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High Precision Spectroscopy of CH 5 + with NICE-OHVMS
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Transcript of High Precision Spectroscopy of CH 5 + with NICE-OHVMS
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)