The Infrared Spectrum of CH 5 + Revisited Kyle N. Crabtree, James N. Hodges, and Benjamin J. McCall

Why study CH 5 + ? Astrochemistry Formed by radiative association of CH H 2 Stable against H 2, so might be detectable in interstellar environments Potential tracer for CH 3 + and gas-phase CH 4, which have no rotational spectrum Possible precursor to gas- phase C-C bond formation Quantum Mechanics Highly symmetric fluxional molecule Challenging to traditional notion of structure 2 Courtesy Joel Bowman

CH 5 + structure and dynamics 3 Z. Jin et al., J. Phys. Chem. A. (2006) 110, A. B. McCoy et al., J. Phys. Chem.A (2004) 108, C s (I) 120 forms 0 cm -1 C s (II) 120 forms ~ 30 cm -1 C 2v 60 forms ~ 300 cm -1 Ground state wavefunction fully delocalized among 120 C s (I) minima G 240 (S 5 * ) permutation-inversion symmetry xkcd.com/55

The unassigned rovibrational spectrum of CH E. T. White et al., Science (1999) 284, Velocity modulation spectroscopy l-N 2 cooled H 2 /CH 4 plasma Line uncertainties MHz 917 transitions

Extracting information from an unassigned rovibrational spectrum Combination differences give rotational energy level spacings, but no assignment! 4-line combination differences (4LCDs) 5 v=0 v=1 J=1 J=2 J=1 J=2 Q(1)P(2)R(1)Q(2) EE EE

Large uncertainties limit 4LCD analysis 6 v=0 v=1 J=1 J=2 J=1 J=2 Q(1) P(2) R(1) Q(2) EE EE

Reducing uncertainties in IR spectroscopy Optical Frequency CombsOptical Parametric Oscillators 7 Laser stabilization & frequency measurement (<10 kHz accuracy) High optical power (1 W)  saturation of rovibrational transitions (linewidth < 50 MHz) Higher bandwidth detectors  FM spectroscopy

OPO-NICE-OHVMS 8 Lamb dips Precision: ~ 300 kHz

Producing CH 5 + in a positive column 9 CH 4 (20 mTorr) + H 2 (1 Torr), minimum possible plasma current

First CH 5 + detection with NICE-OHVMS 10 Plasma current ~ 200 mA (vs. ~ 80 for optimal production) Challenge: decrease plasma current without increasing technical noise Neutral H 2

Technical noise at low plasma current 11 No PlasmaHigh current (40 kHz, 125 mA)Low current (6 kHz, 70 mA)

Comparison with Oka’s spectrum 12 Scan Direction Scan rate > detection system time constant? Line center offset, broad lineshape, asymmetry

Summary 13 CH 5 + high resolution spectrum remains unassigned CH 5 + high resolution spectrum remains unassigned OPO-NICE-OHVMS allows measurement of IR transitions with sub- MHz accuracy OPO-NICE-OHVMS allows measurement of IR transitions with sub- MHz accuracy 4LCD analysis  energy level spacings 4LCD analysis  energy level spacings Spectral acquisition and calibration in progress Spectral acquisition and calibration in progress

Acknowledgements 14 NASA Earth and Space Science Fellowship Program