Internal rotation in a loosely bound ion-rare-gas complex: He-H3+
The ro-vibrational predissociation spectrum of He-H3+ has been recorded via excitation of the ν2 vibrational mode of the H3+ sub-unit in the 22-pole ion trap experiment COLTRAP. The spectrum for the bare H3+ consists of only a few ro-vibrational lines each for the para and ortho nuclear spin configuration, respectively. Instead, the spectrum of the complex is very rich (severalhundred lines) even at the low experimental temperatures (4 K). Part of this complexity is associated with the (almost) free internal rotation of H3+. The experimental results are compared to theoretical predictions of ro-vibrational spectra on the basis of ab-initio calculations of the H3+ potential energy surface. The energy levels result in transitions which agree in many cases with experimental results within a few wavenumbers. In particular the typical band structures of a P- and R-branch associated with an effective diatomic complex seen in the experimental and predicted spectrum help in assigning the rich spectrum. Moreover, an experimental energy term diagram is reconstructed from the observed transitions which can be compared to the rather accurate theoretical predictions. The system is discussed using the approach of Jeremy M. Hutson  typically used to describe the dynamics of Van der Waals molecules. The influence of the Coriolis interaction in a case 2 coupled complex resulting from the H+3 internal rotation in a rotating He-H+3 is discussed.
 Jeremy M. Hutson, Advances in Molecular Vibrations and Collision Dynamics, 1991, 1, 1-45