McCall Research Group Illinois

Sub-Doppler Rovibrational Spectroscopy of Molecular Ions with NICE-OHVMS

NICE-OHVMS: Noise Immune Cavity Enhanced - Optical Heterodyne Velocity Modulation Spectroscopy

Introduction

We have developed a sensitive, accurate, and precise instrument to operate in the mid-IR for spectroscopy on molecular ions. It has applications in astrochemistry, indirect THz spectroscopy, and fundamental chemical physics.

  1. Astrochemistry

    Molecular ions are known to play a role as reactive intermediates in the interstellar medium (ISM), but only ~20 have been observed in the ISM to date. That number represents only ~13% of the > 160 total observed species in space. So few have been observed due to the dearth of laboratory spectra to aid astronomical searches. Furthermore, the more lines that can be observed and correctly identified, the better we can understand the environment of the molecules being observed.

  2. Picture showing combination differences
    An illustration showing how a particular energy level spacing may be determined by combination differences from a pair of transitions sharing a state. In this case, the energy spacing is between J=0 and J=2. To determine the allowed transition, the spacing between J=0 and J=1 must be known.
  3. Indirect THz Spectroscopy

    Small ions which are found in the ISM have rotational transitions in the THz region of the spectrum. There exist many lines that astronomers have observed in the THz region of the spectrum where laboratory work is the most underrepresented. As such, many lines that have been observed are from unknown species. Additionally, telescopes such as Herschel, ALMA, and SOFIA can measure transitions of astronomical species in the THz. Direct THz spectroscopy in the laboratory is a nascent field that is proving to be technologically challenging for molecular ions. However, by measuring rovibrational transitions to high precision and accuracy, rotational energy level spacings can be determined, and rotational transitions can be determined from the spacings. Precision and accuracy reduce the error that is associated with this type of analysis. Since we are trying to calculate an intrinsically small value, low uncertainty is necessary to determine a meaningful value.

  4. Fundamental Chemical Physics

    The spectrum of CH5+, first observed in 1999 by White, Tang, and Oka, remains unassigned. This is attributed to the highly fluxional nature of the molecule. Classically, this can thought of as the protons scrambling freely around the carbon atom, challenging the traditional notion of structure. In an effort to gain some understanding of the spectrum, we seek to do a four line combination differences analysis. By determining pairs of pairs of transitions whose differences in energies are the same, we can infer energy level spacings. The trick to four line combination differences is that they must have small uncertainties to avoid false positives. The uncertainty of Oka's orginal spectrum is not conducive to this type of analysis. Our instrument will be able to measure line centers with unprecedented accuracy and precision thereby reducing the number of false positives in a four line combination difference analysis.

Instrument

This experiment is complementary to the Supersonic Source work. The principal difference between the experiments is the trade-offs inherent in each system. Utilization of the cooled supersonic expansion discharge source is geared toward very low rotation temperatures. Ground state transitions may prove challenging to observe in hotter ion sources due to the scaling of the partition function. On the other hand, our experiment uses a positive column that produces several orders of magnitude greater ions density than an ion beam, though at a higher temperature.

Block diagram of NICE-OHVMS
A diagram of the NICE-OHVMS instrument. PZT: Piezo Transducer, EOM: Electro-Optic Modulator, PS: Phase Shifter, PSD: Phase Sensitive Detector, RF: Radio Frequency Oscillator, OPO: Optical Parametric Oscillator, P: Pump, S: Signal, I: Idler, YDFL: Ytterbium Doped Fiber Laser. A YDFL is fiber coupled to an EOM where locking and heterodyne sidebands are placed on the laser. The laser is then amplified in a fiber amplifier and sent to an OPO. The OPO creates the signal and idler beams. The pump and signal are used for frequency measurements, and the idler in the mid-IR is used for spectroscopy. The back reflection detector is used for Pound-Drever-Hall locking, and the signal from the transmission detector is sent to rf mixers for heterodyne processing. The output of the mixers is sent to PSDs to process the velocity modulation signal. The output is then recorded as our spectrum. Click for full size.

We probe our ions using an Aculight Argos Model 2400 Optical Parametric Oscillator, which provides high power continuous tuning in the mid-IR. It is pumped by a Ytterbium doped fiber laser (1064 nm) that is coupled into a fiber electro-optical modulator which is then fiber amplified. It has a wavelength coverage between 3.2 and 3.9 μm. The light is then coupled into a bow-tie cavity containing a periodically polled lithium niobate (PPLN) crystal. In the crystal, optical parametric oscillation occurs. The pump photon is split into a signal photon and an idler photon. The signal photon is resonant with the cavity, and the idler is the difference of the pump and signal. With additional modules we can increase the coverage to 3-5 μm. The spectroscopic technique we use is called NICE-OHVMS. NICE-OHVMS is the combination of NICE-OHMS with Velocity Modulation Spectroscopy (VMS). VMS has traditionally used phase sensitive detection at the frequency that the ions' velocities are modulated. However, our technique makes use of the advantages of cavity enhancement e.g. enhanced intracavity power and pathlength. The addition of the cavity requires phase sensitive direction to occur at twice the frequency at which the velocity is modulated (2f). This is due to counter-propagating laser beams in the cavity probing the opposite direction of the Doppler shift belonging to the ions, i.e. when the ions move to the right, the transition is red-shifted in one direction and blue-shifted in the other. Operating at 2f was thought impossible due to the competition of concentration modulation occuring at twice the modulation frequency. However, our group has determined that these occur at seperate phases, and can be isolated in seperate channels of the phase sensitive detectors.

Black Widow Liquid Nitrogen Cooled Positive Column Discharge Cell
Black Widow: A Liquid Nitrogen Cooled Positive Column Discharge Cell for producing ions

Ions of interest are produced in a liquid nitrogen cooled, positive column, discharge cell named "Black Widow", located inside an optical cavity. We obtained Black Widow from Prof. Takeshi Oka's laboratory at the University of Chicago. Incidentally, this cell was the same cell that White, Tang, and Oka used to first observe CH5+. High voltage AC is applied across the cell with the gas precursor to the molecular ion of interest being continuously flowed through the cell. The use of liquid nitrogen is useful for cooling the plasma to ~300 K enhancing the signal strength for transitions that start at low energy states such as fundamental bands.

Due to the high optical power that we have with this instrument, we are able to saturate rovibrational transitions. This burns Bennet holes in the population, and causes Lamb dips to appear in the spectra. These features enable us to determine line centers to ~300 KHz precision resulting in low uncertainty in the line center determination. The Lamb dips are fit with a precision of about 70kHz and are calibrated with a MenloSystems Optical Frequency Comb, which can measure the frequency of the laser to < 100kHz. The combined accuracy and precision from this technique enable implementation of indirect THz spectroscopy.

H3+ signal
NICE-OHVMS signal of H3+. These lines are the R(1,0) [left] and R(1,1)u [right] transitions of the ν2 band. We have observed the R(1,0) transition with a S/N ~ 500, and both have Lamb dips in the center of each line. We have achieved a sensitivity of 8 x 10-10 cm-1 Hz-1/2.

Related Content

Papers

91 C. R. Markus, A. J. Perry, J. N. Hodges, and B. J. McCall
"Improving cavity-enhanced spectroscopy of molecular ions in the mid-infrared with up-conversion detection and Brewster-plate spoilers"
Optics Express (2017), 25, 3709-3721.
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90 C. N. Talicska, M. W. Porambo, A. J. Perry, and B. J. McCall
"Mid-infrared concentration-modulated noise-immune cavity-enhanced optical heterodyne molecular spectroscopy of a continuous supersonic expansion discharge source"
Review of Scientific Instruments (2016), 87, 063111.
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89 J. N. Hodges and B. J. McCall
"Quantitative velocity modulation spectroscopy"
Journal of Chemical Physics (2016), 144, 184201.
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87 C. R. Markus, J. N. Hodges, A. J. Perry, G. S. Kocheril, H. S. P. Müller and B. J. McCall
"High precision rovibrational spectroscopy of OH+"
The Astrophysical Journal (2016), 817, 138-142.
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85 A. J. Perry, J. N. Hodges, C. R. Markus, G. S. Kocheril and B. J. McCall
"High-precision R-branch transitions in the ν2 fundamental band of H3+"
Journal of Molecular Spectroscopy (2015), 317, 71-73.
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82 A. J. Perry, J. N. Hodges, C. R. Markus, G. S. Kocheril, and B. J. McCall
"Communication: High-precision sub-Doppler infrared spectroscopy of the HeH+ ion"
Journal of Chemical Physics (2014), 141, 101101-1-101101-5.
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79 J. N. Hodges, A. J. Perry, P. A. Jenkins II, B. M. Siller, and B. J. McCall
"High-Precision and High-Accuracy Rovibrational Spectroscopy of Molecular Ions"
Journal of Chemical Physics (2013), 139, 164201.
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76 B. M. Siller, J. N. Hodges, A. J. Perry, and B. J. McCall
"Indirect Rotational Spectroscopy of HCO+"
Journal of Physical Chemistry A (2013), 117, 10034-10040.
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72 K. N. Crabtree, J. N. Hodges, B. M. Siller, A. J. Perry, J. E. Kelly, P. A. Jenkins II, and B. J. McCall
"Sub-Doppler Mid-Infrared Spectroscopy of Molecular Ions"
Chemical Physics Letters (2012), 551, 1-6.
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63 B. M. Siller, M. W. Porambo, A. A. Mills, and B. J. McCall
"Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy"
Optics Express (2011), 19, 24822–24827.
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53 A. A. Mills, B. M. Siller, and B. J. McCall
"Precision Cavity Enhanced Velocity Modulation Spectroscopy"
Chemical Physics Letters (2010), 501, 1-5.
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48 B. M. Siller, A. A. Mills, and B. J. McCall
"Cavity-Enhanced Velocity Modulation Spectroscopy"
Optics Letters (2010), 35, 1266-1268.
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Talks

139 C. R. Markus, J. E. McCollum, T. S. Dieter, P. A. Kocheril, and B. J. McCall
"Sub-doppler rovibrational spectroscopy of the H3+ cation and isotopologues"
Seventy Second International Symposium on Molecular Spectroscopy, University of Illinois at Urbana-Champaign, 2017.
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140 C. R. Markus, J. E. McCollum, T. S. Dieter, J. N. Hodges, A. J. Perry, and B. J. McCall
"Cavity-enhanced spectroscopy of molecular ions in the mid-infrared with up-conversion detection and Brewster-plate spoilers"
Seventy Second International Symposium on Molecular Spectroscopy, University of Illinois at Urbana-Champaign, 2017.
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142 C. R. Markus, J. E. McCollum, T. S. Dieter, P. A. Kocheril, and B. J. McCall
"Cavity-enhanced spectroscopy of molecular ions in the mid-infrared with up-conversion detection and Brewster-plate spoilers"
12th International User Meeting on Cavity Enhanced Spectroscopy, Radboud University, Nijmegen, Netherlands, 2017.
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134 J. N. Hodges and B. J. McCall
"Quantitative determination of lineshape parameters from velocity modulation spectroscopy"
Seventy First International Symposium on Molecular Spectroscopy, The University of Illinois at Urbana-Champaign, 2016.
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135 J. N. Hodges and B. J. McCall
"Using NICE-OHVMS lineshapes to study relaxation rates and transition dipole moments"
Seventy First International Symposium on Molecular Spectroscopy, The University of Illinois at Urbana-Champaign, 2016.
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136 A. J. Perry, G. S. Kocheril, J. N. Hodges, C. R. Markus and B. J. McCall
"Highly accurate and precise infrared transition frequencies of the H3+ cation"
Seventy First International Symposium on Molecular Spectroscopy, The University of Illinois at Urbana-Champaign, 2016.
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137 C. R. Markus, J. N. Hodges, A. J. Perry, B. J. McCall
"Improved spectroscopy of molecular ions in the mid-infrared with up-conversion detection"
Seventy First International Symposium on Molecular Spectroscopy, The University of Illinois at Urbana-Champaign, 2016.
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129 J. N. Hodges, B. M. Siller and B. J. McCall
"Elucidating the Complex Lineshapes Resulting from the Highly Sensitive, Ion Selective Technique NICE-OHVMS"
Seventieth International Symposium on Molecular Spectroscopy, University of Illinois, Urbana-Champaign, IL, 2015.
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130 J. N. Hodges, A. J. Perry, C. R. Markus, P. A. Jenkins II, G. S. Kocheril and B. J. McCall
"Advances in Sensitive, Accurate, Precise Ion Spectroscopy through Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy"
Eleventh International User Meeting & Summer School on Cavity-Enhanced Spectroscopy, NIST / NOA, Boulder, CO, 2015.
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132 C. R. Markus, A. J. Perry, J. N. Hodges, G. S. Kocheril, P. A. Jenkins II, and B. J. McCall
"High Precision Infrared Spectroscopy of OH+"
Seventieth International Symposium on Molecular Spectroscopy, University of Illinois, Urbana-Champaign, IL, 2015.
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126 A. J. Perry, J. N. Hodges, C. R. Markus, G. S. Kocheril, P. A. Jenkins II, and B. J. McCall
"High-Precision Sub-Doppler Infrared Spectroscopy of HeH+"
Sixty-Ninth International Symposium on Molecular Spectroscopy, University of Illinois, Urbana-Champaign, IL, 2014.
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121 J. N. Hodges, A. J. Perry, C. R. Markus, P. A. Jenkins II, G. S. Kocheril, and B. J. McCall
"New High Precision Linelist of H3+"
Sixty-Ninth International Symposium on Molecular Spectroscopy, University of Illinois, Urbana-Champaign, IL, 2014.
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119 A. J. Perry, J. N. Hodges, B. M. Siller, and B. J. McCall
"Indirect Rotational Spectroscopy of HCO+"
Sixty-Eighth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2013.
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115 J. N. Hodges, A. J. Perry, B. M. Siller, and B. J. McCall
"Sub-Doppler Spectroscopy of H3+"
Sixty-Eighth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2013.
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116 J. N. Hodges, A. J. Perry, and B. J. McCall
"High Precision Spectroscopy of CH5+ using NICE-OHVMS"
Sixty-Eighth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2013.
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110 K. N. Crabtree, J. N. Hodges and B. J. McCall
"The Infrared Spectrum of CH5+ Revisited"
Sixty-Seventh International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2012.
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109 J. N. Hodges, K. N. Crabtree and B. J. McCall
"Sub-Doppler Spectroscopy of Molecular Ions in the Mid-IR"
Sixty-Seventh International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2012.
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101 B. M. Siller, A. A. Mills, M. W. Porambo and B. J. McCall
"Cavity Enhanced Velocity Modulation Spectroscopy"
Cavity Enhanced Spectroscopy User Meeting, Queen's University, Kingston, Ontario, Canada, 2011.
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94 A. A. Mills, M. W. Porambo, B. M. Siller, B. J. McCall
"Lineshape and Sensitivity of Spectroscopic Signals of N2+ in a Positive Column Collected Using NICE-OHVMS"
Sixty-Sixth International Symposium on Molecular Spectroscopy, The Ohio State University,Columbus, OH, 2011.
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93 B. M. Siller, A. A. Mills, M. W. Porambo and B. J. McCall
"Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy"
Sixty-Sixth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2011.
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90 B. M. Siller, A. A. Mills, M. W. Porambo and B. J. McCall
"Cavity Enhanced Velocity Modulation Spectroscopy"
Midwest Astrochemistry Meeting, University of Illinois, Urbana, IL, 2010.
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83 A. A. Mills, B. M. Siller and B. J. McCall
"Precision Cavity Enhanced Velocity Modulation Spectroscopy"
Sixty-Fifth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2010.
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80 B. M. Siller, A. A. Mills and B. J. McCall
"Cavity Enhanced Velocity Modulation Spectroscopy"
Sixty-Fifth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2010.
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65 A. A. Mills, K. B. Ford, H. Kreckel, M. Perera, K. N. Crabtree and B. J. McCall
"Indirect Terahertz Spectroscopy of Molecular Ions Using Highly Accurate and Precise Mid-IR Spectroscopy"
Sixty-Fourth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2009.
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10 B. J. McCall
"Laboratory Spectroscopy of H3+"
Royal Society Discussion Meeting on, London, England, 2000.
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Posters

33 G. S. Kocheril, A. J. Perry, C. R. Markus, J. N. Hodges and B. J. McCall
"High Precision Spectroscopy of Molecular Ions"
Chemistry REU Annual Poster Session, The University of Illinois at Urbana-Champaign, 2015.
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31 C. R. Markus, A. J. Perry, J. N. Hodges, G. S. Kocheril and B. J. McCall
"High precision and high accuracy spectroscopy of molecular ions"
Midwest Astrochemistry Meeting, The University of Northern Iowa, Cedar Falls, IA, 2015.
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26 J. N. Hodges, K. N. Crabtree, B. M. Siller, M. W. Porambo, A. A. Mills, and B. J. McCall
"Indirect THz Spectroscopy of Molecular Ions"
Midwest Astrochemistry Meeting, University of Illinois, Urbana, IL, 2011.
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Other Publications

42 G. S. Kocheril
"An Introduction to Noise-Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy"
B. S. Thesis, University of Illinois, 2016.
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38 C. R. Markus
"Progress in High-Precision Spectroscopy of Molecular Ions in the Mid-Infrared"
Research Prospectus for Preliminary Examination, University of Illinois, 2015.
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35 A. J. Perry
"Advances in High-Resolution Spectroscopic Measurements of Molecular Ions in the Mid-Infrared"
Research Prospectus for Preliminary Examination, University of Illinois, 2014.
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45 B. M. Siller and B. J. McCall
"Applications of NICE-OHMS to Molecular Spectroscopy"
Book Chapter, Cavity-Enhanced Spectroscopy and Sensing (Springer), 2014.
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33 B. M. Siller
"New Techniques for Sub-Doppler Spectroscopy of Molecular Ions"
Ph.D Thesis, University of Illinois, 2013.
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31 J. N. Hodges
"Fluxional Molecules with Five Equivalent Atoms: Mid-infrared Spectroscopy of CH5+ and H5+"
Research Prospectus for Preliminary Examination, University of Illinois, 2013.
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29 K. N. Crabtree
"Chemical Physics and Astrochemistry of H3+ and Sub Doppler Mid Infrared Spectroscopy of Molecular Ions"
Ph. D. Thesis, University of Illinois, 2012.
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