The frequency-domain infrared spectrum of ammonia encodes changes in molecular dynamics caused by a DC electric field.
basic_science · Level V
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- Record sourced from PubMed, PMID 31690662.
- Also identified by DOI 10.1073/pnas.1914432116 and PMC identifier 6876197.
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Abstract
Ammonia is special. It is nonplanar, yet in <i>v</i> = 1 of the umbrella mode (ν<sub>2</sub>) its inversion motion is faster than <i>J</i> = 0↔1 rotation. Does the simplicity of the Chemist's concept of an electric dipole moment survive the competition between rotation, inversion, and a strong external electric field? NH<sub>3</sub> is a favorite pedagogical example of tunneling in a symmetric double-minimum potential. Tunneling is a dynamical concept, yet the quantitative characteristics of tunneling are expressed in a static, eigenstate-resolved spectrum. The inverting-umbrella tunneling motion in ammonia is both large amplitude and profoundly affected by an external electric field. We report how a uniquely strong (up to 10<sup>8</sup> V/m) direct current (DC) electric field causes a richly detailed sequence of reversible changes in the frequency-domain infrared spectrum (the <i>v</i> = 0→1 transition in the ν<sub>2</sub> umbrella mode) of ammonia, freely rotating in a 10 K Ar matrix. Although the spectrum is static, encoded in it is the complete inter- and intramolecular picture of tunneling dynamics.