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Updated: Jan 4, 2026

Infrared Degenerate Four-wave Mixing with Upconversion Detection for Quantitative Gas Sensing
Published on: March 22, 2019
The frequency-domain infrared spectrum of ammonia encodes changes in molecular dynamics caused by a DC electric field
Youngwook Park1, Hani Kang1, Robert W Field2
1Department of Chemistry, Seoul National University, 08826 Seoul, Republic of Korea.
Abstract:
Ammonia is special. It is nonplanar, yet in v = 1 of the umbrella mode (ν2) its inversion motion is faster than J = 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? NH3 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 108 V/m) direct current (DC) electric field causes a richly detailed sequence of reversible changes in the frequency-domain infrared spectrum (the v = 0→1 transition in the ν2 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.
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