High Resolution Spectroscopy of protonated methane: Structure and dynamics of floppy molecules

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This project focuses on the spectroscopy of the enigmatic CH5+ molecular ion. It is the prototype of a strongly bound yet very fluxional molecule. As a result the general picture of a molecular structure represented by a ball and stick diagram is in question for CH5+ and similar molecules. In the first funding period infrared spectra at very high resolution have revealed a first view at the energy term diagram of the lowest energy states. We succeeded to associate transitions to the three distinct nuclear spin states of this molecule with five equivalent protons. This is the first step in assigning the spectrumof this unconventional molecule. Based on the experimental developments in this project in the last six years it will be possible to record the THz spectrum of CH5+ and thus to find the lowest energy transitions, may they be pure rotational or ro-vibrational in nature. Our experimental development is complemented by a powerful, new theoretical description of the molecular energy terms and their transitions. This fundamentally different approach is triggered by the recent finding that the ro-vibrational wave-function no longer can be separated into a rotational and vibrational part. Instead the new model, partly adoptedfrom nuclear physics will reveal term diagrams beyond the traditional distorted rigid rotor approach. Measuring and interpreting the spectrum of CH5+ in this combined experimental and theoretical project potentially will lead to a general, proper description of non-rigid molecules from weakly bound clusters to more strongly bound structures. Therefore the project aims at fundamentally improved understanding of molecular physics.