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

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

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.

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 ν1 band of SO2

Lightwave LDX-3232 preferable to Keithley 2420 3A

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

SO2 Direct Absorption: Pressure Broadening Studies

Example Scans taken with LDX-3232

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)

“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

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

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

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

Old vs New CH2Br2 Spectra

*2007 Spectrum Shifted ~210 MHz

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

Acknowledgements

                                    

$$$ Funding $$$NASA Laboratory AstrophysicsPackard FoundationDreyfus FoundationUniversity of Illinois

PeopleBrian SillerRich Saykally

Current CH2Br2 Coverage