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 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 Credit: NASA, 2009 Scientific American 296, (2007) Methane isotopes CH 3 D Applied Optics, Vol. 33, Issue 33, pp (1994) 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 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 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). 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 FITR: Mol. Phys Comparison of CH 3 D CRDS spectrum with software simulation and FTIR spectrum 1. Spectrum simulation - comparison 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. + 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 3. Temperature dependence - test Methane spectrum line intensity temperature dependence (CH 4 Q branch) CH 4 Q-branch 3. Temperature dependence - test Methane CH 4 ground state rotational energy comparison between approximation and temperature dependence experimental result CH 4 Q-branch 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. 1MHz Probe laser: DFB diode laser output wavenumber ~6000cm -1 output power ~10mW. 10MHz pump laser probe laser Decrease absorption Increase absorption 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 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) 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) =10 transition at cm -1. Broad negative peaks: Correspond to that the probe laser excites molecules =20. Cell: ~1torr pure CH 4. Sharp positive peaks: The pump laser excites CH 4 R(0) =10 transition at cm -1, and the probe laser excites molecules =31. Broad positive peaks: Vibrational heating effect of pump laser beam ParameterGround state (8) (4) A (1)5.2134(7) B (1)3.8506(3) D J / (4) 104.8(7) D JK / (2)102.6(2) D K / (2)10-2.3(3) H J / (5)10 2 H JK / (2)10 2 H KJ / (4)10 2 H K/ (4)10 3 A (1) J/ (2) K / (5) q eff / (2) q eff K / / Vibration-rotational parameters for simulatingand bands (unit cm -1 ) )(2)( AA 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 Future plans CH 3 D spectrum at an intermedia temperature Ladder double resonance assignment Liquid nitrogen cooled double resonance Double resonance in CRDS Acknowledgements NSF NASA University of Virginia Yongxin Tang 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