Observation of a rotational transition in trapped and sympathetically cooled molecular ions

TL;DR

This paper reports the first direct observation of a fundamental rotational transition in sympathetically cooled molecular ions, demonstrating rotational excitation at millikelvin temperatures using THz radiation and laser cooling techniques.

Contribution

It introduces a novel method for directly observing rotational transitions in molecular ions cooled to very low temperatures, expanding capabilities in molecular spectroscopy.

Findings

First direct observation of the fundamental rotational transition in HD+

Successful rotational laser cooling to enhance signal detection

Detection of black-body-radiation-induced rotational excitation

Abstract

We demonstrate rotational excitation of molecular ions that are sympathetically cooled by laser-cooled atomic ions to a temperature as low as ca. 10 mK. The molecular hydrogen ions HD+ and the fundamental rotational transition $(v=0,\, N=0)\rightarrow(v'=0,\, N'=1)$ at 1.3 THz, the most fundamental dipole-allowed rotational transition of any molecule, are used as a test case. This transition is here observed for the first time directly. Rotational laser cooling was employed in order to increase the signal, and resonance-enhanced multiphoton dissociation was used as detection method. The black-body-radiation-induced rotational excitation is also observed. The extension of the method to other molecular species is briefly discussed.