Interstellar cyanoacetylene: Cationic and cloudy

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  • Published: Aug 15, 2015
  • Author: David Bradley
  • Channels: Atomic
thumbnail image: Interstellar cyanoacetylene: Cationic and cloudy

ZEKE

High resolution photoelectron spectrum of HCCCN (Credit-YMO/Tsinghua)

Researchers in China have used zero-kinetic energy photoelectron spectroscopy to determine the quantum energy levels of the cyanoacetylene cation with unprecedented detail, the work has fundamental merit in organic chemistry but might also improve our understanding of interstellar clouds and the atmosphere of Saturn's largest moon Titan where this ion is present in abundance.

Zuyang Dai, Wei Sun, Jia Wang, and Yuxiang Mo of the Department of Physics and State Key Laboratory of Low-Dimensional Quantum Physics, at Tsinghua University, in Beijing, China, explain how they have measured the spin-vibronic energy levels of the cyanoacetylene cation, in the Journal of Chemical Physics.

Born again, Oppenheimer

It is usually assumed, when describing molecular motion, that vibrational motion in the nucleus doesn't have an effect on the electronic states of an atom, largely because of the enormous disparity between the size of the electrons and neutrons and protons in the nucleus. This is the well-known Born-Oppenheimer approximation and is an assumption that underpins much of chemical physics. However, it seems that the assumption breaks down when we are confronted with the photoinduced energy states of cyanoacetylene moieties.

Now, by "interrogating" the energy levels in cyanoacetylene and related organic molecules with zero-kinetic energy photoelectron spectroscopy, the Tsinghua team has offered us a better understanding of the processes and potentially a way to move forward from Born-Oppenheimer approximation.

"[It] is an ideal tool to study the energy structure of the cation," explains physicist Mo, who led the research "At the present, there is no other experimental tool that can accomplish this task."

Exceptional

It was known that exceptions to the Born-Oppenheimer approximation exist, of course, for every rule an exception, after all. These are seen in linear, symmetric molecules with degenerate electronic states, such as cyanoacetylene (one hydrogen, one nitrogen, and three carbon atoms). In this molecule, the nuclear and electronic motions are vibronically coupled, which means that a small change in one will affect the others - the Renner-Teller effect.

In order to study this in detail, the team used a tuneable, nanosecond, pulsed vacuum ultraviolet laser to pump the sample cyanoacetylene molecules to highly electronically excited Rydberg states. Two beams focus on to a pulsed jet of xenon gas, for four-wave mixing. A small, pulsed electric field then ionized the excited molecules for detection purposes.

This technique of ZEKE spectroscopy is well suited to measuring vibrational energy of cations and gave the Tsinghua team a full, high-resolution spectrum showing the energy levels for cations from their vibrational ground state to excited states several thousand wavenumbers, higher. These energy levels agreed with theoretical vibronic energy levels for the cyanoacetylene cation generated using a diabatic model. The team also obtained data on the spin-vibronic energy levels of fluoromethane, chloromethane, and monochloroacetylene cations, which are highly symmetric or linear molecules that also have strong electron and nuclear coupling effects. "From these results, it seems that now we understand the main physics of the vibronic coupling for these benchmark molecules," Mo adds.

Related Links

J Chem Phys, 2015, 143, 054301: "The Renner-Teller effect in HCCCN+(X2N) studied by zero-kinetic energy photoelectron spectroscopy and theoretical calculations"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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