High Resolution Mid-IR Spectroscopy With Quantum Cascade Lasers

Introduction

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

In the McCall group we have constructed a Fabry-Perot quantum cascade laser (FP-QCL) based continuous-wave cavity ringdown (cw-CRD) spectrometer coupled to a high-temperature oven supersonic expansion source. Development of the instrument is the result of a collaboration with Professor Claire Gmachl's group at Princeton University. The primary purpose of the spectrometer is to enable high resolution rovibrational spectroscopy of a rotationally and vibrationally cold gas phase sample of Buckminsterfullerene (C₆₀) around ~8.5 microns. In addition to the C₆₀ spectroscopic search, we are continuing to develop our instrument for other high resolution gas phase mid-IR spectroscopy projects.

C₆₀ Spectroscopic Interest

C₆₀ is of fundamental spectroscopic interest because of its high degree of symmetry. C₆₀ is a member of the I_h point group. Because of the 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 composed entirely 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. Because of these symmetry considerations we expect missing transitions in the rovibrational band due to missing levels in the ground state with forbidden symmetry.

C₆₀ Astronomical Interest

C₆₀ was discovered in experiments of carbon-rich stellar outflows. Carbon-rich stars are therefore a likely source of C₆₀ that could diffuse into interstellar space. After the laboratory discovery, C₆₀ has been found in meteorite impact sediments and associated with impact craters on the Long Duration Exposure Facility. Though the C₆₀ found in both of these sources cannot conclusively be identified as extraterrestial in origin, they do provide indirect evidence for the possibility of C₆₀ outside of the solar system. Recently, the first astronomical detection of C₆₀ was made through infrared emission observations from the protoplanetary nebula Tc1. The detected C₆₀ was found to be embedded in dust grains. A high-resolution gas-phase spectrum will provide an essential roadmap for identifying spectral features belonging to gas-phase C₆₀ in an astronomical search. Detections of gas phase C₆₀ in other environments will aid in understanding the interstellar chemistry of this unique molecular species.

High Resolution Molecular Spectroscopy

Comparison between CH₂Br₂ spectra recorded with a slit nozzle (top) and a pinhole nozzle (bottom). Click for full size.

Methylene Bromide (CH₂Br₂) Spectroscopy

The ν₈ band of methylene bromide (-CH₂ wag ~1197 cm^-1) has been observed to test the performance of the cw-CRD spectrometer. It has also been developed as a tool to optimize the overlap between the supersonic expansion and the high-finesse cavity axis. The heavy mass of the bromine atoms in the molecule make methylene bromide a near prolate top, providing an easily assignable spectrum. However, due to the nearly 1:1 isotopic abundance of ⁷⁹Br to ⁸¹Br, any spectrum from an non-isopotically pure sample of the liquid will result in three separate spectral bands closely overlaid on one another. Spectroscopy of a the P and Q branch of the vibrational band has been carried out with a pinhole nozzle. Sufficient assignment work has been carried out on those spectra to allow for a determination of the different vibrational band centers and upper state A rotational constants for all three isotopomers. Additional spectra covering the P, Q, and R branches of the vibrational band have been acquired using a slit nozzle. Spectral assignment of slit-jet spectra is ongoing.

Section of pyrene R branch acquired with slit nozzle. Click for full size.

Pyrene (C₁₆H₁₀) Spectroscopy

To continue the instrument development we have begun studying the seeding and vibrational cooling of larger target molecules in our heated oven supersonic expansion source. One such molecule that is within the spectral coverage of our FP-QCLs is the polycyclic aromatic hydrocarbon pyrene. Sections of the R-branch of the ν₁₉ vibrational mode (in plane hydrogen wag) have been recorded using a slit nozzle with the oven heated to 120 Celsius.

Instrument Development

High Temperature Supersonic Expansion Source

High temperature oven at 600 °C.

To generate a detectable amount of gas-phase C₆₀ for our spectrometer we need to heat our sample to very high temperatures because the vapor pressure of C₆₀ is very low at normal temperatures. We have constructed a high temperature oven that is capable of operating continuously at temperatures up to 700°C for at least 24 hours. After generating hot C₆₀ gas we then cool the molecules using a supersonic expansion with Ar as the carrier gas. Our design allows us to use different nozzle types for our supersonic expansion by simply replacing a flange on the front of the oven.

Ringdown Mirror Mounts

Cavity ringdown spectroscopy is sensitive to slight changes in alignment of the ringdown mirrors. Because of this, heating of the vacuum chamber due to the presence of the hot oven spoils our alignment if the high reflectivity mirrors are rigidly mounted to the vacuum chamber. We have constructed ringdown mirror mounts designed by Andrew Mills to avoid this problem. The new mounts are rigidly attached to the platforms on either side of the vacuum chamber rather than to the chamber itself. The mounts connect to the chamber via flexible bellows to reduce vacuum chamber vibrations from our pumping system from being coupled into the ringdown mirror mounts. The mounts also mitigate the thermal expansion issues that spoil the alignment of the ringdown cavity.

Diagram of new ringdown mirror mounts attached to vacuum chamber.

Fresnel Rhomb

One problem with using QCLs with cavity ringdown spectroscopy is that QCLs become unstable when exposed to optical back-reflection. When performing cavity ringdown spectroscopy, much of the time the light from the laser is reflected from the ringdown mirror back at the QCL, which destabilizes the laser and can cause mode-hops. When this happens, the laser is not useful for high-resolution spectroscopy. With visible and near-IR lasers, this problem can be solved by using an optical isolator based on a Faraday rotator. Faraday rotators are not well-developed in the mid-IR, which makes a typical optical isolator impractical for our purposes. Instead, we have implemented a ZnSe Fresnel rhomb in conjunction with a wire-grid polarizer to act as a mid-IR optical isolator. First, the light from the laser is passed through a wire-grid polarizer which transmits the QCL light because it is linearly polarized. Next, the light enters the Fresnel rhomb, which allows us to convert linearly polarized light into circularly polarized light. When this light is reflected from the ringdown mirror, the direction of the circular polarization switches and when the back-reflected light enters the rhomb, it is reconverted to linearly polarized light rotated 90 degrees from the original polarization. This light is then blocked by the wire-grid polarizer which prevents it from reaching the QCL and destabilizing the laser.

Diagram showing experimental setup of Fresnel rhomb-based optical isolator. Click for full size.

Related Content

Papers

58	B. E. Brumfield, J. T. Stewart, and B. J. McCall "High-Resolution Spectroscopy of the ν₈ Band of Methylene Bromide Using a Quantum Cascade Laser" Journal of Molecular Spectroscopy (2011), 266, 57-62.
49	B. E. Brumfield, J. T. Stewart, S. L. Widicus Weaver, M. D. Escarra, S. S. Howard, C. F. Gmachl, and B. J. McCall "A Quantum Cascade Laser cw Cavity Ringdown Spectrometer Coupled to a Supersonic Expansion Source" Review of Scientific Instruments (2010), 81, 063102.

Proceedings

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

100	J. T. Stewart, B. E. Brumfield and B. J. McCall "Rotationally-resolved Infrared Spectroscopy of the Polycyclic Aromatic Hydrocarbon Pyrene (C₁₆H₁₀) Using a Quantum Cascade Laser-based Cavity Ringdown Spectrometer" Sixty-Sixth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2011.
96	J. T. Stewart, B. E. Brumfield and B. J. McCall "High-resolution Mid-infrared Spectroscopy of Deuterated Water Clusters Using a Quantum Cascade Laser-based Cavity Ringdown Spectrometer" Sixty-Sixth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2011.
82	J. T. Stewart, B. E. Brumfield, M. D. Escarra, C. F. Gmachl and B. J. McCall "High-Resolution Spectroscopy of the ν₈ Band of Methylene Bromide Using a Quantum Cascade Laser-Based Cavity Ringdown Spectrometer" Sixty-Fifth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2010.
73	B. E. Brumfield, J. T. Stewart, M. D. Escarra, C. F. Gmachl and B. J. McCall "Astrochemistry and Spectroscopy of C₆₀: The search for the 8.5 μm Vibrational Band" 239th American Chemical Society National Meeting, San Francisco, CA, 2010.
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

25	J. T. Stewart, B. E. Brumfield and B. J. McCall "Rotationally-Resolved Infrared Spectroscopy of the Polycyclic Aromatic Hydrocarbon Pyrene (C₁₆H₁₀)" Midwest Astrochemistry Meeting, University of Illinois, Urbana, IL, 2011.
21	B. E. Brumfield, J. T. Stewart and B. J. McCall "Continued Development of a Sensitive Mid-IR Cavity Ring-down Spectrometer Using a Quantum Cascade Laser" Midwest Astrochemistry Meeting, University of Illinois, Urbana, IL, 2010.
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

26	B. E. Brumfield "Development of a Quantum Cascade Laser Based Spectrometer for High Resolution Spectroscopy of Gas Phase C₆₀" Ph.D. Thesis, University of Illinois, 2011.
25	J. T. Stewart "High-Resolution Gas Phase Infrared Spectroscopy of Water Clusters and Cluster Ions Using Quantum Cascade Lasers" Research Prospectus for Preliminary Examination, University of Illinois, 2011.
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.