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Michael Porambo , Andrew Mills, Brian Siller, Benjamin J. McCall

Lineshape and Sensitivity of Spectroscopic Signals of N 2 + in a Positive Column Collected Using NICE-OHVMS. Michael Porambo , Andrew Mills, Brian Siller, Benjamin J. McCall University of Illinois at Urbana-Champaign 20 June 2011. Outline. Introduction

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Michael Porambo , Andrew Mills, Brian Siller, Benjamin J. McCall

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  1. Lineshape and Sensitivity of Spectroscopic Signals of N2+ in a Positive Column Collected Using NICE-OHVMS Michael Porambo, Andrew Mills, Brian Siller, Benjamin J. McCall University of Illinois at Urbana-Champaign 20 June 2011

  2. Outline • Introduction • Lineshape Description, Analysis, Ultra-high Resolution Spectroscopy • Sensitivity Comparison • Summary, Conclusions, Present and Future Work

  3. Spectroscopic Techniques Velocity Modulation Spectroscopy (VMS)1,2 1Gudeman and Saykally, Ann. Rev. Phys. Chem.1984. 2Stephenson and Saykally, Chem. Rev.2005. Optical Heterodyne3 Velocity Modulation Spectroscopy (OHVMS)4 EOM 3Bjorklund and Levenson, Appl. Phys. B1983. 4Lindsay, Ph.D. Thesis, University of Chicago, 2002. Cavity Enhanced Velocity Modulation Spectroscopy4,5 4Siller et al. Optics Lett.2010. 5Mills et al. Chem. Phys. Lett.2010. Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS)6,7 6Ye et al. J. Opt. Soc. Am. B1998. 7Foltynowicz et al. Appl. Phys. B, 2008. Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy (NICE-OHVMS) Sample EOM

  4. NICE-OHVMS

  5. N2+ Signal with NICE-OHVMS Lamb dips from optical saturation Sideband-carrier interaction ~1 GHz A. U. ~500 MHz Sideband-sideband interaction Carrier-carrier interaction NICE-OHVMS spectrum of Q11(14) of N2+ acquired with 1 GHz heterodyne detection bandwidth.

  6. Heterodyne Detection Bandwidth Relative Frequency (MHz) As cavity length is scanned, FSR changes. Laser sidebands do not couple into the cavity as efficiently, noise immunity suffers. 1.02 GHz (9 × FSR) – 9 kHz shift in longitudinal mode with respect to sideband. 113 MHz (1 × FSR) – 1 kHz shift in longitudinal mode with respect to sideband.

  7. Absorption and Dispersion Absorption Dispersion - +

  8. Absorption and Dispersion Absorption and dispersion related by the Kramers-Kronig relations. Example for Gaussian absorption profile:

  9. Heterodyne Detection Bandwidth Detector PZT Ti:Sapph Laser EOM EOM Absorption 90° Phase Shift X 9 × Cavity FSR 1.02 GHz 1 × Cavity FSR 113 MHz Y Lock-In Amplifier Lock-In Amplifier 40 kHz Plasma Frequency Absorption Signal Dispersion Signal Dispersion X Y X Y X Y

  10. 113 MHz Detection Dispersion Absorption 113 MHz Sidebands 1 Cavity FSR Dispersion Absorption Lock-In X Lock-In Y

  11. Sub-Doppler Spectra Dispersion Absorption Lock-In X Lock-In Y No center Lamb dip in absorption Spectra calibrated with optical frequency comb Frequency precision to ~1 MHz!

  12. Ultra-High Resolution Spectroscopy Dispersion Absorption Red – Data Blue - Fit Red – Fit Blue - Data 113 MHz Sub-Doppler fitting equation modeled as convolution of Gaussian and Lorentzian absorption and dispersion profiles (2 absorption/each, 3 dispersion/each) Line center from fit: 326,187,572.2 ± 0.1 MHz After correcting for systematic problems, line center measured to within uncertainty of ~300 kHz!

  13. Signal and Noise Calculations NICE-OHVMS (1 GHz) OHVMS (1 GHz) VMS CEVMS Signal-to-noise ratio calculated for different detection techniques under the same conditions. NICE-OHVMS S/N factor of 2 greater than the next sensitive technique!

  14. Technique Comparison VMS OHVMS CEVMS NICE-OHVMS

  15. Summary and Conclusions • NICE-OHVMS addresses well challenges in direct absorption/dispersion spectroscopy of ions. • Distinctive, absorption/dispersion lineshape with Lamb dips. • Precise line centers obtained using Lamb dips and calibrating to optical frequency comb (~1 MHz precision). • S/N greatly improved over VMS, OHVMS, and CEVMS.

  16. Present and Future Work Vibrational spectroscopy in the mid-IR • Positive column discharge setup with CW OPO (Aculight Argos). • Study molecular ions of astronomical, fundamental chemical interest (e.g., CH5+). Highly sensitive technique for molecular ion beam detection McCall group ion beam instrument Aculight Argos CW OPO http://www.lockheedmartin.com/data/assets/ms2/pdf/ArgosSF.pdf • Direct absorption/dispersion spectroscopy of N2+ in a fast ion beam. • Stay tuned for next talk (MI11) on ion beam.

  17. Acknowledgments • McCall Research Group Ben McCall Andrew Mills Brian Siller • Sources of Funding • Air Force – Research Corp. • NASA – Univ. of Illinois • Dreyfus • Packard • NSF • Sloan

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