We have trapped and sympathetically cooled the molecular ions HCO ϩ and N 2 H ϩ in a Penning trap.Through their Coulomb interaction with laser-cooled Mg ϩ ions the molecules were sympathetically cooled to cryogenic temperatures. We identify the molecules through a measurement of their characteristic massdependent breathing mode frequencies. From a measurement of the temperature of the Mg ϩ ions we estimate that the final temperature of the sympathetically cooled molecules is 4 K. ͓S1050-2947͑99͒04611-9͔

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