Transcript Slide 1
Brian Siller, Andrew Mills, Michael Porambo & Benjamin McCall Chemistry Department, University of Illinois at Urbana-Champaign 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 C6H7+ H2 3 C2H2 C4H3+ H C4H2+ C3H2 C3H3 OH 3 e H2 C+ C2H2 C2H e C2H3 C2H5 + + e C+ CH3+ CH4 e CH5+ 3 C2H5CN H2 CH3CN H2 CH3NH2 CH3+ CH2+ CH HCN H2 + H2 + C3H+ 2 e C3H C e 3 2 C6H6 C6H5+ 4 e e CH+ H2O+ H2 C OH+ O H3 + H2 H2+ HCO+ CO High ion column density Ion-neutral discrimination Low rotational temperature Narrow linewidth Compatible with cavity-enhanced spectroscopy Velocity Modulation Hollow Cathode Supersonic Expansion Positive column discharge cell ◦ High ion density, rich chemistry ◦ Cations move toward the cathode +1kV -1kV Plasma Discharge Cell 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 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 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 0 1 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 Single-pass direct absorption 0 1 Single-pass Heterodyne @ 1GHz 2 Doppler-broadened lines ◦ Blended lines ◦ Limited determination of line centers Sensitivity ◦ Limited path length through plasma 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 Ti:Sapph Laser PZT Polarizing Beamsplitter Detector EOM Detector AOM 30MHz Cavity Transmission Quarter Wave Plate Lock Box Error Signal Audio Amplifier Transformer Laser Cavity Mirror Mounts 40 kHz Lock-In Amplifier 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) V (kV) Absorption t (μs) Relative Frequency 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 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) 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. McCall, Chem. Phys. Lett., 501, 1-5. (2010) Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy Large Signal Small Noise Cavity Enhancement Heterodyne Spectroscopy NICE-OHMS Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy Cavity Modes Laser Spectrum Ti:Sapph Laser PZT Polarizing Beamsplitter EOM Detector AOM 30MHz Quarter Wave Plate Lock Box Detector Ti:Sapph Laser PZT EOM Detector Ti:Sapph Laser Detector PZT EOM EOM N × Cavity FSR (113 MHz) Lock-In Amplifier Signal 40 kHz Plasma Frequency N I C E O H V M S oise mmune avity nhanced ptical eterodyne elocity odulation pectroscopy 0 See talk MI10 for more thorough analysis 1 2 3 • Sidebands spaced at 9 cavity FSRs (1 GHz) • 3rd derivative-like Doppler lineshape • Lamb dips from each laser frequency and combination of laser frequencies N2* N2+ • Retain ion-neutral discrimination 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 McCall Group ◦ Ben McCall ◦ Andrew Mills ◦ Michael Porambo Funding