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 measure for molecules and ions with small dipole moments (e.g. CH₅⁺, C₆H₇⁺). 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, a mid-infrared continuous-wave cavity ringdown spectrometer, 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, which is a result of the thermal velocity spread to velocity ratio being low when the ions are traveling very fast.

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 overlap the drift region with a laser inside high-finesse optical cavity and perform continuous-wave cavity ringdown spectroscopy on the ion beam. The laser used in this experiment is a home-built difference frequency laser, made by combining the continuous-wave outputs of a Nd:YAG laser (1064 nm) and a tunable 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 ~1x10^-7) and high spectral resolution (3 x 10^-5 cm^-1). Finally, because the light in the cavity propagates both with and against the direction of the ion beam, a mass-dependent Doppler splitting will be present in the spectrum, further aiding in spectral assignment.

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 will 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 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 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. By combining this information with the aforementioned mass-dependent Doppler splitting, we will be able to definitively prove that the spectrum we observe actually comes from the ion we want.

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

Current Work

The development of SCRIBES is presently split into several projects. The supersonic ion source is being developed and characterized separately and will be integrated at a later stage. The major challenge for the initial measurements with SCRIBES is to produce a well-defined ion beam with high number density to be observed with our cavity ringdown spectrometer, which has already been delevolped and used in a separate experiment (Paper #38). Therefore, we are currently working with a cold cathode ion source to produce an N₂⁺ beam of sufficent intensity as a test case for the instrument before we switch to ions of astrophysical relevance like H₃⁺, CH₅⁺ and C₃H₃⁺.

A bender used to steer the ion beam by 90 degrees.

Once the ion beam is extracted from the source and accelerated to 4 keV it will be focused by an Einzel lens and guided into a cylindrical 90 degree deflector. After the deflector, the interaction region will be defined by two circular apertures, and in this section cavity ringdown spectroscopy will take place by overlapping the ion beam and the laser cavity longitudinally over a distance of ~20 cm.

Following the interaction region a second cylindrical bender is used to deflect the ion beam out of the cavity and measure the current in a Faraday cup. At a future date it is foreseen to utilize time-of-flight mass spectroscopy in order to study the composition of the ion beams in more detail.

Related Content

Talks

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

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

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.