1. 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

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 Optical Heterodyne 3 Velocity Modulation Spectroscopy (OHVMS) 4 Cavity Enhanced Velocity Modulation Spectroscopy 4,5 Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy (NICE- OHMS) 6,7 6 Ye et al. J. Opt. Soc. Am. B 1998. 7 Foltynowicz et al. Appl. Phys. B, 2008. EOM 1 Gudeman and Saykally, Ann. Rev. Phys. Chem. 1984. 2 Stephenson and Saykally, Chem. Rev. 2005. 3 Bjorklund and Levenson, Appl. Phys. B 1983. 4 Lindsay, Ph.D. Thesis, University of Chicago, 2002. 4 Siller et al. Optics Lett. 2010. 5 Mills et al. Chem. Phys. Lett. 2010. Sample Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy (NICE- OHVMS)

4. NICE-OHVMS

5. N 2 + Signal with NICE-OHVMS NICE-OHVMS spectrum of Q 11 (14) of N 2 + acquired with 1 GHz heterodyne detection bandwidth. ~500 MHz ~1 GHz Lamb dips from optical saturation Carrier-carrier interaction Sideband-carrier interaction Sideband-sideband interaction A. U.

6. Heterodyne Detection Bandwidth 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. Relative Frequency (MHz)

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

10. 113 MHz Detection AbsorptionDispersion Lock-In X Lock-In Y DispersionAbsorption 113 MHz Sidebands 1 Cavity FSR

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

12. Ultra-High Resolution Spectroscopy Red – Data Blue - Fit 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! Red – Fit Blue - Data AbsorptionDispersion 113 MHz

13. Signal and Noise Calculations OHVMS (1 GHz) VMSCEVMS NICE-OHVMS (1 GHz) 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 VMSOHVMS 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., CH 5 + ). Highly sensitive technique for molecular ion beam detection Direct absorption/dispersion spectroscopy of N 2 + in a fast ion beam. Stay tuned for next talk (MI11) on ion beam. Aculight Argos CW OPO http://www.lockheedmartin.com/data/assets/ms 2/pdf/ArgosSF.pdf McCall group ion beam instrument

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

18. Mathematical Description Doppler broadened lineshape function Sub-Doppler lineshape function  fm : FM detection phase d : Detuning of carrier from transition frequency m : RF modulation frequency (1 GHz, 113 MHz)  abs : General absorption lineshape function  disp : General dispersion lineshape function A 0, A 1, A 2 : Power and modulation depth dependent

19. Frequency Comb Explanation? Mode-locked fs laser equally spaced lines in frequency (F REP ) Phase offset can be locked providing absolute calibration of comb lines. The comb can now be used as an absolute frequency reference. Unknown laser comb  beat frequency Beat frequency, repetition frequency, offset frequency and comb mode contribution to get laser frequency  Beat Frequency F REP F Offset Original Comb Doubled Comb Ti:Sapph Laser Frequency

20. Spectroscopic Techniques VMS Ion-neutral discrimination OHVMS Ion-neutral discrimination Zero-background detection/higher bandwidth CEVMS Ion-neutral discrimination Cavity increases signal Sub-Doppler features for ultra-high resolution spectroscopy NICE-OHVMS Ion-neutral discrimination High sensitivity Sub-Doppler spectroscopy Relative noise immunity EOM