The Influence of Free-Running FP- QCL Frequency Jitter on Cavity Ringdown Spectroscopy of C 60 Brian E. Brumfield* Jacob T. Stewart* Matt D. Escarra** Claire Gmachl ** Benjamin McCall ‡ *Department of Chemistry, University of Illinois, Urbana, IL **Department of Electrical Engineering, Princeton University, Princeton Institute for the Science and Technology of Materials, Princeton, NJ ‡Departments of Chemistry and Astronomy, University of Illinois, Urbana, IL

Why do we care about FP-QCL frequency jitter?  Why high resolution C 60 spectroscopy? Supporting evidence for presence in ISM  Found in experiments simulating carbon star outflows  44 eV to break cage  Presence in meteorite impact sediments How to verify?  IR allowed vibrational band ~1184 cm -1  Allowed band is in atmospheric window allowing ground based observations  What’s necessary? High temperature oven to generate C 60 vapor An expansion source capable of producing “cold” C 60 molecules Tunable laser source ~1184 cm -1

Why do we care about FP-QCL frequency jitter?  Laser linewidth requirements Big molecule, small rotational constant ~ cm -1 (~90 MHz) Doppler linewidths from expansion source sub-100 MHz  To achieve the best desired resolution and sensitivity we want a narrow laser linewidth.

QCL Laser Linewidths  Distributed feedback (DFB)-QCLs 1-10 MHz linewidths  Heterodyne measurements  External cavity (EC)-QCL ~30 MHz linewidths  Analysis of lineshapes in absorption spectra  Frequency stability issues Temperature stability Power supply stability

Power Supply Stability Issues  Not initially apparent- back reflection issues  Mid-IR wavemeter provided real time information  Characterize linewidth using rovibrational transitions in ν 1 band of SO 2  Lightwave LDX-3232 preferable to Keithley A

Experimental Set-up Quantum Cascade Laser Mid-IR cw Wavemeter High finesse cavity Oven & Supersonic Expansion 1 Acousto-Optic Modulator Detector Bristol Mode-matching optics Direct Absorption Cell

SO 2 Direct Absorption: Pressure Broadening Studies Example Scans taken with LDX-3232

SO 2 Direct Absorption: Pressure Broadening Studies Frequency (cm -1 ) J’ Ka’,Kc’ J” Ka”,Kc” γ self expt (HWHM cm -1 /atm) uncertainty HITRAN γ self , , , , , , , , , , , , , , Using LDX-3232 Sumpf, B. J Mol. Struct., 599, 39 (2001)

“Low” Pressure SO 2 Spectra Lightwave LDX MHz Steps 880 mTorr SO 2 Keithley A 54 MHz Steps 660 mTorr SO 2

Measurement of N 2 O Line Noise Stdev~1.4x10 -8 cm R(49f)

Revisiting CH 2 Br 2 ?  Why CH 2 Br 2 Vibrational band falls within frequency coverage of QCL ( cm -1 ) Test molecule for beam overlap optimization Rotational temperature probe v 8 –CH 2 wag ~1197 cm -1

CH 2 Br 2 Example Scanning Window q Q 6 79_81 q Q 7 79_81 q Q 7 79_79 q Q 8 79_79 q Q 6 81_81 q Q 7 81_81

Old vs New CH 2 Br 2 Spectra *2007 Spectrum Shifted ~210 MHz

Future Directions  Optimize overlap of expansion source with laser beam.  Adjust expansion conditions and collect additional CH 2 Br 2 spectra  Turn on high temperature oven and observe impact on CH 2 Br 2 T rot  Begin scanning for C 60

Acknowledgements $$$ Funding $$$ NASA Laboratory Astrophysics Packard Foundation Dreyfus Foundation University of Illinois People Brian Siller Rich Saykally

Current CH 2 Br 2 Coverage