The Infrared Spectrum of CH5+ Revisited

Kyle N. Crabtree, James N. Hodges, and Benjamin J. McCall

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Why study CH5+?

Astrochemistry

• 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

Quantum Mechanics

• Highly symmetric fluxional molecule

• Challenging to traditional notion of structure

Courtesy Joel Bowman

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CH5+ structure and dynamics

Z. Jin et al., J. Phys. Chem. A. (2006) 110, 1569-1574A. B. McCoy et al., J. Phys. Chem .A (2004) 108, 4991-4994

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) minimaG240 (S5

*) permutation-inversion symmetry

xkcd.com/55

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The unassigned rovibrational spectrum of CH5+

E. T. White et al., Science (1999) 284, 135-137

Velocity modulation spectroscopyl-N2 cooled H2/CH4 plasmaLine uncertainties 90-180 MHz917 transitions

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Extracting information from an unassigned rovibrational spectrum

• Combination differences give rotational energy level spacings, but no assignment!

• 4-line combination differences (4LCDs)

v=0

v=1

J=1

J=2

J=1

J=2

Q(1) P(2) R(1) Q(2)

DE DE

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Large uncertainties limit 4LCD analysis• 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

v=0

v=1

J=1

J=2J=1

J=2

Q(1) P(2) R(1) Q(2)

DE DE

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Reducing uncertainties in IR spectroscopy

Optical Frequency Combs Optical Parametric OscillatorsLaser stabilization & frequency

measurement (<10 kHz accuracy)High optical power (1 W) saturation of

rovibrational transitions (linewidth < 50 MHz)Higher bandwidth detectors FM

spectroscopy

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OPO-NICE-OHVMS

Lamb dipsPrecision: ~300 kHz

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Producing CH5+ in a positive column

CH4 (20 mTorr) + H2 (1 Torr), minimum possible plasma current

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First CH5+ detection with NICE-OHVMS

Plasma current ~200 mA (vs. ~80for optimal production)

Challenge: decrease plasma current without increasing technical noise

Neutral H2

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Technical noise at low plasma current

No PlasmaHigh current (40 kHz, 125 mA)Low current (6 kHz, 70 mA)

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Comparison with Oka’s spectrum

Scan DirectionScan rate > detection system time constant?Line center offset, broad lineshape, asymmetry

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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

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Acknowledgements

NASA Earth and Space Science Fellowship Program