  First results are presented obtained with an experimental setup developed to
record IR spectra of rotationally state-selected ions. The method we use is a
state-selective version of a method developed by S. Schlemmer, D. Gerlich and
coworkers (Int. J. Mass. Spec. 185, 589 (1999); J. Chem. Phys. 117, 2068
(2002)) to record IR spectra of ions. Ions are produced in specific rotational
levels using mass-analyzed-threshold-ionization spectroscopy. The
state-selected ions generated by pulsed-field ionization of Rydberg states of
high principal quantum number ($n\approx200$) are extracted toward an octupole
ion guide containing a neutral target gas. Prior to entering the octupole, the
ions are excited by an IR laser. The target gas is chosen so that only excited
ions react to form product ions. These product ions are detected mass
selectively as a function of the IR laser wavenumber. To illustrate this
method, we present IR spectra of C$_2$H$_2^+$ in selected rotational levels of
the $^2\Pi_{\textrm{u,}3/2}$ and $^2\Pi_{\textrm{u,}1/2}$ spin-orbit components
of the vibronic ground state.
