The Crabtree lab currently focuses on studying kinetics of reactions that are relevant in astrochemistry. We're particularly interested in gas-phase reactions between radical and neutral species that can occur in the low temperature, low density environment of molecular clouds in the interstellar medium (ISM). Our goal is to provide rate constants and branching ratios to improve existing chemical models of the ISM and establish new ones.
We can accomplish this with a combination of a Chirped-Pulse Fourier Transform Microwave (CP-FTMW) spectrometer, supersonic molecular beam methods that employ a pulsed Laval nozzle, and an excimer laser. All of our chemistry happens in a vacuum chamber and begins when the reactant gases are pumped into a reservoir at one end of the chamber. The reactants are then pumped into the chamber through the Laval nozzle in 100 microsecond pulses, forming a uniform molecular beam (see the Pulsed Laval Nozzle page for specifics). As the gases enter the Laval nozzle, they are hit by the excimer laser in order to form radicals via photolysis. The molecular beam travels the length of the chamber and its composition is analyzed with the CP-FTMW spectrometer (see CP-FTMW page). Since the longer it takes the beam to reach the spectrometer, the more time the species have to react after photolysis, repeated microwave analysis allows us to track changes in abundance of reactants and the appearance of products.
In order to determine rate constants from this data, we use principles of pseudo first-order kinetics. This way we only have to monitor the change in relative concentration of one reactant in a bimolecular reaction. In a radical-neutral reaction, the radical will always have a much smaller concentration; this is the species that we monitor in each individual gas pulse in order to find its rate constant. By varying the starting concentration of the neutral species over many repeated experiments, we can extract its rate constant, as well.
Eventually, we plan on adding Laser Induced Fluorescence (LIF) to this system in order to increase our ability to monitor changes in the composition of the molecular beam.