High Precision Spectroscopy of CH5+ with

NICE-OHVMSJames N. Hodges, Adam J. Perry and Benjamin J. McCall

Outline• Motivation CH5

+ • Experimental Challenges• Current Data• Future Direction

Infrared Spectroscopy of CH5

+• First Observed in 1999 by White ,

Tang & Oka• Observed by Velocity Modulation

Spectroscopy• To this day remains completely

unassigned

E.T. White, J. Tang & T. Oka. Science, 284, 135 (1999).

Above: CH5+

Right: Infrared Spectrum of CH5

+.

Infrared Spectroscopy of CH5

+

• 917 Lines Observed • Assignment by

Subtraction – Removed the

spectrum of other species: H3

+, CH3+,

C2H3+, HCO+, HCNH+,

CH4 and Rydberg H2

E.T. White, J. Tang & T. Oka. Science, 284, 135 (1999).

Potential Energy Surface• The potential energy surface

– 120 mimina of Cs(I)– 120 Cs(II) saddlepoints ~ 40 cm-1 above

minimum– 60 C2v saddlepoints ~ 300 cm-1 above

minimum

E.T. White, J. Tang & T. Oka. Science, 284, 135 (1999).X. Wang & T. Carrington Jr. J. Chem. Phys., 129, 234102 (2008).

Cs(I) Cs(II) C2V

Potential Energy Surface

E.T. White, J. Tang & T. Oka. Science, 284, 135 (1999).X. Wang & T. Carrington Jr. J. Chem. Phys., 129, 234102 (2008).

Zero Point Energy 10917 cm-1

~ 300 cm-1

~ 40 cm-1

Instrumental Layout

OPO

YDFL

EOMLock-In

Amplifier

X & YSignal

Lock-In Amplifier

X & YSignal

Wave-meter

40 kHzPlasma

Frequency

80 MHz1 × Cavity FSR

90o Phase Shift

IPS

2f

ni = np - ns

Freq. CombAOM

K. N. Crabtree, et al. Chem. Phys. Lett. (2012), 551, 1-6.

Comb Calibration

Wave-meter

Freq. CombAOM

[…]

Signal Pump

Comb Calibration

Wave-meter

Freq. CombAOM

[…]

Signal Pump

Comb Calibration

Wave-meter

Freq. CombAOM

[…]

Signal PumpSignal

Production of CH5+

• Velocity modulated, l-N2 cooled, positive column • H3

+ + CH4 CH5+ + H2

• Low current: ~ 80 mA• 6 kHz modulation frequency• Ratio 50:1 H2:CH4

• Total pressure ~ 1 TorrE.T. White, J. Tang & T. Oka. Science, 284, 135 (1999).

Technical Challenges• Lower modulation frequency lower

current

• Lower frequency greater noise

• Higher frequencies lower pressures

Technical Challenges

No PlasmaHigh current (40 kHz)Low current (6 kHz)

Last Year’s Line

Wavenumber (cm-1)

S/N ~ 25

Experimental ComparisonParameter Oka Us

Current (mA) 80 200

Frequency (kHz) 6 40

Pressure (Torr) 1 1

H2:CH4 50:1 ~50:1

Mirror Absorbance

Mirror Absorbance

𝑆𝑖𝑔𝑛𝑎𝑙 ∝𝐶𝑜𝑢𝑝𝑙𝑖𝑛𝑔𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦∝ 1𝐴𝑏𝑠𝑜𝑟𝑝𝑡𝑖𝑜𝑛

Improvements• Baked Mirrors

– Operating in dry purge box

– Prevented rapid degradation of performance

Improvements• Included a -emitter

– 63Ni– Less plasma noise in lock– Attain lower current

Recent WorkCH5

+ line @ 2898 cm-1. Approximately 0.5 Intensity as Oka’s line @ 2926 cm-1 .S/N ~ 30

Experimental ComparisonParameter Oka Us

Current (mA) 80 110

Frequency (kHz) 6 46

Pressure (Torr) 1 0.250

H2:CH4 50:1 ~30:1

Calibrated Scans and Fits

Linecenter (MHz)

SNR (MHz) Oka Uncertainty (MHz)

Obs.- Oka (MHz)

86880179 50 5 MHz 90-180 85 MHz

Calibrated Scans and Fits

Linecenter (MHz)

SNR fit (MHz) Oka Uncertainty (MHz)

Obs.- Oka (MHz)

87720253 40 ~2 MHz 90-180 200 MHz

Future Outlook• New Mirrors

– Have specialized coating– Improved Performance Lamb dips

• Complete 917 lines• 4-line combination differences with

complete data set.

Acknowledgements

Springborn FellowshipNSF GRF (DGE 11-44245 FLLW)