Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation

Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy Brian Siller, Andrew Mills, Michael Porambo & Benjamin Mc. Call 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 CH Only ~20 are ions CH OCH Need laboratory data to CH OH provide astronomers with CH CO HO spectral targets HO 2 4 e 3 3 3 2 2 OH e 3 C 6 H 7 + H 2 C 6 H 6 C 6 H 5+ C 2 H 2 C 4 H 3+ H C 4 H 2+ C 3 H 2 C C 3 H 3 e + e C 3 H H 2 C 3 H+ C+ C 2 H 2 e C 2 H 5 + C 2 H 3 + e C 2 H C+ CH 3+ CH 4 e CH 3 OH , e H 2 O, e CH 5+ H 2 CH 3+ CO, e e H 2 CH 2+ CH e CN, CH 3 HCN, e NH 3, e N, e CH+ H 2 O+ H 2 OH+ O H 3 C + H 2+ C 2 H 5 CN CH 3 CN H 2 + H 2 e HCO+ CO CH 3 NH 2

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

Velocity Modulation Spectroscopy Positive column discharge cell ◦ High ion density, rich chemistry ◦ Cations move toward the cathode +1 k. V -1 k. V 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 +1 k. V -1 k. V 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 -1 k. V +1 k. V 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 0 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 N 2+ Single-pass direct absorption 0 1 Single-pass Heterodyne @ 1 GHz 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 Laser Detector

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

CEVMS Setup Audio Amplifier Transformer Laser Cavity Mirror Mounts 40 k. Hz Lock-In Amplifier

CEVMS Setup

Extracting N 2+ Absorption Signal Absorption Strength (Arb. Units) 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 Relative Frequency (GHz)

Extracting N 2+ Absorption Signal V (k. V) Absorption t (μs) Relative Frequency

Extracting N 2+ Absorption Signal Demodulate detected signal at twice the modulation frequency (2 f) 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 30 m. W laser power N 2+ Meinel Band N 2* 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 N 2+/N 2* B. M. Siller, A. A. Mills and B. J. Mc. Call, Opt. Lett. , 35, 1266 -1268. (2010) 1 M. Havenith, M. Schneider, W. Bohle, and W. Urban; Mol. Phys. 72, 1149 (1991)

Precision & Accuracy 0 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. Mc. Call, Chem. Phys. Lett. , 501, 1 -5. (2010)

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 Polarizing Beamsplitter EOM Detector AOM 30 MHz Quarter Wave Plate Lock Box Detector

Experimental Setup Ti: Sapph Laser PZT EOM Detector

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

NICE-OHVMS 0 See talk MI 10 for more thorough analysis 1 2 3 • Sidebands spaced at 9 cavity FSRs (1 GHz) • 3 rd derivative-like Doppler lineshape • Lamb dips from each laser frequency and combination of laser frequencies

NICE-OHVMS N 2* N 2+ • 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 Mc. Call Group ◦ Ben Mc. Call ◦ Andrew Mills ◦ Michael Porambo Funding