Back to Table of Contents

42580-G6
Spectroscopy of Carbocations and C60

Benjamin J. McCall, University of Illinois (Urbana-Champaign)

^{_{We are in the midst of developing two new experiments aimed at studying molecules and molecular ions of importance to both chemistry and astronomy.}}

Our first experiment is aimed at obtaining a rotationally resolved infrared spectrum of gas-phase C₆₀. Solid C₆₀ is heated in an oven to ~600 C, and then pre-heated argon is passed through the oven, leaking through a small orifice into a vacuum chamber, thereby generating a supersonic expansion. In this expansion, the C₆₀ is cooled to ~25 K while remaining in the gas phase. A custom-made quantum cascade laser with emission around 8.4 microns will then be used to perform absorption spectroscopy using a continuous-wave cavity ringdown scheme. To date, we have built the oven and demonstrated that it is capable of both volatilizing the C₆₀ (as evidenced by deposition of C₆₀ in our vacuum chamber) and that it cools the gas to low temperatures (by performing cavity ringdown spectroscopy of the N₂⁺ ion in the near-infrared). We have obtained the quantum cascade laser, and have successfully obtained cavity ringdown spectra of a test molecule, CH₂Br₂. We are currently working with a vendor to test a prototype Faraday isolator (not commercially available in this wavelength range) to prevent back-reflections from our ringdown cavity that tend to induce mode hops in the quantum cascade laser. We expect to obtain the spectrum of C₆₀ in the very near future, as all of the major pieces are now in place.

Our second experiment is targeting complex carbocations, such as CH₅⁺ and C₆H_{7+. We are developing a new technique called SCRIBES (Sensitive, Cooled, Resolved, Ion BEam Spectroscopy) that will overcome the limitations of previous experimental techniques used for high-resolution spectroscopy of molecular ions. SCRIBES will use a supersonic expansion discharge source to produce rotationally cold molecular ions, which will then be pulled out of the expansion using electrostatic optics and accelerated into fast ion beam. The ion beam, after being turned by an electrostatic quadrupole, will be collinearly probed with a tunable infrared laser using cavity ringdown spectroscopy. To date, we have built and characterized a test (non-supersonic) source, assembled the ion optics and the quadrupole, and have successfully produced an intense and well collimated ion beam. We have decided to characterize the system using the well-known spectrum of N2⁺, as a test, and then to move onto more complex molecular ions. We have obtained cavity ringdowns of the ion beam, but found that electrons were impinging on one of our cavity mirrors, degrading the quality factor of the cavity. We have recently implemented a deflector to prevent this, and will shortly return to the N2⁺ spectroscopy. In other developments, we have recently completed the construction of a new building to house our large Roots pump, and the vacuum system is now fully functional. After optimizing the system with N2⁺ with our warm ion source, we will couple the ion beam system to a supersonic ion source and begin our experiments on carbocations.}

The funding from the ACS PRF has been extremely helpful in jump-starting the development of these experiments, and has been successfully leveraged into several federal grants. I am very grateful for this early support from the ACS.

Back to top