1. Charles R. Markus, Adam J. Perry, James N. Hodges, Benjamin J. McCall
Improved Spectroscopy of Molecular Ions in the Mid-Infrared with Up-Conversion Detection
Charles R. Markus, Adam J. Perry, James N. Hodges, Benjamin J. McCall
71st International Symposium on Molecular Spectroscopy University of Illinois at Urbana-Champaign 21 June, 2016
TC07

2. Overview NICE-OHVMS technique Removing parasitic etalons
Description
Limitations
Removing parasitic etalons
Mechanism of detection
Brewster-Spoiler plate
Up-conversion detection
Design
Results
Future directions

3. Molecular Ions H
Over 190 molecules identified in the interstellar medium (ISM)
Reactions must overcome cold and diffuse environment
Ion-neutral reactions dominate chemistry of ISM
Aleman, et al., A&A,2014, 566, A79. 𝜇m 𝜇m

4. Molecular Ions H
Many of the simplest molecules are molecular ions
Accurate ab initio calculations must go beyond Born-Oppenheimer approximation
Laboratory spectra act as benchmarks for state-of-the-art methods H He H

5. Ion Spectroscopy
Ion are orders of magnitude less abundant than neutral species

6. NICE-OHVMS
Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy

7. Increased Signal and Precision
NICE-OHVMS
Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy
Increased Signal and Precision
K. N. Crabtree, et al., Chem. Phys. Lett., 2012, 551, 1-6.

8. Increased Signal and Precision
NICE-OHVMS
Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy
Increased Signal and Precision
Improved Sensitivity
K. N. Crabtree, et al., Chem. Phys. Lett., 2012, 551, 1-6.

9. Increased Signal and Precision Ion-Neutral Discrimination
NICE-OHVMS
Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy
Increased Signal and Precision
Ion-Neutral Discrimination
Improved Sensitivity
K. N. Crabtree, et al., Chem. Phys. Lett., 2012, 551, 1-6.

10. Velocity Modulation Velocity
C. S. Gudeman, et al., J. Chem. Phys., 1983, 78, 5873.

11. - - + + Velocity Modulation Velocity Lock-in Amplifier
C. S. Gudeman, et al., J. Chem. Phys., 1983, 78, 5873.

12. - - + + Velocity Modulation Velocity Lock-in Amplifier
B. M. Siller, et al., Opt. Lett., 2010, 35, 1266.

13. - - + + Velocity Modulation 𝜈 Lock-in Amplifier Lock-in Amplifier
Referenced at 2×𝑓 𝜈
B. M. Siller, et al., Opt. Lett., 2010, 35, 1266

14. Cavity Enhancement
Signal increase of 2F/𝜋
FSR =𝑐/2𝑛𝑙
𝐼 𝑡𝑟𝑎𝑛𝑠 𝜈

15. Heterodyne Detection

16. NICE-OHMS 𝐼 𝑡𝑟𝑎𝑛𝑠 𝜈 Fheterodyne = n×FSR
J.Ye, et al., J. Opt. Soc. Am. B, 1998, 15, 6.

17. Saturation Spectroscopy
𝜈− 𝜈 𝑟𝑒𝑠𝑡 (MHz)
Velocity (m/s)

18. Saturation Spectroscopy
𝜈− 𝜈 𝑟𝑒𝑠𝑡 (MHz)
Velocity (m/s)

19. Saturation Spectroscopy
𝜈− 𝜈 𝑟𝑒𝑠𝑡 (MHz)
Velocity (m/s)

20. Saturation Spectroscopy
𝜈− 𝜈 𝑟𝑒𝑠𝑡 (MHz)
Velocity (m/s)

21. Saturation Spectroscopy
𝜈− 𝜈 𝑟𝑒𝑠𝑡 (MHz)
Velocity (m/s)

22. Saturation Spectroscopy
𝜈− 𝜈 𝑟𝑒𝑠𝑡 (MHz)
Velocity (m/s)

23. NICE-OHVMS Spectrometer
Frequency Calibration
40 kHz
to PZT
OPO
PZT
nidler = npump - nsignal
~3 MHz
EOM
Lock-In Amplifier
Lock-In Amplifier ν
2xf = 80 kHz
80 MHz
YDFL
X & Y Channels
X & Y Channels
90o Phase Shift
Fast
Lock Box
Slow
B. M. Siller, et al., Opt. Express,2011, 19,

24. NICE-OHVMS Results H3+ R(1,0), 2725.898 cm-1 80 MHz Mixer 1 Mixer 2
2×40 kHz
Lock-in 1
Lock-in 2 0o
90o
Mixer 1 Output
Mixer 2 Output
H3+ R(1,0), cm-1

25. Parasitic Etalons 𝜈 𝐼 𝜈 𝐼 𝜈 𝐼
FSR = 𝑐 2𝒏𝑙
𝑛 𝑡 =𝛼×sin⁡(2 𝜔 𝑣𝑚𝑠 𝑡)

26. Spoiling Parasitic Etalons𝜈 𝐼

27. Spoiling Parasitic Etalons𝜈 𝐼
C. Webster, et al., J. Opt. Soc. Am., 1985, 2, 1464.

28. Spoiling Parasitic Etalons
Mixer 1 Output
H3+ R(1,0), cm-1
Mixer 2 Output
Galvo
S/N (best channel)
Off 25 On
763
Galvo
S/N (best channel)
Off 25

29. Heterodyne Detection 𝑓 𝐻𝑒𝑡 = 20% FWHM Overlapping sub-Doppler features
Optimized signal
Resolved Lamb dips
Improved fits
Improved Δ𝜆

30. Difference Frequency Generation
𝜂 𝐷𝐹𝐺 = 0.5% /W532
APD
V10
Vigo PVM-10.6
Menlo APD 220 Δ𝜈
150 (MHz)
1000 MHz
NEP
4800 (pW/Hz1/2)
0.4 (pW/Hz1/2)
Vigo PVM-10.6 Δ𝜈
150 (MHz)
NEP
4800 (pW/Hz1/2)
Vigo PVM-10.6
Menlo APD 220 Δ𝜈
150 (MHz)
1000 MHz
NEP
4800 (pW/Hz1/2)
0.4 (pW/Hz1/2)
NEP/( 𝜂 𝐷𝐹𝐺 × 𝑃 𝑝𝑢𝑚𝑝 )
10 (pW/Hz1/2)

31. Up-conversion Results
H3+ R(1,0), cm-1
Mixer 1 Output
Mixer 2 Output
Detection Method
S/N (Best Channel)
78 MHz mid-IR
337
78 MHz up-conversion
2029
390 MHz up-conversion
1211
Detection Method
S/N (Best Channel)
78 MHz mid-IR
337
Detection Method
S/N (Best Channel)
78 MHz mid-IR
337
78 MHz up-conversion
2029

32. Up-conversion Results
4.2× 10 −9 cm-1
3.5× 10 −10 cm-1
Detection Method
78 MHz mid-IR
78 MHz up-conversion
390 MHz up-conversion
1.6× 10 −10 cm-1

33. Future Directions H Apply up-conversion to survey of H3+
Measure fundamental band (3 – 4.5 𝜇m)
Hotband transitions
Extend to measure overtone transitions (1.9 – 2.1 𝜇m)
Observe resolved Lamb dips
Increase intracavity power
Lock sidebands using DeVoe-Brewer locking H
ν (c m −1 )

34. Conclusions
Removed parasitic etalons in NICE-OHVMS experiment with Brewster-spoiler plate
Improved sensitivity by an order of magnitude with DFG up-conversion of mid-IR beam

35. Acknowledgments Advisor: Ben McCall Special thanks to: James Hodges
Adam Perry
G. Stephen Kocheril