, C. Gmachl and B. J. McCall "The Influence of Free-Running FP-QCL Frequency Jitter on Cavity Ringdown Spectroscopy of C60" Sixty-Fourth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2009.
53	B. E. Brumfield, S. S. Howard, C. F. Gmachl, D. K. Wilson, M. Percevault, and B. J. McCalll "Development and Implementation of Optical Isolation for a Continuous Wave Fabry Perot Quantum Cascade Laser (CW-FP-QCL) at 8.5 um Using an Experimental Faraday Rotator" Sixty-Third International Symposium on Molecular Spectroscopy, Ohio State University, Columbus, OH, 2008.
40	S. L. Widicus Weaver, B. E. Brumfield, A. A. Mills, S. S. Howard, C. F. Gmachl, and B. J. McCall "A Search for the 8.5 Micron Vibrational Spectrum of C60 in the Laboratory and Space" Sixty-Second International Symposium on Molecular Spectroscopy, Ohio State University, Columbus, OH, 2007.
39	B. E. Brumfield, S. L. Widicus Weaver, S. S. Howard, C. F. Gmachl, and B. J. McCall "Cavity Ringdown Spectrum of the v8 Band of Methylene Bromide Using a Quantum Cascade Laser" Sixty-Second International Symposium on Molecular Spectroscopy, Ohio State University, Columbus, OH, 2007.
32	S. L. Widicus Weaver, B. E. Brumfield, S. Howard, C. Gmachl, and B. J. McCall "A Laboratory and Observational Search for the Vibrational Spectrum of C60" Molecules in Space and Laboratory, Paris, France, 2007.
29	S. L. Widicus Weaver, M. C. Zwier, Y. Ding, and B. J. McCall "Laboratory and Observational Studies of C60 and C60+" American Chemical Society National Meeting, Atlanta, Georgia, 2006.

Posters

15	B. E. Brumfield, J. T. Stewart and B. J. McCall "Mid Infrared Continuous Wave Cavity Ringdown Spectrometer for Acquisition of the High-Resolution Spectrum of C60" Second Midwest Astrochemistry Meeting, University of Illinois, 2009.
10	B. E. Brumfield, B. Siller and B. J. McCall "Towards Acquisition of a High Resolution Gas Phase Spectrum of C60" Inaugural Midwest Astrochemistry Meeting, University of Illinois, 2008.
5	B. E. Brumfield, S. L. Widicus Weaver, S. S. Howard, C. F. Gmachl, and B. J. McCall "Continuous Wave Cavity Ringdown Spectroscopy of C60 at 8.5 Microns Using a Quantum Cascade Laser and a Supersonic Expansion Source" American Chemical Society National Meeting, Chicago, Illinois, 2007.

Other Publications

14

B. E. Brumfield "High-Resolution Spectroscopic Studies of C60 and C6H7+: Molecules of Fundamental Spectroscopic and Astrochemical Importance" Research Prospectus for Preliminary Examination, University of Illinois, 2007.