Rovibrational spectroscopy of the CH$^+$-He and CH$^+$-He$_4$ complexes
  A cryogenic 22-pole ion trap apparatus is used in combination with a
table-top pulsed IR source to probe weakly bound CH$^+$-He and CH$^+$-He$_4$
complexes by predissociation spectroscopy at 4 K. The infrared
photodissociation spectra of the C-H stretching vibrations are recorded in the
range of 2720-2800 cm$^{-1}$. The spectrum of CH$^+$-He exhibits perpendicular
transitions of a near prolate top with a band origin at 2745.9 cm$^{-1}$, and
thus confirms it to have a T-shaped structure. For CH$^+$-He$_4$, the C-H
stretch along the symmetry axis of this oblate top results in parallel
transitions.
