Understanding the chemistry of the frozen universe

To better understand the chemistry of the universe, scientists must be able to accurately characterize and quantify complex organic molecules formed and desorbed from simulated interstellar ices, a feat that was previously difficult to accomplish due to limitations in detection methods. Radhakrishnan et al. developed a new apparatus to accurately measure the yield of gaseous molecules from interstellar ice with a spectroscopic fingerprint for each.

The apparatus also yields the rotational temperature of molecules cooled within a buffer gas cell — corresponding to interstellar ice gases — and correctly obtains branching ratios despite inconsistent spectrometer throughput across a broad frequency range.

“This work employs chirped-pulse rotational spectroscopy in the milimeter-wave regime to probe molecules that are cooled within a buffer gas cell,” author Bernadette Broderick said. “This work in itself is not a dramatic advance; however, coupling it to probe molecules sublimed from ices, which is currently underway, will bring new capabilities for astrochemistry and chemistry in ices in general.”

To simulate and characterize the interstellar gas, the team cooled propyl cyanide with pre-cooled neon, excited the gaseous molecules with radiation, and measured their free-induction decay, correlating these signals with specific chemical forms — even specific isomers or conformers — of the radiated gas.

The authors are building on this work by more closely simulating the formation and desorption of organic molecules formed in interstellar ices.

“Paper two of this series is currently in preparation as we have shown that we can generate ices at 4K, perform temperature-programmed desorption where molecules enter the gas phase, are injected into the buffer gas cell where they are cooled, and probed with broadband rotational spectroscopy,” Broderick said.

Source: “Buffer gas cooled ice chemistry. I: Buffer gas cell and mm-wave spectrometer,” by S. Radhakrishnan, T. Hager, A. Kanaheraarachchi, C. Williams, G. E. Hall, and B. M. Broderick, Journal of Chemical Physics (2022). The article can be accessed at https://doi.org/10.1063/5.0111792 .