mid-ir saturation spectroscopy of heh + molecular ion hsuan-chen chen,chung-yun hsiao institute of...

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 : shy@phys.nthu.edu.tw

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