Collision-Dependent Line Areas in the a 1 Δ g ←X 3 Σ − g Band of O 2 Vincent Sironneau, Adam J. Fleisher,* and Joseph T. Hodges Material Measurement Laboratory National Institute of Standards & Technology Gaithersburg, MD 20899, USA

Electronic States of O 2 Low-lying electronic states Improved spectral parameters required for advanced atmospheric chemistry experiments Line intensities Line shapes Line mixing Collision-induced absorption A band 760 nm 1 Δ band 1.27 µm Noxon band 1.91 µm R.A. Washenfelder et al., JGR 111, D22305 (2006)

Atmospheric Monitoring R.A. Washenfelder et al., JGR 111, D22305 (2006) NASA, The Earth Observer, August 2014, Volume 26, Issue 4 Total Carbon Column Observataion Network (TCCON)OCO-2 and GOSAT Evenly mixed in Earth’s atmosphere Used to understand systematic fluctuations that occur during atmospheric retrievals of CO 2 and CH 4 Ambitious, high cost, high profile satellites require an unprecedented level of precision for reference data to fulfill mission goals

Frequency-Stabilized CRDS Length servo – frequency ruler 1 part in Probe servo – high acquisition rate 1 part in 10 4 – 10 6 External cavity diode laser 150 cm −1 bandwidth D.A. Long et al., CPL 536, 1 (2012)

High pressure data

Frequency-Stabilized CRDS BOA to trigger decays Broadband OFC referenced Mirror losses 50 ppm D.A. Long et al., CPL 536, 1 (2012)

Scanning in FS-CRDS March from one mode of the cavity to the next, with high fidelity Tuning temperature, current, or external cavity Maintain cavity transmission with low-bandwidth transmission lock or with stabilization to a high-resolution wavelength meter frequency

Line Areas S.M. Newman et al., JPCA 104, 9467 (2000) R.A. Washenfelder et al., JGR 111, D22305 (2006) O. Leshchishina et al., JQSRT 111, 2236 (2010) *V.T. Sironneau and J.T. Hodges, JQSRT 152, 1, (2015)

Line Areas High QF obtained using partially correlated quadratic-speed-dependent Nelkin-Ghatak profile (pCqSDNGP, the so-called “Hartmann-Tran Profile”) J. Tennyson et al., Pure Appl. Chem. 86, 1931 (2014).

An Unexpected Trend

Buffer Gas Dependence Constant partial pressure of O 2 P O 2 = 2 Torr R7 R7 transition Line area is never a constant vs. pressure, regardless of buffer gas

Collision-Induced Absorption Secondary CIA model The fate of the 1 Δ state … spontaneous emission (A spont ) collisional deactivation (Γ p ) diffusion out of laser beam (Γ tt ) secondary CIA (CI-B Noxon ) O 2 while in the a 1 Δ g state undergoes a collision which reduces its electronic symmetry and subsequently allows for the strong absorption of a second photon A band 760 nm 1 Δ band 1.27 µm Noxon band 1.91 µm CIA in 1 Δ g

Rate Equations Write out the steady-state rate equations

Steady-State Model At low pressures, decay commensurate with transit time loss out of beam Can be extrapolated to obtain zero- pressure area S int (0) = 8.108x10 −26 cm/molecule

Include Previous Measurements Requires large oscillator strength for the collisional-induced absorption. Evidence from large CIA that this is possible. Evidence from fluorescence measurements of the Noxon band. S.M. Newman et al., JPCA 104, 9467 (2000) O. Leshchishina et al., JQSRT 111, 2236 (2010)

Power Dependence Absorption of a second photon should yield a strong power dependence to the observed line areas. Extract power law from model Yields a power law of two

Preliminary Data Negligible power dependence over about 1 order of magnitude … … more work required … if saturated, the Noxon-like absorption P sat < 25 mW circulating power ≈0.2% uncertainty

Non-Exponential Cavity Decays K.K. Lehmann, Appl. Phys. B 116, 147 (2014).

Summary Precision measurements of 1 Δ g O 2 line areas using sophisticated line shape profiles reveal a subtle yet significant collisional dependence We postulate a steady-state model which includes the following pathways for 1 Δ g O 2 loss: A spont, Γ p, Γ tt, and CI-B Noxon A potentially more interesting question arises... are these observations independent of optical power?

Acknowledgements Joseph Hodges, David Long, Zach Reed K. Bielska, M. Ghysels, H. Lin, Q. Liu, V. Sironneau, S. Wójtewicz, H. Yi NIST Greenhouse Gas Measurements and Climate Research Program Good postdocs always wanted:

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Instrument background – water paper OFC-FS-CRDS absolute frequency accuracy, high precision and fidelity lineshape models, QF CIA, LM important as heard in previous talk O2 measurements under low pressure previous measurements from two groups line areas change in unexpected ways pressure, partner CI2PA model model using collision-induced two-photon absorption a long way from saturation - Yariv Future tests of model Power-dependent tests