High-Resolution Spectroscopy of the ν 8 Band of Methylene Bromide Using a Quantum Cascade Laser-Based Cavity Ringdown Spectrometer Jacob T. Stewart and Brian E. Brumfield, Department of Chemistry, University of Illinois at Urbana-Champaign Matthew D. Escarra and Claire F. Gmachl, Department of Electrical Engineering, Princeton University Benjamin J. McCall, Departments of Chemistry and Astronomy, University of Illinois at Urbana-Champaign

Why a Quantum Cascade Laser-Based Spectrometer? C 60 spectroscopy at ~8.5 μm in order to perform an astronomical search Quantum cascade lasers (QCLs) offer performance necessary for high-resolution spectroscopy at this wavelength

How do QCLs work? Semiconductor laser based on stacks of quantum wells Lasing occurs through transitions within the conduction band Different frequencies possible by changing thickness of quantum wells Each QCL has limited tunability

Testing our QCL Spectrometer Decided on methylene bromide as a test molecule No previous high- resolution work in IR Probe of temperature conditions in supersonic jet ν 8 band of CH 2 Br 2 ~1197 cm -1 Tuning range of our QCL ~ cm -1

Initial Layout of QCL Spectrometer PC-MCT 800 μm pinhole

Experimental Spectra and Spectrometer Performance Reference (SO 2 ) Spectrum Wavemeter CH 2 Br 2 Spectrum Noise equivalent absorption = 1.4×10 -8 cm -1 Linewidth = MHz Step size = ~21 MHz ( cm -1 ) Sensitivity = 5×10 -8 cm -1 Hz -1/2

Assigning the Spectrum Two Br isotopes with almost equal abundance ( 79 Br & 81 Br) Near-prolate top Ground state rotational constants known from microwave Fitting done using PGOPHER Three isotopologues: Abundance CH 2 79 Br 2 1 CH 2 79 Br 81 Br2 CH 2 81 Br PGOPHER, a Program for Simulating Rotational Structure, C. M. Western, University of Bristol,

Room Temperature Spectra

Simulated spectrum T rot = 7 K T rot = 300 K Combination Jet-Cooled Spectra Experimental Spectrum of Jet-Cooled Sample B. E. Brumfield, J. T. Stewart, S. L. Widicus Weaver, M. D. Escarra, S. S. Howard, C. F. Gmachl, B. J. McCall, Rev. Sci. Instrum. (2010), 81,

Molecular Constants Molecule ν 0 (cm -1 )A’ (cm -1 )Avg. |o-c| (cm -1 ) # lines assigned CH 2 79 Br (99) (22) CH 2 79 Br 81 Br (12) (28) CH 2 81 Br (12) (23)

Improvements to the Spectrometer Faster detector (PV- MCT from Kolmar) Using a 150 μm×12 mm slit instead of 800 μm pinhole Ten times smaller current step size New piezo driver to increase ringdowns per second New mirror mounts with flexible bellows PC-MCT PV-MCT

New and Improved Spectra Noise equivalent absorption = ~4×10 -9 cm -1 Linewidth = ~10 MHz Step size = ~2.4 MHz ( cm -1 ) Sensitivity = ~8×10 -9 cm -1 Hz -1/2 Not Noise!

Conclusions Quantum cascade lasers are useful light sources for high-resolution infrared spectroscopy We have constructed the first QCL-based cw- ringdown spectrometer coupled with a supersonic expansion source We have obtained and assigned the previously unobserved rotational structure of the ν 8 band of methylene bromide

What’s Next? Collect high-resolution spectrum of pyrene (C 16 H 10 ) Collect high-resolution spectrum of C 60

Dealing with Back-Reflection Fresnel rhomb uses total internal reflections to act like a quarter wave plate Laser Polarizer Ringdown Cavity ZnSe Frsenel Rhomb

Acknowledgments Brian Siller Andrew Mills Richard Saykally Kevin Lehmann Funding NASA Packard Foundation Dreyfus Foundation University of Illinois