The Influence of Free-Running FP-QCL Frequency Jitter on Cavity Ringdown Spectroscopy of C 60
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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
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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
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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.
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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
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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
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Experimental Set-upQuantum Cascade Laser
Mid-IR cw Wavemeter
High finesse cavity
Oven &Supersonic Expansion
1
Acousto-Optic Modulator
Detector
Bristol
Mode-matching optics
Direct A
bsorption Cell
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SO2 Direct Absorption: Pressure Broadening Studies
Example Scans taken with LDX-3232
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SO2 Direct Absorption: Pressure Broadening Studies
Frequency (cm-1)
J’Ka’,Kc’ J”Ka”,Kc”
γself expt (HWHM cm-1/atm)
uncertaintyHITRAN γself
1197.061 348,26 337,27 0.156 0.022 0.39
1196.969 446,38 435,39 0.191 0.020 0.39
1196.953 437,37 426,36 0.216 0.024 0.39
1196.844 2110,12 209,11 0.153 0.007 0.39
1196.811 279,19 268,18 0.168 0.010 0.39
1196.982 1814,4 1813,5 0.115 0.019 0.39
1196.925 1914,6 1913,7 0.096 0.016 0.39
Using LDX-3232
Sumpf, B. J Mol. Struct., 599, 39 (2001)
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“Low” Pressure SO2 Spectra2Laser
2Doppler
2Exptl ΔνΔνΔν
Lightwave LDX-3232 23 MHz Steps880 mTorr SO2
Keithley 2420 3A54 MHz Steps660 mTorr SO2
MHz 12120ΔνExptl MHz 55ΔνDoppler
2Doppler
2ExptlLaser ΔνΔνΔν
MHz 11108ΔνLaser
MHz 562ΔνExptl
MHz 229ΔνLaser MHz 55ΔνDoppler
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Measurement of N2O Line
Noise Stdev~1.4x10-8 cm-1 042 0
-022 0
R(4
9f)
MHz 1~20ν
MHz 67ν
MHz 270ν
Laser
Doppler
Exptl
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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
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CH2Br2 Example Scanning Window
q Q6 79
_81
q Q7 7
9_81
q Q7 7
9_79
q Q8 79
_79
q Q6 8
1_81
q Q7 81
_81
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Old vs New CH2Br2 Spectra
*2007 Spectrum Shifted ~210 MHz
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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
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Acknowledgements
$$$ Funding $$$NASA Laboratory AstrophysicsPackard FoundationDreyfus FoundationUniversity of Illinois
PeopleBrian SillerRich Saykally
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Current CH2Br2 Coverage