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

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

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. www.nasa.gov/mission_pages/hubble 𝜇m 𝜇m

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

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

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

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.

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.

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.

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

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

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

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

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

Heterodyne Detection

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

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

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

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

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

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

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

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, 24822-7.

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), 2725.898 cm-1

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

Spoiling Parasitic Etalons 𝜈 𝐼

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

Spoiling Parasitic Etalons Mixer 1 Output H3+ R(1,0), 2725.898 cm-1 Mixer 2 Output Galvo S/N (best channel) Off 25 On 763 Galvo S/N (best channel) Off 25

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

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)

Up-conversion Results H3+ R(1,0), 2725.898 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

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

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 )

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

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