The Near-IR Spectrum of CH3D Shaoyue Yang, Kevin K. Lehmann, Robert J Hargreaves, Peter Bernath Michael Rey, Andrei V. Nikitin, Vladimir Tyuterev

Credit: NASA, 2009

Methane isotopes CH3D Applied Optics, Vol. 33, Issue 33, pp. 7704-7716 (1994)

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

They reported 56 vibrational band centers between 3342-6428 cm-1 Most Complete Previous analysis of CH3D in the NIR was published by O.N. Ulenikov , E.S. Bekhtereva , S. Albert , S. Bauerecker , H. Hollenstein & M. Quack, Molecular Physics 108, 1209-1240 (2010) They reported 56 vibrational band centers between 3342-6428 cm-1 Tabulate wavenumber for 5496 individual transitions, with J” max of 11. No line intensities given No spectroscopic constants nor fit residuals given

Typical CRDS spectrum of CH3D at near infra-red region Cavity ring-down spectroscopy of ~177 ppm CH3D in ~ 8.3 Torr N2 buffer gas, in comparison with FTIR spectrum (76.7 Torr pressure, 105 m absorption path length ). FTIR spectrum : K. Deng et al, Molec. Phys., VOL. 97, NO. 6 (1999).

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: And the ground state energy E would be: The ground state energy, mostly rotational energy, can be estimated: E=BJ(J+1)

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

Methane spectrum line intensity temperature dependence (CH4 Q branch) 3. Temperature dependence - result CH4 Q branch Methane spectrum line intensity temperature dependence (CH4 Q branch)

3. Temperature dependence - result Methane CH4 ground state rotational energy comparison between approximation and temperature dependence experimental result

Old Dominion Spectra Took FTIR Spectra of CH3D in “hot cell” 50 cm long 50 torr RT vapor pressure 0.0025 – 0.009 cm-1 FWHM broadening expected. 0.02 cm-1 resolution on Bruker with soft Norton-Beer Apodization Spectra & backgrounds taken from 294 – 900 K Lines determined from program that peak finds and then autofits lines to Voigt Profile Spectra taken from 5150 – 8950 cm-1

Summary of CH3D results 5150-8950 cm-1 Lower State Term values estimated from temperature dependence for transitions observed in 3 or more spectra. (16,267 unique transitions)

Comparison of Lower State Term values with Previous Assignments By Ulenikov et al.

Theory comes to rescue! Extending and improving upon previously theoretical published work, Phys. Chem. Chem. Phys., 2013, 15, 10049—10061, Michael Rey, Andrei V. Nikitin and Vladimir G. Tyuterev have calculated the spectrum of CH3D Provided me list of 416,437 transitions with n between 5000-6300 cm-1, J” = 0 -22 with assignments and intensities.

Residuals of all but one assigned transitions by Ulenikov et al with theoretical Predictions of Rey, Nikitin, and Tyuterev.

4. Double resonance -setup C-H stretching bands of CH3D : Fundamental band, near 3000 cm-1, well studied … pump laser probe : First overtone band, near 6000 cm-1, much more complicated and not well studied Pump laser: CW Optical parametric oscillation (OPO) output wavenumber ~3000cm-1 output power > 1W Probe laser: DFB diode laser output wavenumber ~6000cm-1 output power ~10mW

4. Double resonance - four features Cell: ~300 mtorr pure CH3D Sharp negative peaks: The pump laser excites CH3D RR1(1) ν=1←0 transition at 3034.688 cm-1 . Broad negative peaks: correspond the probe laser excites molecules ν=2←0. Cell: ~1torr pure CH4. Sharp positive peaks: The pump laser excites CH4 R(0) ν=1←0 transition at 3028.726 cm-1, and the probe laser excites molecules ν=3←1. Broad positive peaks: Possibly thermal heating effect of pump laser beam

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.

Future plans Improve Empirical Lower state Term values, Include spectra taken at 77 and 198 K Combine theoretical predictions with ground state combination-difference searches to automate assignment of dense CH3D spectrum. Produce a file that can be used to accurately simulate CH3D spectrum for Temperatures up to 900 K.

Acknowledgements NSF NASA University of Virginia Thank you!