PerspectiveMETROLOGY

High-precision molecular measurement

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Science  04 Sep 2020:
Vol. 369, Issue 6508, pp. 1160-1161
DOI: 10.1126/science.abb9186

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Summary

The hydrogen molecular ion (H2 + ≡ p+ + p+ + e) is the simplest molecule with two protons bound by an electron. Historically, it was the first molecule to be studied by using quantum mechanics, and it remains on the short list of experimentally accessible molecules for which a truly precise theoretical understanding is possible. However, several characteristics make precision optical spectroscopy of H2+ a formidable challenge in laboratory experiments. Hydrogen deuteride molecular ion (HD+ ≡ p+ + d+ + e), in which one of the protons of H2+ is replaced by a deuteron (13), has an asymmetric dipolar structure that allows numerous vibrational-rotational transitions. These “rovibrational” transitions (νrv) have exceptionally narrow relative widths of less than 10−13 and occur at much higher rates when compared to the even narrower H2+ transitions. On page 1238 of this issue, Patra et al. report two frequencies with a precision of 2.9 parts per trillion and determine the mass ratio between the proton and electron (2).

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