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 (3x10^-5 cm^-1).

A cold cathode discharge source used to produce hot ions. It was 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-OHMS signal of N₂⁺ in the fast ion beam. The black dots are the experimental data, while the smooth lines are the fit functions. The red and blue coloring correspond to the red-shifted and blue-shifted signals respectively.

Near-IR Work

Using our tunable Ti:Sapphire laser, we were able to obtain dispersion NICE-OHMS spectra of N₂⁺ using an uncooled cold cathode source in the ion beam spectrometer. We used our optical frequency comb to precisely calibrate the spectra of N₂⁺, determining the line center of the rest frequency to within ~8 MHz. Our calculated sensitivity was ~2x10^-11 cm^-1 Hz^-1/2.

Mid-IR Work

After the spectrometer was optimized for N₂⁺ spectroscopy in the near-infrared, we implemented the mid-infrared DFG system. Our system is designed to perform NICE-OHMS on various molecular ions of astrochemical interest. We tested the mid-IR capabilities of our DFG system on methane. We were able to acquire both Doppler-broadened spectra and sub-Doppler features with sensitivities of ~2x10^-7 cm^-1 Hz^-1/2 and ~6x10^-9 cm^-1 Hz^-1/2 respectively. This achievement represents the first ever NICE-OHMS DFG system.

Current Work

We are currently working to integrate our supersonic expansion discharge ion source into our DFG system. Initially, we plan to characterize and optimize the new source by studying HN₂⁺. Additionally, we plan to use the optical frequency comb and an iodine cell for stabilizing the Ti:Sapphire and the Nd:YAG lasers respectively. These two steps will allow us to acquire precisely calibrated spectra with the ability to determine the line centers of the probed transitions to a high degree of accuracy. The supersonic expansion discharge source will enable us to study the supersonically cooled spectra of CH₅⁺, C₃H₃⁺, and other nonlinear molecular ions. Following the completion of the new source, the building of SCRIBES should be complete, and will hopefully function as a powerful tool for high resolution rotationally cooled spectroscopy of molecular ions.