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The Infrared Spectrum of CH 5 + Revisited

The Infrared Spectrum of CH 5 + Revisited. Kyle N. Crabtree , James N. Hodges, and Benjamin J. McCall. Why study CH 5 + ?. Astrochemistry. Quantum Mechanics. Highly symmetric fluxional molecule Challenging to traditional notion of structure.

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The Infrared Spectrum of CH 5 + Revisited

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  1. The Infrared Spectrum of CH5+ Revisited Kyle N. Crabtree, James N. Hodges, and Benjamin J. McCall

  2. Why study CH5+? Astrochemistry Quantum Mechanics Highly symmetric fluxional molecule Challenging to traditional notion of structure • Formed by radiative association of CH3+ + H2 • Stable against H2, so might be detectable in interstellar environments • Potential tracer for CH3+ and gas-phase CH4, which have no rotational spectrum • Possible precursor to gas-phase C-C bond formation http://bjm.scs.illinois.edu Courtesy Joel Bowman

  3. CH5+ structure and dynamics xkcd.com/55 Cs(I) 120 forms 0 cm-1 Cs(II) 120 forms ~30 cm-1 C2v 60 forms ~300 cm-1 Ground state wavefunction fully delocalized among 120 Cs(I) minima G240 (S5*) permutation-inversion symmetry Z. Jin et al., J. Phys. Chem. A. (2006) 110, 1569-1574 A. B. McCoy et al., J. Phys. Chem .A (2004) 108, 4991-4994 http://bjm.scs.illinois.edu

  4. The unassigned rovibrational spectrum of CH5+ Velocity modulation spectroscopy l-N2 cooled H2/CH4 plasma Line uncertainties 90-180 MHz 917 transitions E. T. White et al., Science (1999) 284, 135-137 http://bjm.scs.illinois.edu

  5. Extracting information from an unassigned rovibrational spectrum J=2 v=1 J=1 Q(1) P(2) R(1) Q(2) J=2 v=0 DE DE J=1 http://bjm.scs.illinois.edu Combination differences give rotational energy level spacings, but no assignment! 4-line combination differences (4LCDs)

  6. Large uncertainties limit 4LCD analysis J=2 v=1 J=1 Q(2) P(2) R(1) Q(1) J=2 v=0 DE DE J=1 http://bjm.scs.illinois.edu Each transition has uncertainty s Valid 4 LCD means: Large s false positives With 917 observed transitions, total possible 4LCDs: = 175619245830 (1.8 x 1011) With 90-180 MHz uncertainty, ~107 4LCDs found Most of these are likely false positives; require more precise frequencies

  7. Reducing uncertainties in IR spectroscopy Optical Frequency Combs Optical Parametric Oscillators High optical power (1 W)  saturation of rovibrational transitions (linewidth < 50 MHz) Higher bandwidth detectors  FM spectroscopy Laser stabilization & frequency measurement (<10 kHz accuracy) http://bjm.scs.illinois.edu

  8. OPO-NICE-OHVMS Lamb dips Precision: ~300 kHz http://bjm.scs.illinois.edu

  9. Producing CH5+ in a positive column CH4 (20 mTorr) + H2 (1 Torr), minimum possible plasma current http://bjm.scs.illinois.edu

  10. First CH5+ detection with NICE-OHVMS Plasma current ~200 mA (vs. ~80 for optimal production) Neutral H2 Challenge: decrease plasma current without increasing technical noise http://bjm.scs.illinois.edu

  11. Technical noise at low plasma current High current (40 kHz, 125 mA) Low current (6 kHz, 70 mA) No Plasma http://bjm.scs.illinois.edu

  12. Comparison with Oka’s spectrum Scan Direction Scan rate > detection system time constant? Line center offset, broad lineshape, asymmetry http://bjm.scs.illinois.edu

  13. Summary • CH5+ high resolution spectrum remains unassigned • OPO-NICE-OHVMS allows measurement of IR transitions with sub-MHz accuracy • 4LCD analysis  energy level spacings • Spectral acquisition and calibration in progress http://bjm.scs.illinois.edu

  14. Acknowledgements NASA Earth and Space Science Fellowship Program http://bjm.scs.illinois.edu

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