Brian Siller, Michael Porambo & Benjamin McCall Chemistry Department University of Illinois at Urbana-Champaign

 ◦ ◦ Applications Astrochemistry Fundamental physics  ◦ ◦ Goals Completely general (direct absorption) High resolution

     Molecular ions are important to interstellar chemistry Ions important as reaction intermediates >150 Molecules observed in ISM C 2 H 4 Only ~20 are ions Need laboratory data to provide astronomers with spectral targets H 2 O e OH e CH 3 OCH 3 CH 3 OH CH 2 CO H 3 O + H 2 C 3 H 2 C 2 H 5 + CH H 2 O + H 2 e OH + CH 3 + e C 2 H 3 + C + CH 4 O C 6 H 7 + H 2 C 6 H 5 + C 2 H 2 C 4 H 3 + H C 4 H 2 + C C 3 H 3 + H 2 C 3 H + C + C 2 H 2 e CH 5 + H 2 CH 3 + H 2 CH 2 + CH + H 2 H 3 + C CO e e e C 6 H 6 C 3 H C 2 H HCO + C 2 H 5 CN CH 3 CN HCN CH 3 NH 2 H 2 + H 2

  Combination differences to compute THz transitions by observing rovibrational transitions in the mid-IR Support for Herschel, SOFIA, and ALMA THz observatories 60-670 µm 0.3-1600 µm 3-400 µm

3320 3300 3280 3260 3240 80 -1 . 0 60 40 20 0 - 1. 0 3320 3300 3280 -0 . 5

IR Transitions Even Combination differences Odd Combination Differences 1-0 Rotational Transition Reconstructed Rotational Transitions

3260 3240 80 -1.0

0. 0 -0.5

60 40 20 0.0

- 0. 5 0 -0.5

0.0

0.5

1.0

J’ 4 3 2 1 0 1.5

2.0

0. 5 6 0.5

1.0

5 4 3 2 1 0 J” 1.5

1. 0 1 . 5 2 . 0 1. 0 1 .5

2 . 0

 ◦ ◦ ◦ CH 5 + is a prototypical carbocation S N 1 reaction intermediates Highly fluctional structure Spectrum completely unassigned E.T. White, J. Tang, and T. Oka, “CH5+: The Infrared Spectrum Observed”, Science, 284, 135-137 (1999).

Animation from Joel Bowman, Emory University

 ◦ ◦ Positive Column High ion density Simple setup  ◦ ◦ Ion Beam Rigorous ion-neutral discrimination Mass-dependent Doppler shift

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

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

 ◦ ◦ ◦ Positive column discharge cell High ion density, rich chemistry Cations move toward the cathode Ions absorption profile is Doppler-shifted Laser -1kV +1kV 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  Single-pass Heterodyne @ 1GHz 0 1 2

  ◦ Doppler-broadened lines ◦ ◦ Blended lines Limited determination of line centers Sensitivity Limited path length through plasma  Improve by combining with cavity enhanced absorption spectroscopy

Ti:Sapph Laser Polarizing Beamsplitter Quarter Wave Plate AOM 30MHz 0.1-60kHz EOM Detector PZT Lock Box <100Hz Detector Cavity Transmission Error Signal

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

   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)

   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   N 2 + N 2 * Meinel Band first positive band  Second time a Lamb dip of a molecular ion has been observed (first was DBr technique) 1 + in laser magnetic resonance  Used 2 lock-in amplifiers for N 2 + /N 2 * B. M. Siller, A. A. Mills and B. J. McCall, Opt. Lett.

, 35, 1266-1268. (2010) 1 M. 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 Cavity Modes Laser Spectrum J. Ye, L. S. Ma, and J. L. Hall, JOSA B, 15, 6-15 (1998)

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

Ti:Sapph Laser EOM PZT Detector

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

113 MHz Sidebands 1 Cavity FSR

Dispersion

Lock-In X Lock-In Y

Absorption

Dispersion

Lock-In X Lock-In Y

Absorption

No center Lamb dip in absorption

Spectra calibrated with optical frequency comb Frequency precision to <1 MHz!

Ultra-High Resolution Spectroscopy

Dispersion Absorption 113MHz Sub-Doppler fit based on pseudo-Voigt absorption and dispersion profiles (6 absorption, 7 dispersion) Line center from fit: 326,187,572.2 ± 0.1 MHz After accounting for systematic problems, line center measured to within uncertainty of ~300 kHz!

VMS CEVMS OHVMS

   Better sensitivity than traditional VMS ◦ ◦ Increased path length through plasma Decreased noise from heterodyne modulation Retained ion-neutral discrimination ◦ ◦ Sub-Doppler resolution Better precision & absolute accuracy with comb Resolve blended lines  Can use same optical setup for ion beam spectroscopy

Ti:Sapph Laser EOM EOM PZT Ion Beam Instrument Detector Lock-In Amplifier Lock-In Amplifier X Y X Y Absorption Signal Dispersion Signal 40 kHz Plasma Frequency

Laser

S _ R I Be S

Brewster window Einzel lens 2 retractable Faraday cup TOF beam modulation electrodes Faraday cup electrostatic deflector 2 drift tube (overlap) variable apertures electrostatic deflector 1 steerers Einzel lens 1 ion source Ion source Ion optics wire beam profile monitors Current measurements Co-linearity with laser Mass spectrometer Laser coupling Velocity modulation ±5V ~ ±100MHz Brewster window electron multiplier TOF detector Ground 4kV 2kV

   Ion density ~5×10 6 Cavity finesse ~450 Lock-in τ=10s cm -3   '  1     4kV float voltage ±5V modulation ~120MHz linewidth Float voltage 2

qV Mc

2 Ion mass

 ◦ ◦ ◦ Positive Column High ion density Simpler setup Direct measurement of transition rest frequency  ◦ ◦ ◦ ◦ Ion Beam Rigorous ion-neutral discrimination Simultaneous mass spectroscopy Mass identification of line each spectral No Doppler-broadened component of lineshape

 ◦ ◦ Positive Column Mid-IR OPO system  ~1W mid-IR idler power  Pump and signal lasers referenced to optical frequency comb Liquid-N 2 cooled discharge cell  ◦ ◦ Ion Beam Mid-IR DFG laser  Ti:Sapph referenced to comb  Nd:YAG locked to I 2 hyperfine transition Supersonic expansion discharge source

  McCall Group ◦ ◦ Ben McCall Michael Porambo Funding