The Influence of Free-Running FP-QCL Frequency Jitter on Cavity Ringdown Spectroscopy of C 60

1. The Influence of Free-Running FP-QCL Frequency Jitter on Cavity Ringdown Spectroscopy of C60 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

2. Why do we care about FP-QCL frequency jitter? • Why high resolution C60 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 C60 vapor • An expansion source capable of producing “cold” C60 molecules • Tunable laser source ~1184 cm-1

3. Why do we care about FP-QCL frequency jitter? • Laser linewidth requirements • Big molecule, small rotational constant ~0.0028 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.

4. 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

5. Power Supply Stability Issues • Not initially apparent-back reflection issues • Mid-IR wavemeter provided real time information • Characterize linewidth using rovibrational transitions in ν1 band of SO2 • Lightwave LDX-3232 preferable to Keithley 2420 3A

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

7. SO2 Direct Absorption: Pressure Broadening Studies Example Scans taken with LDX-3232

8. SO2 Direct Absorption: Pressure Broadening Studies Using LDX-3232 Sumpf, B. J Mol. Struct., 599, 39 (2001)

9. “Low” Pressure SO2 Spectra Lightwave LDX-3232 23 MHz Steps 880 mTorr SO2 Keithley 2420 3A 54 MHz Steps 660 mTorr SO2

10. Measurement of N2O Line 0420-0220 R(49f) Noise Stdev~1.4x10-8 cm-1

11. Revisiting CH2Br2? • Why CH2Br2 • Vibrational band falls within frequency coverage of QCL (1178-1201 cm-1) • Test molecule for beam overlap optimization • Rotational temperature probe v8 –CH2 wag ~1197 cm-1

12. CH2Br2 Example Scanning Window qQ7 79_81 qQ6 79_81 qQ7 81_81 qQ7 79_79 qQ8 79_79 qQ6 81_81

13. Old vs New CH2Br2 Spectra *2007 Spectrum Shifted ~210 MHz

14. Future Directions • Optimize overlap of expansion source with laser beam. • Adjust expansion conditions and collect additional CH2Br2 spectra • Turn on high temperature oven and observe impact on CH2Br2 Trot • Begin scanning for C60

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

17. Current CH2Br2 Coverage