Sympathetic cooling and detection of molecular ions in a Penning trap

We have trapped and sympathetically cooled the molecular ions HCO⁺ and N₂H⁺ in a Penning trap. Through their Coulomb interaction with laser-cooled Mg⁺ ions the molecules were sympathetically cooled to cryogenic temperatures. We identify the molecules through a measurement of their characteristic mass-dependent breathing mode frequencies. From a measurement of the temperature of the Mg⁺ ions we estimate that the final temperature of the sympathetically cooled molecules is 4 K.