Frequency-comb referenced spectroscopy of v 4 =1 and v 5 =1 hot bands in the 1. 5 µm spectrum of C 2 H 2 Trevor Sears Greg Hall Talk WF08, ISMS 2015 Matt Cich Chris McRaven

Motivation Accurate measurement of vibration-rotation transitions in C 2 H 2 for remote sensing applications and evaluation of models[WF09 Talk] of energies and dynamics. Explore the effects of weak underlying features on line shape modeling. Instrument development.

ωrωr ω0ω0 Frequency comb referenced measurements ω rep and ω o are fixed to the GPS atomic clock standard, so optical frequency is accurately known. ω opt = n ω rep + ω 0 3 Lock external laser frequency to comb component ωrωr ω laser = n ω rep + ω 0 + ω laser-off ω0ω0 Use the comb as a standard frequency ruler by phase locking a spectroscopic laser to a single comb line. Currently limited to optical wavelengths between 1 and 2 microns.

Comb-Referenced Experiment Lock external laser to the cavity Cavity referenced to comb Stable spectrometer with precision and accuracy better than one part in Tune spectrometer by slightly varying the comb repetition frequency ECDL Comb Absorption cell 4 Resonant Cavity Finesse(f): ~ 430 Free spectral range (FSR): MHz

Comb-Referenced Experiment

Data Quality Typical measured derivative signal recorded at 1mTorr Fit to line profile model, Axner a or similar, to determine line center. Confirm the accuracy of the spectrometer: 3 kHz a O. Axner, et al. J. Quant. Spectrosc. Radiat. Transf. 68 (2001) b A.A. Madej, et al. J. Opt. Soc. Amer. B 23 (2006) MHz GHz Previous work b : GHz

Saturation Power Estimate saturation intensities, I s as: where Γ TT &  2 are 2π ×475 kHz and 6.22 × C 2 m 2 I s = 207 mW/mm 2, with beam waist of 0.36 mm (from transit-time broadening), the estimated saturation power is 83 mW.

Lineshape broadens with intracavity power Estimated saturation power is consistent with experimental observation Asymmetric line shape at high intracavity power is clear Saturation Power

Sub-Doppler measurements of hot bands in the 1. 5 µm region of C 2 H 2 Typical measured derivative signal recorded at 20 mTorr and intra-cavity power of mW. Can use higher powers because the linestrength and absorption are weaker. Fit to model line shape to determine line center. The accuracy between 3kHz and 22kHz

Sub-Doppler measurements of v 4 =1 and v 5 =1 hot bands 135 Transitions have been measured, limited by frequency cut off of the spectrometer

Comparison to HITRAN and FTS data J-Dependent differences of orders of MHz at certain J values. Differences larger in the v 4 =1 hot band than in v 5 =1. Fit the unperturbed levels alone to get estimated rotational parameters.

Fitted Parameters

Summary and Future Summary : –Highly accurate and precise measurements of v 4 =1 and v 5 =1 hot bands in the 1.5  m region up to J=30. –Broadening and asymmetry increase with intra-cavity power. –Comparison to HITRAN/FTS data shows there are J- dependent differences in frequencies of the order of MHz. –Fits of unperturbed terms values provide better molecular constants than have been available to date Future: –Exploration of broadening due to amplitude modulation and also pressure broadening. –Apply the same method to a different molecular system (i.e CH ) in 1.6  m region.

Acknowledgements Greg Hall Anh Le Chris McRaven Damien Forthomme Trevor Sears Hua Gen Yu Hong Xu Matt Cich