Many ion/molecule reactions relevant to interstellar and atmospheric chemistry proceed under conditions where classical kinetic models break down. Among these are radiative association reactions—barrierless processes in which two species (in our case a neutral molecule and an ion) collide and stabilize by emitting a photon rather than undergoing three-body collisions.
To study such reactions in the lab, our group developed the CODEx (Cryogenic Optical cavity for Direct-absorption Experiments) instrument, using buffer-gas cooling techniques to prepare cold, state-resolved molecules in the gas phase. We use cavity-enhanced spectroscopy to probe rotational and translational temperatures with high sensitivity and resolution, enabling direct evaluation of low-temperature reactivity, line strengths, and population distributions.
Figure 1. Buffer-gas cell used for cavity ringdown spectroscopy at cryogenic temperatures.
In recent work, we demonstrated that hydrogen cyanide (HCN) can be rotationally and translationally cooled to temperatures near 150 K, and used these conditions to measure Doppler-broadened spectral lines with high accuracy. These studies validated our ability to quantify molecular temperatures under conditions relevant to ion/molecule kinetics.
We have also extended these techniques to cyanoacetylene (HC3N), a key prebiotic molecule detected in planetary and cometary atmospheres. Our group produced the first low-temperature spectrum of its 2ν1 overtone near 1.5 μm—a region accessible to the James Webb Space Telescope and upcoming infrared observatories. These results support its use as a target for exoplanet spectroscopy and offer experimental data for benchmarking theory.
Figure 2. Cyanoacetylene is synthesized in our laboratory, and the near-infrared spectrum measured at 20 K.
Together, these studies form the foundation for our long-term goal: to investigate radiative association reactions between cold neutral molecules (like HC3N, HCN) and atomic ions under astrophysically relevant conditions. CODEx will ultimately be combined with our glow-discharge ion-trap (GDIT) kinetics instrument to make these measurements.
Figure 3. The glow-discharge ion-trap instrument for measuring ion/molecule reaction kinetics.
Selected Publications
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Howard, T. et al. “The 1.5 μm band of cyanoacetylene as a spectroscopic target in astrochemistry,”
J. Phys. Chem. Lett., 2025, 16, 3748–3753.
DOI: 10.1021/acs.jpclett.5c00724 -
Howard, T. et al. “Buffer-gas cooling of hydrogen cyanide quantified by cavity-ringdown spectroscopy,”
J. Mol. Spec., 2024, 406, 111953.
DOI: 10.1016/j.jms.2024.111953