Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy Brian Siller, Andrew Mills, Michael Porambo & Benjamin McCall Chemistry Department, University of Illinois at Urbana-Champaign

Ions & Astrochemistry Molecular ions are important to interstellar chemistry Ions important as reaction intermediates >150 Molecules observed in ISM Only ~20 are ions Need laboratory data to provide astronomers with spectral targets H2+ H3+ CH+ CH2+ CH3+ CH5+ CH4 C2H3+ C2H2 C3H+ C3H3+ C4H2+ C4H3+ C6H5+ C6H7+ C6H6 H2 C e C+ H OH+ H2O+ H3O+ H2O OH O HCO+ CO HCN CH3NH2 CH3CN C2H5CN N, e NH3, e HCN, e CH3CN, e CO, e H2O, e CH3OH, e CH CH2CO CH3OH CH3OCH3 C2H5+ C2H4 C3H2 C3H C2H

Ion Spectroscopy Techniques Velocity Modulation Hollow Cathode Supersonic Expansion    High ion column density    Ion-neutral discrimination     Low rotational temperature     Narrow linewidth  Compatible with cavity-enhanced spectroscopy  

Velocity Modulation Spectroscopy Positive column discharge cell High ion density, rich chemistry Cations move toward the cathode +1kV -1kV Plasma Discharge Cell

Velocity Modulation Spectroscopy Positive column discharge cell High ion density, rich chemistry Cations move toward the cathode Ions absorption profile is Doppler-shifted +1kV -1kV Laser Plasma Discharge Cell Detector

Velocity Modulation Spectroscopy Positive column discharge cell High ion density, rich chemistry Cations move toward the cathode Ions absorption profile is Doppler-shifted -1kV +1kV Laser Plasma Discharge Cell Detector

Velocity Modulation Spectroscopy Positive column discharge cell High ion density, rich chemistry Cations move toward the cathode Ions absorption profile is Doppler-shifted Drive with AC voltage Ion Doppler profile alternates red/blue shift Laser at fixed wavelength Demodulate detector signal at modulation frequency Laser Plasma Discharge Cell Detector

Velocity Modulation Spectroscopy 1

Velocity Modulation Spectroscopy Want strongest absorption possible Signal enhanced by modified White cell Laser passes through cell unidirectionally Can get up to ~8 passes through cell Laser Plasma Discharge Cell Detector Also want lowest noise possible, so combine with heterodyne spectroscopy

Velocity Modulation of N2+ Single-pass direct absorption Single-pass Heterodyne @ 1GHz 1 2

Velocity Modulation Limitations Doppler-broadened lines Blended lines Limited determination of line centers Sensitivity Limited path length through plasma

Cavity Enhanced Absorption Spectroscopy (CEAS) Optical cavity acts as a multipass cell Number of passes = For finesse of 300, get ~200 passes Must actively lock laser wavelength/cavity length to be in resonance with one another DC signal on detector is extremely noisy Velocity modulation with lock-in amplifier minimizes effect of noise on signal detection Cavity Detector Laser

Pound-Drever-Hall Locking Ti:Sapph Laser PZT Detector Polarizing Beamsplitter EOM Detector Cavity Transmission Error Signal AOM 30MHz Quarter Wave Plate Lock Box

CEVMS Setup Audio Amplifier Lock-In Amplifier Transformer Laser 40 kHz Lock-In Amplifier Transformer Laser Cavity Mirror Mounts

CEVMS Setup

Extracting N2+ Absorption Signal Doppler profile shifts back and forth Red-shift with respect to one direction of the laser corresponds to blue shift with respect to the other direction Net absorption is the sum of the absorption in each direction Absorption Strength (Arb. Units) Relative Frequency (GHz)

Extracting N2+ Absorption Signal V (kV) t (μs) Absorption Relative Frequency

Extracting N2+ Absorption Signal Demodulate detected signal at twice the modulation frequency (2f) Can observe and distinguish ions and neutrals Ions are velocity modulated Excited neutrals are concentration modulated Ground state neutrals are not modulated at all Ions and excited neutrals are observed to be ~75° out of phase with one another

Typical Scan of Nitrogen Plasma Cavity Finesse 150 30mW laser power N2+ Meinel Band N2* first positive band Second time a Lamb dip of a molecular ion has been observed (first was DBr+ in laser magnetic resonance technique)1 Used 2 lock-in amplifiers for N2+/N2* B. M. Siller, A. A. Mills and B. J. McCall, Opt. Lett., 35, 1266-1268. (2010) 1M. Havenith, M. Schneider, W. Bohle, and W. Urban; Mol. Phys. 72, 1149 (1991)

Precision & Accuracy 1 2 Line centers determined to within 1 MHz with optical frequency comb Sensitivity limited by plasma noise A. A. Mills, B. M. Siller, and B. J. McCall, Chem. Phys. Lett., 501, 1-5. (2010)

Heterodyne Spectroscopy NICE-OHMS Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy Large Signal Small Noise Cavity Enhancement Heterodyne Spectroscopy NICE-OHMS

NICE-OHMS Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy Cavity Modes Laser Spectrum

Experimental Setup Ti:Sapph Laser PZT Lock Box Detector Polarizing EOM Beamsplitter EOM Detector AOM 30MHz Quarter Wave Plate Lock Box

Experimental Setup Ti:Sapph Laser EOM PZT Detector

Experimental Setup Ti:Sapph Laser N oise I mmune C avity E nhanced - PZT Detector Ti:Sapph Laser EOM EOM N × Cavity FSR (113 MHz) N oise I mmune C avity E nhanced - O ptical H eterodyne V elocity M odulation S pectroscopy Lock-In Amplifier Signal 40 kHz Plasma Frequency

NICE-OHVMS See talk MI10 for more thorough analysis 1 2 3 See talk MI10 for more thorough analysis Sidebands spaced at 9 cavity FSRs (1 GHz) 3rd derivative-like Doppler lineshape Lamb dips from each laser frequency and combination of laser frequencies

NICE-OHVMS N2* N2+ Retain ion-neutral discrimination

Velocity Modulation Techniques

NICE-OHVMS Summary Increased path length through plasma Better sensitivity due to heterodyne modulation Retained ion-neutral discrimination Sub-Doppler resolution due to optical saturation 50 MHz Lamb dip widths Resolve blended lines Better precision & absolute accuracy with comb

Acknowledgements McCall Group Funding Ben McCall Andrew Mills Michael Porambo Funding