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

We demonstrate rotational excitation of molecular ions that are sympathetically cooled by laser-cooled atomic ions to a temperature as low as approximately 10 mK. The molecular hydrogen ions HD⁺ and the fundamental rotational transition (v=0,N=0)→(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 has not been observed before. 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.