Cold, Ion-Neutral, Chemical Reactions in Coulomb Crystals With Thermal Neutral Sources and Progress Towards Stark-Decelerated Sources

Laser-cooled ions in radio frequency traps provide a unique environment in which to study ion-neutral gas-phase reactions. Such systems form Coulomb crystals, which offer a translationally cold and inherently flexible system for studying reaction kinetics and dynamics. In combination with a time-of-flight mass spectrometer, our linear ion trap has been optimized to monitor gas-phase chemical reactions one molecule at a time with high resolution detection of products. In addition, the low pressures and cold conditions offered by this environment mimic important aspects of the astrochemical environment; this expands the relevance of our studies to chemical dynamics throughout multiple regions of space, including the interstellar medium and planetary atmospheres. 

In my dissertation, I present experimental data on three important reactions of interstellar interest studied in our linear ion trap: CCl++CH3CN, CCl++C6H6, and C2H2 ++CH3CN. Each of these reactions has demonstrated fast kinetics with branching to multiple products of interest to astrochemical modelers. In two of these reactions (CCl++CH3CN and C2H2 ++CH3CN) we were able to demonstrate the reaction dynamics with a full potential energy surface; this also indicated interesting underlying mechanics that influence the outcome of these reactions. 

My work also involved development of future directions of this experiment, including a new neutral molecular beam source. This will provide collisional energy and quantum state control over the neutral reactant with a traveling wave Stark decelerator (TWSD). In my dissertation, I describe the characterization and integration of a TWSD with our linear ion trap. I demonstrate detection of a decelerated molecular beam of ND3 in krypton at the location of the ion trap with tunable final velocities down to 20m/s. This advance allows future reactions in our linear ion trap to be energy-resolved such that ion-neutral kinetic theories may be empirically compared over the energy ranges of ~1K – 50 K. This combined setup broadens the astrochemical relevance of our measurements to a critical range of temperatures, as well as providing an excellent opportunity to understand the underlying impacts of collisional energy on reaction dynamics.