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MID-IR SATURATION SPECTROSCOPY OF HeH+ MOLECULAR IONHSUAN-CHEN CHEN,CHUNG-YUN HSIAOInstitute of Photonics Technologies, National Tsing Hua University, TaiwanJIN-LONG PENGCenter of Measurement Standards, Industrial Technology Research Institute, TaiwanTAKAYOSHI AMANODepartment of Physics and Astronomy and Department of Chemistry, University of Waterloo, CanadaJOW-TSONG SHYDepartment of Physics, National Tsing Hua University, Taiwan*Mail Address : [email protected]
2011/ 6 / 21 – International Symposium of Molecular Spectroscopy
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Motivation• HeH+ : one of the simplest two-electron diatomic molecules. • Difference between the theoretical calculation and the
experimental results is about 60 MHz .• More precise measurements – testing the quantum mechanical
calculations. • Test the breakdown of Born-Oppenheimer approximation – 3HeH+,
3HeD+, 4HeH+, 4HeD+.
http://en.wikipedia.org/wiki/NGC_7027
• It has been suggested that HeH+ would be present in some areas of interstellar space.
NGC 7027
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Previous Work - 4HeH+ Authors Measured Transitions Uncertainty
[1979] D.E. Tolliver, G. A. Kyrala and W.H. Wing
Fundamental Band : P(12)-P(13)ν = 2-1 : P(9)-P(11) )
[1982] P. Bernath and T. Amano Fundamental Band : R(0)-R(4) and P(1)-P(4))
[1987] D. J. Liu, W. C. Ho and T. Oka Pure Rotation ν = 0 R(6))
[1989] M.W. Crofton, R.S. Altman, N.N. Haese and T. Oka
ν=1-2 R(1)-R(5), R(8), P(1) and its isotopes for strong line )
[1997] F. Matsushima, T. Oka and K. Takagi
Low J pure rotation J=1 ← 0 and J=2 ← 1 [FIR])
[1997] Z. Liu and P.B. Davies Quasi-bound to quasi-bond and bond to quasi-bond )
• Difficulties of experiments of HeH+ : Low concentration of HeH+
• We plan to improve the accuracy of ro-vibrational transitions by 2 orders of magnitude using saturation spectroscopy.
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How to achieve? > Light Source – CW Optical Parametric Oscillator ---- Enough power for saturation.
> Extended Negative Glow Discharge Tube ---- Low pressure, high concentration of ion.
> Optical Frequency Comb ---- Frequency ruler for precision measurement.
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Singly Resonant OPO
• Increasing the signal of the saturation spectroscopy of molecular ion. • Idler Wavelength :2.7 – 3.9 μm• Average Power : > 300 mW @ 7 W pump• Frequency Tuning : > 40 GHz (Mode-Hop-Free)• Free-Running Stability : < 500 MHz @ 8 hours
𝜔𝑝𝑢𝑚𝑝=𝜔𝑠𝑖𝑔𝑛𝑎𝑙+𝜔𝑖𝑑𝑙𝑒𝑟
𝑠𝑖𝑔𝑛𝑎𝑙𝑝𝑢𝑚𝑝𝑖𝑑𝑙𝑒𝑟
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Glow Discharge
Characteristics of negative glow region:
Largest glow intensity
Highest concentration of positive
ions
Nearly field free
The region is relatively shortn+
n-
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Extended Negative Glow Discharge
- +
Negative glow Region
Positive column
Anode
• Flow mixture of He:H2 = 98 : 2 (120 mtorr)• Discharge Current : 16 mA• Ethanol cooled at -70 oC
Benefits of extended negative glow :• Lower gas pressure (~100 mtorr)• Narrower linewidth for precision measurement
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Extended Negative Glow Discharge
B field (300 Gauss) - +
Negative glow Region
Positive column
Anode
• Flow mixture of He:H2 = 98 : 2 (120 mtorr)• Discharge Current : 16 mA• Ethanol cooled at -70 oC
Benefits of extended negative glow :• Lower gas pressure (~100 mtorr)• Narrower linewidth for precision measurement
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Extended Negative Glow Discharge
B field (300 Gauss) - +
Negative glow Region
Anode
• Flow mixture of He:H2 = 98 : 2 (120 mtorr)• Discharge Current : 16 mA• Ethanol cooled at -70 oC
Benefits of extended negative glow :• Lower gas pressure (~100 mtorr)• Narrower linewidth for precision measurement
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Optical Frequency Comb
• Repetition Rate : 250 MHz• Supercontiuum : 1030 ~ 2200 nm • RF Reference : GPS-locked Rubidium clock• Accuracy : < 10-11 @ 1000 sec (~few kHz in the MIR region)
𝜔𝑝𝑢𝑚𝑝=𝜔𝑠𝑖𝑔𝑛𝑎𝑙+𝜔𝑖𝑑𝑙𝑒𝑟
1.062 μm 1.4-1.8 μm 2.7-3.9 μm
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Experimental Setup
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Results- Direct Absorption
• B Field Modulation - Concentration Modulation• Linewidth (FWHM): 420 MHz (Temperature at -64 oC)• Absorption : 0.0128 %
-900 -600 -300 0 300 600 900
-8.0x10-5
-4.0x10-5
0.0
4.0x10-5
8.0x10-5
1.2x10-4
1.6x10-4
FWHM ~ 420.4 MHz
HeH+ R(1) Gaussian Fit
Sig
nal (
a.u
.)
Frequency Tuning (MHz)
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3rd Harmonics Signal - Saturation Dip
-20 -15 -10 -5 0 5 10 15 20-1.0x10-3
-8.0x10-4
-6.0x10-4
-4.0x10-4
-2.0x10-4
0.0
2.0x10-4
4.0x10-4
Sig
nal I
nten
sity
(V
)
Frequency Tuning (MHz)
HeH+ R(1) Transition
• Frequency Modulation Method – Idler wave• Modulation Frequency : 31 kHz• Modulation Depth : 7.6 MHz• Laser Intensity : ~ 10 kW/m2
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Linewidth Investigation
Linewidth = 7.07 +/- 0.45 MHz
* Gas Pressure = 120 mtorrFitting Function : Hui-Mei Fang et. al., Opt. Comm., 257, 1, 76-83(2006)
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Preliminary Measurement
• ν [HeH+ R(1)] = 90 788 394.58 0.20 MHz
0 5 10 15 20
-4
-3
-2
-1
0
1
2
3
4Day 2Day 1
S.T
.D.
Fre
quen
cy D
evia
tion
(MH
z)
Times
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Conclusion and Future Work
• The first observation of saturation absorption spectra of HeH+ has been demonstrated.
• The linewidth had also been investigated (7 MHz).• The absolute frequency of HeH+ R(1) transition was determined to
be 90 788 394.58 0.20 MHz. [ 3028.374869(7) cm-1 ] (300 times better than previous work.)
Future work:• Improving our spectroscopic system. • Measuring other transitions of HeH+ and H3
+ in the discharge tube.
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Acknowledgements
• We thank to the National Science Council (NSC), Ministry of Education, Natural Science and Engineering Research Council of Canada ( NSERC ) and Industrial Technology Research Institute.
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Thanks For your Attention
Prof. Shy
Prof. AmanoHsiao
Dr. Peng
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Mean S.T.D.
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Broadening and Shift
• Zeeman Shift ~ 10 kHz
• Power Broadening can be ignored.
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Outline
• Motivation > The Importance of HeH+
> Previous work• Our Spectroscopic System > Light Source –CW Optical Parametric Oscillator > Extended Negative Glow Discharge Tube > Fiber-Based Optical Frequency Comb • Experimental Arrangement and Result• Summary and Future Work• Acknowledgement
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Experimental Setup