1. CH 3 D Near Infrared Cavity Ring-down Spectrum Reanalysis and IR-IR Double Resonance S. Luna Yang George Y. Schwartz Kevin K. Lehmann University of Virginia 06/2015

2. Outline  Motivation for studying methane isotopomers  Experimental setup of cavity ring-down spectroscopy (CRDS)  Spectrum analysis of CH 3 D  Future plan and acknowledgement - spectrum simulation - combination differences - temperature dependence - double resonance

3. Credit: NASA, 2009

4. Scientific American 296, 42 - 51 (2007)

5. Methane isotopes CH 3 D Applied Optics, Vol. 33, Issue 33, pp. 7704-7716 (1994)

6. Outline Motivation for studying methane isotopomers  Experimental setup of cavity ring-down spectroscopy (CRDS)  Spectrum analysis of CH 3 D  Future plan and acknowledgement - spectrum simulation - combination differences - temperature dependence - double resonance

7. Experimental setup for CRDS methane detection HeNe DFB diode Lasers Pulse Generator Detector #1 MOS PZT SM1 SM2 Cavity M M OPM TECs #1 #2 #3 #4 Current Sources Detector #2 Single Pass Cell Mixer 70MHz PC EOM SOA Beam Splitter

8. Cavity ring-down spectroscopy of ~177 ppm CH 3 D in ~ 8.3 Torr N 2 buffer gas, in comparison with FTIR spectrum (76.7 Torr pressure, 105 m absorption path length ). Typical CRDS spectrum of CH 3 D at near infra-red region FTIR spectrum : K. Deng et al, Molec. Phys., VOL. 97, NO. 6 (1999).

9. Outline Motivation for studying methane isotopomers Experimental setup of cavity ring-down spectroscopy (CRDS)  Spectrum analysis of CH 3 D  Future plan and acknowledgement - spectrum simulation - combination differences - temperature dependence - double resonance

10. FITR: Mol. Phys. 1999 Comparison of CH 3 D CRDS spectrum with software simulation and FTIR spectrum 1. Spectrum simulation - comparison

11. 2. Combination differences - theory If two transitions are from different ground states but both go to the same overtone band state, and, their frequency difference is: The relative intensities of combination differences can also be predicted. +…

12. 3. Temperature dependence - theory Line intensity of a certain transition for spherical top molecules have the following dependence on temperature (when neglecting vibrational energies and perturbations): Therefore, if we have the line intensity under two different temperatures, their ratio would be: The ground state energy, mostly rotational energy, can be estimated: E=BJ(J+1)+(A-B)K 2

13. 3. Temperature dependence - test Methane spectrum line intensity temperature dependence (CH 4 Q branch) CH 4 Q-branch

14. 3. Temperature dependence - test Methane CH 4 ground state rotational energy comparison between approximation and temperature dependence experimental result CH 4 Q-branch

15. 4. Double resonance -setup : Fundamental band, near 3000 cm -1, well studied : First overtone band, near 6000 cm -1, much more complicated and not well studied C-H stretching bands of CH 3 D Pump laser: CW Optical parametric oscillation (OPO) output wavenumber ~3000cm -1 output power > 1W. ∆ν1MHz Probe laser: DFB diode laser output wavenumber ~6000cm -1 output power ~10mW. ∆ν10MHz … pump laser probe laser Decrease absorption Increase absorption

16. 4. Double resonance - result Found about 100 sharp positive&negative peaks from double resonances for different transitions. Improved simulations to be more accurate – the simulation and calibrated wavenumber of assigned peaks are within 0.004 cm -1 difference. Found transitions from other bands that are not yet simulated. Pump laser transition R Q 0 (8) Pump laser transition R Q 0 (5)

17. 4. Double resonance - four features Cell: ~300 mtorr pure CH 3 D Sharp negative peaks: The pump laser excites CH 3 D R R 1 (1) ν=1←0 transition at 3034.688 cm -1. Broad negative peaks: Correspond to that the probe laser excites molecules ν=2←0. Cell: ~1torr pure CH 4. Sharp positive peaks: The pump laser excites CH 4 R(0) ν=1←0 transition at 3028.726 cm -1, and the probe laser excites molecules ν=3←1. Broad positive peaks: Vibrational heating effect of pump laser beam

18. ParameterGround state ν-5980.404(8)6022.203(4) A5.2508215.242(1)5.2134(7) B3.8801953.862(1)3.8506(3) D J /10 -5 5.2614-1.5(4) ×104.8(7) D JK /10 -4 1.26287-1.6(2)×102.6(2) D K /10 -4 -0.788443.4(2)×10-2.3(3) H J /10 -9 1.394-1.0(5)×10 2 H JK /10 -8 1.146-3.9(2)×10 2 H KJ /10 -9 -6.58-3.9(4)×10 2 H K/ 10 -9 -1.05--2.9(4)×10 3 Aζ---0.1021(1) η J/ 10 -3 --1.26(2) η K /10 -3 --1.20(5) q eff /10 -2 ---1.59(2) q eff K /10 -3 ---  /10 -3 --- Vibration-rotational parameters for simulatingand bands (unit cm -1 ) )(2)(2 1414 AA

19. Outline Motivation for studying methane isotopomers Experimental setup of cavity ring-down spectroscopy (CRDS) Spectrum analysis of CH 3 D  Future plan and acknowledgement - spectrum simulation - combination differences - temperature dependence - double resonance

20. Future plans CH 3 D spectrum at an intermedia temperature ‘Ladder’ double resonance assignment Liquid nitrogen cooled double resonance Double resonance in CRDS

21. Acknowledgements NSF NASA University of Virginia Yongxin Tang

22. gas I/O Liquid nitrogen tank Safe vent window Glass tube with both ends sealed with windows Conflat flange bellow 3. Temperature dependence - setup of cold cell