SCRIBES

Sensitive, Cooled, Resolved Ion BEam Spectroscopy

A diagram of the SCRIBES instrument. Click for full size (opens in a new window/tab).

Introduction

Molecular ions play vital roles in many diverse areas of chemistry and astronomy, and are particularly relevant to chemistry in the interstellar medium (ISM). Because the ISM has a low number density (~100 cm^-3) and temperature (~30 K), reactions with small barriers (such as ion/molecule reactions) dominate the chemistry. Because spectroscopy is the only tool available for probing astronomical environments, it is important to obtain laboratory spectra of these ions so that they may be detected in space. Vibrational spectroscopy is a particularly effective tool for this, as a molecule's vibrational spectrum contains a unique fingerprint based on its structure. Furthermore, a vibrational spectrum can be used to extract the rotational spectrum of a molecule, which is difficult to obtain because of difficulties in producing sufficient numbers of molecules and the microwave search problem. With SCRIBES, we are developing cutting-edge laboratory techniques for laser spectroscopic study of molecular ions in the gas phase under astrophysically relevant conditions. SCRIBES consists of an ion source, a fast ion beam, highly sensitive cavity enhanced spectroscopies, and a mass spectrometer.

The SCRIBES experiment as of 7 October 2009. The source chamber is in the background; the benders and drift region are to the right. The long tube in the foreground is the time-of-flight mass spectrometer.

Ion Source

Direct current discharges have commonly been used to produce ions for spectroscopy. However, the ions are produced with high rotational and vibrational temperatures. This is problematic not only because of increased spectral congestion, but also because, for weak transitions, the band strength is spread out over a large number of transitions instead of only a few, making the ion more difficult to observe. To overcome this, we are implementing a supersonic expansion discharge source, which will produce translationally and rotationally cold ions (<20 K). This not only solves the previously-mentioned issues, but also allows us to observe the spectrum as it would appear in the interstellar medium.

Ion Beam

A typical plasma is only about 1x10^-6 ionized, so the vast majority of the plasma consists of un-ionized molecules. These can complicate the spectrum by absorbing in the same region that ions of interest absorb. In order to reduce this spectral confusion, we are using a fast ion beam to spatially separate the ions from the neutrals using electrostatic ion optics. An additional benefit of a fast ion beam is a reduction in the absorption linewidth through an effect called kinematic compression.

Spectrometer

When the ions are spatially separated from the neutrals, they are turned and sent into a drift region, where they are available to be probed by laser spectroscopy. We have attempted several types of cavity enhanced spectroscopies, including continuous-wave Cavity RingDown Spectroscopy (cw-CRDS), Cavity Enhanced Absorption Spectroscopy (CEAS), and Noise Immune Cavity Enhanced Optical Heterodyne Molecular Spectroscopy (NICE-OHMS). Currently we use a tunable cw Ti-Sapphire as our laser source in the near-infrared. But we also have a home-built difference frequency generation (DFG) laser which can be used to produce mid-infrared light. A DFG laser is made by combining the continuous-wave outputs of a Nd:YAG laser (1064 nm) and the Ti:Sapphire laser (700-900 nm) in a periodically-poled LiNbO₃ nonlinear crystal. With this laser spectrometer, we are able to achieve high sensitivity (minimum detectable absorbance between ~1x10^-7 and ~1x10^-10) and high spectral resolution (3 x 10^-5 cm^-1).

A cold cathode discharge source used to produce hot ions. It is currently being used as a test source for aligning the ion beam.

Mass Spectrometer

When using a plasma source to produce ions for spectroscopy, it can be difficult to determine whether the observed spectrum comes from the ion of interest, or some other ionic species. We use a beam modulated time-of-flight mass spectrometer (BM-TOF-MS) to identify the species that are produced in our continuous ion source in SCRIBES. The BM-TOF-MS device uses quickly-pulsed deflecting plates to sweep the ion beam over a slit aperture placed near a dual micro-channel plate detector, thereby creating a small packet of ions. The ions in this packet are separated by mass during flight through a 1.5 meter drift region, resulting in a mass resolution on the order of 1 amu. By recording the mass spectrum of the ion beam, we can confirm the presence of our ion of interest in the beam, and also use the spectrum as a feedback mechanism for optimizing the production of that ion in the plasma. Additionally, the mass spectrometer can give information about the beam energy of the ion beam, as well as the energy spread. By measuring the spread of arrival times of single ions, one can determine the energy spread of the ion beam, which corresponds to a linewidth in the spectroscopy. Knowing the beam energy allows us to determine where lines will appear as a consequence of the Doppler-shift of the ions. Also, we have used the mass spectrometer to diagnose the performance of the uncooled cold cathode that produces the ions of interest. By using hydrogen as our discharge gas, we have found that the source contains different degrees of ion collisions depending on the orientation of the electrodes. This information is useful for determining the optimum source setup to produce ions of various electronic and vibrational energies. Having this mass spectrometer has proven invaluable in deducing the barriers to spectroscopy.

Highly Accurate and Precise Spectra

Traditionally, the accuracy of work in the mid-infrared is limited by the frequency standards that are used. Wavemeters typically have an absolute accuracy of 60-200 MHz, thereby limiting the ultimate accuracy of the measurement. This level of precision does not allow the pure rotational transitions to be inferred from the mid-infrared measurements with sufficient precision to enable radioastronomical searches. This problem can be reduced by using state of the art frequency measurements on the mid-infrared transitions. We do this using a MenloSystems Optical Frequency Comb. By using a GPS disciplined high stability crystal oscillator as our frequency reference, and a 70 MHz accuracy wavemeter, in conjunction with the frequency comb, we are able to determine line centers on transitions to better than 1 MHz. By improving the accuracy from 60 to 1 MHz, we ultimately increase the accuracy of measurements, further allowing for microwave observations to be enabled by mid-infrared spectroscopy.

NICE-OHVMS signal of N₂⁺ in the fast ion beam. The light blue trace is the raw signal from the experiment, while the dark trace is the same signal smoothed.

Current Work

We have recently acquired our first signal of N₂⁺ produced by an uncooled cold cathode source in the ion beam using velocity modulated NICE-OHMS. Presently, we are optimizing the sensitivity and lineshape of the signal. We can successfully acquire spectra of the N₂⁺ ion beam using two modes of modulation: velocity modulation and concentration modulation. We also shortly aim to acquire N₂⁺ ion beam spectra precisely calibrated to the optical frequency comb. With these precisely calibrated spectra, the line centers of the probed transitions should be measured to a high degree of accuracy.

After the spectrometer is optimized for N₂⁺ spectroscopy in the near-infrared, we plan to build the mid-infrared DFG system and use the ion beam spectrometer to acquire vibrational spectra of various molecular ions of astrochemical interest. We will begin by studying a simple test ion, HN₂⁺. Later, we aim to record ultra-high resolution spectra of astronomically interesting molecular ions, such as HCO⁺, HOC⁺, and others. Further in the future, we will integrate the supersonic expansion discharge ion source to the instrument to study the supersonically cooled spectra of CH₅⁺, C₃H₃⁺, and other nonlinear molecuar ions. At this point, the building of SCRIBES should be complete, and will hopefully function as a powerful tool for high resolution rotationally cooled spectroscopy of molecular ions.

Related Content

Papers

64	A. A. Mills, B. M. Siller, M. W. Porambo, M. Perera, H. Kreckel, and B. J. McCall "Ultra-Sensitive High-Precision Spectroscopy of a Fast Molecular Ion Beam" Journal of Chemical Physics (2011), 135, 224201.
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.
53	A. A. Mills, B. M. Siller, and B. J. McCall "Precision Cavity Enhanced Velocity Modulation Spectroscopy" Chemical Physics Letters (2010), 501, 1-5.
50	K. N. Crabtree, C. A. Kauffman and B. J. McCall "A Modular and Robust Continuous Supersonic Expansion Discharge Source" Review of Scientific Instruments (2010), 81, 086103.
48	B. M. Siller, A. A. Mills and B. J. McCall "Cavity-Enhanced Velocity Modulation Spectroscopy" Optics Letters (2010), 35, 1266-1268.

Talks

104	M. W. Porambo, H. Kreckel, A. Mills, M. Perera, B. M. Siller and B. J. McCall "High Precision, Sensitive, Near-IR Spectroscopy in a Fast Ion Beam" Midwest Astrochemistry Meeting, University of Illinois, Urbana, IL, 2011.
102	B. J. McCall "New Approaches to High Resolution Spectroscopy of Molecular Ions" 31st International Symposium on Free Radicals, Port Douglas, Australia, 2011.
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.
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.
95	M. W. Porambo, A. A. Mills, B. M. Siller, H. Kreckel, M. Perera and B. J. McCall "Progress and Recent Developments in Sensitive, Cooled, Resolved Ion Beam Spectroscopy (SCRIBES)" Sixty-Sixth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2011.
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-six International Symposium on Molecular Spectroscopy, The Ohio State University,Columbus, OH, 2011.
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.
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.
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.
81	A. A. Mills, B. M. Siller, H. Kreckel, M. Perera and B. J. McCall "Towards High Resolution Cavity Enhanced Spectroscopy with Fast Ion Beams" Sixty-Fifth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2010.
71	H. Kreckel, A. A. Mills, M. Perera, B. M. Siller, K. N. Crabtree, C. A. Kauffman and B. J. McCall "High Resolution Spectroscopy of Molecular Ion Beams" 239th American Chemical Society National Meeting, San Francisco, CA, 2010.
70	H. Kreckel, A. A. Mills, M. Perera, B. M. Siller, K. N. Crabtree, C. A. Kauffman and B. J. McCall "SCRIBES: Sensitive, Cooled, Resolved Ion-BEam Spectroscopy" Second Midwest Astrochemistry Meeting, University of Illinois, 2009.
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.
64	M. Perera, K. N. Crabtree, K. B. Ford, H. Kreckel, A. A. Mills and B. J. McCall "Progress in the Development of an Infrared Ion Beam Spectrometer" Sixty-Fourth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH, 2009.
47	A. A. Mills, K. N. Crabtree, and B. J. McCall "Progress on the Development of an Infrared Ion Beam Spectrometer" Sixty-Third International Symposium on Molecular Spectroscopy, Ohio State University, Columbus, OH, 2008.
35	S. L. Widicus Weaver, A. A. Mills, and B. J. McCall "Cavity Ringdown Spectroscopy of Molecular Ions in a Fast Ion Beam" Sixty-Second International Symposium on Molecular Spectroscopy, Ohio State University, Columbus, OH, 2007.
34	A. A. Mills, S. L. Widicus Weaver, and B. J. McCall "Development of a Fast Ion Beam Spectrometer for Molecular Ion Spectroscopy" Sixty-Second International Symposium on Molecular Spectroscopy, Ohio State University, Columbus, OH, 2007.

Posters

23	M. W. Porambo, H. Kreckel, A. A. Mills, M. Perera, B. M. Siller and B. J. McCall "Sensitive, Cooled, Resolved, Ion BEam Spectroscopy: SCRIBES" Midwest Astrochemistry Meeting, University of Illinois, Urbana, IL, 2010.
20	A. A. Mills, B. M. Siller, H. Kreckel, M. Perera, M. W. Porambo and B. J. McCall "New Approaches to Molecular Ion Spectroscopy" Spectroscopy of Molecular Ions in the Laboratory and in Space, Kos, Greece, 2010.
14	K. N. Crabtree, K. B. Ford, C. A. Kauffman, H. Kreckel, A. A. Mills, M. Perera, B. M. Siller and B. J. McCall "SCRIBES: Sensitive, Cooled, Resolved, Ion BEam Spectroscopy" Advancing Chemical Understanding Through Astronomical Observations, Green Bank Telescope, Green Bank, WV, 2009.
11	A. A. Mills, H. Kreckel, M. Perera, K. N. Crabtree, B. M. Siller, K. B. Ford and B. J. McCall "Ultrasensitive Infrared Spectroscopy of Molecular Ions" Air Force Molecular Dynamics Contractors Meeting, San Diego, CA, 2009.
9	A. A. Mills, K. B. Ford, K. N. Crabtree and B. J. McCall "High Resolution Spectroscopy of Molecular Ions: Development of an Instrument" Inaugural Midwest Astrochemistry Meeting, University of Illinois, 2008.
6	S. L. Widicus Weaver, A. A. Mills, and B. J. McCall "Continuous-wave Cavity Ringdown Spectroscopy of Molecular Ions in a Fast Ion Beam" American Chemical Society National Meeting, Chicago, Illinois, 2007.

Other Publications

28	A. A. Mills "Design, Construction, and Characterization of an Ultra-sensitive, High-precision Fast Ion-Beam Spectrometer for the Study of Molecular Ions" Ph.D. Thesis, University of Illinois, 2011.
23	B. M. Siller "Cavity Enhanced Velocity Modulation Spectroscopy of C₂H₅⁺" Research Prospectus for Preliminary Examination, University of Illinois, 2010.
20	K. N. Crabtree "Design of a Continuous Supersonic Expansion Discharge Source for the Acquisition of a Rotationally-Cold Vibrational Spectrum of CH₅⁺ with the SCRIBES Instrument" Research Prospectus for Preliminary Examination, University of Illinois, 2009.
15	A. A. Mills "Construction of the SCRIBES (Sensitive Cooled Resolved Ion BEam Spectroscopy) Instrument for the Detection of Astrochemically Important Molecular Ions" Research Prospectus for Preliminary Examination, University of Illinois, 2007.
14	B. E. Brumfield "High-Resolution Spectroscopic Studies of C₆₀ and C₆H₇⁺: Molecules of Fundamental Spectroscopic and Astrochemical Importance" Research Prospectus for Preliminary Examination, University of Illinois, 2007.