  A novel method of spectroscopy in ion traps termed leak-out spectroscopy
(LOS) is presented. Here, mass selected, cold ions are excited by an infrared
laser. In a subsequent collision with a neutral buffer gas particle their
internal energy is then transferred to kinetic energy. As a result, these ions
leak out from the ion trap and are detected. The LOS scheme is generally
applicable, very sensitive and close to background free when operated at low
temperature. The potential of this method is demonstrated and characterized
here for the first time by recording the rotationally resolved spectrum of the
C-H stretching vibration $\nu_1$ of linear C$_3$H$^+$. Besides performing
high-resolution spectroscopy, this method opens up the way for analyzing the
composition of trap content, e.g., determining isomer ratios, by selectively
expelling isomers or other isobaric ions from the trap. Likewise, LOS can be
used to prepare clean samples of structural and nuclear spin isomers.
