Guiding astronomical observations by understanding vibronic coupling interactions

The diffuse interstellar bands are a series of over 500 lines in the visible part of the electromagnetic spectrum that are observed in space. Only one molecule has been confirmed to create one of these lines, so the composition of molecules in space remains mysterious. To find these molecules and match them to astronomical observations, simulations of their spectra are required.

Ethynyl radicals are one of the most abundant molecules in the universe and one of the key building blocks for molecules in combustion and interstellar chemistry. Understanding the simplest carbon monohydride and their spectra can guide astronomical observations and laboratory experiments.

However, simulating such spectra is difficult due to the effects of vibronic coupling. Mixing between electronic and vibrational motions complicate approximations that treat the two motions separately.

Laws et al. obtained high resolution velocity-map imaged photoelectron spectra of ethynyl anions. They also performed simulations to account for vibronic coupling interactions and replicate the spectra.

“The photoelectron spectrum shows the electronic and vibrational structures of the neutral molecule. Sharp, complex structures are resolved, which are direct signatures of the vibronic coupling interactions,” said author Benjamin Laws. “Because we obtain excellent agreement between the experimental data and simulated spectrum, we can confirm that our proposed quasidiabatic approach accurately simulates the vibronic coupling effects.”

By creating a benchmarked model, the authors now have a technique to apply to larger molecules and in different environmental conditions.

“Extending this work to much larger carbon chains may help determine if we can observe these species in space,” said Laws.

Source: “Velocity map imaging spectroscopy of C2H− and C2D−: A benchmark study of vibronic coupling interactions,” by Benjamin A. Laws, Zachariah D. Levey, Andrei Sanov, John F. Stanton, Timothy W. Schmidt, and Stephen T. Gibson, Journal of Chemical Physics (2022). The article can be accessed at https://doi.org/10.1063/5.0100297 .