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

• Applicants: Dr. Oskar Asvany; Professor Dr. Stephan Schlemmer
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
• Term: from 2009 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 133140313

• Final Report Year: 2021

Final Report Abstract

This project started out as the experimental work on high-resolution spectroscopy of the enigmatic CH+5 molecular ion in the first funding period. In the second funding period we split the work into an experimental and a theoretical part because the theoretical part also became highly successful and needed dedicated support. Major advances of the project concerned (i) the development of the super-rotor model which treats the floppy motion of CH+5 as a rotation in five dimensions. An energy term diagram based on this zeroth order SO(5) model agrees surprisingly well with the experimentally determined term diagram already determined during the first funding period. (ii) Following Ritz combination principle for atomic spectroscopy we further developed a method to reconstruct the energy term diagram of the lowest rotational states of (any) molecule. This method has been very successfully applied to the measured ro-vibrational spectrum of CH+5 where now a large fraction of the observed lines are associated with the term diagram. (iii) Two of the three nuclear spin isomers of CH+5 are found as a result of this very cumbersome work. However, the most intense lines of the I=1/2 (A1 ) nuclear spin state have not yet been assigned. Further experimental investigations are needed and pure rotational transitions are still missing for this very floppy molecule. New experimental and theoretical methods for this purpose have been developed in this project. (iv) Spectra of other floppy molecules have been explored in order to enlarge the suite of experimental methods. In particular first measurements on the cationic hydrogen-helium complexes Hn He+m have been conducted. Among these systems the H+3-He complex shows a striking similarity to the CH+5 molecule in that the in-plane motion of He about H+3 is without a barrier and the out-of-plane motion has only a small barrier. Therefore, He explores an almost free internal rotation which resembles the free internal rotation of the CH+3 sub-unit and the H2 moiety in CH+5. More work in this direction is the subject of the ERC advanced grant (MissIons). (v) Detailed kinetics studies of the endothermic reaction CH+5 + CO2 and its isotopic variants allowed us to determine the zero-point vibrational energies (ZPVEs) of CH+5 and its isotopologs. The experimental values agree rather well with theoretical predictions.

Publications

• “Symmetry of extremely floppy molecules: Molecular states beyond rotation-vibration separation,” J. Chem. Phys., vol. 143, p. 154302, 2015.
  H. Schmiedt, S. Schlemmer, and P. Jensen
  
• “Collective molecular super-rotation: A model for extremely floppy molecules applied to protonated methane,” Phys. Rev. Lett., vol. 117, p. 223002, 2016
  H. Schmiedt, P. Jensen, and S. Schlemmer
  
• “Unifying the rotational and permutation symmetry of nuclear spin states: Schur-Weyl duality in molecular physics,” J. Chem. Phys., vol. 145, no. 7, p. 074301, 2016
  H. Schmiedt, P. Jensen, and S. Schlemmer
  
• “Rotation-vibration motion of extremely flexible molecules – the molecular superrotor,” Chem. Phys. Lett., vol. 672, pp. 34–46, 2017
  H. Schmiedt, P. Jensen, and S. Schlemmer
  
• “Searching for new symmetry species of CH+5 - From lines to states without a model,” J. Mol. Spectrosc., vol. 342, pp. 73 – 82, 2017
  S. Brackertz, S. Schlemmer, and O. Asvany
  
• “Double Resonance Rotational Spectroscopy of He-HCO+,” Phys. Chem. Chem. Phys., vol. 21, p. 3440, 2018
  T. Salomon, M. Töpfer, P. Schreier, S. Schlemmer, H. Kohguchi, L. Surin, and O. Asvany
  
• “Double resonance rotational spectroscopy of Weakly Bound Ionic Complexes: the case of floppy CH+3 -He,” Phys. Rev. Lett., vol. 121, p. 143001, 2018
  M. Töpfer, T. Salomon, S. Schlemmer, O. Dopfer, H. Kohguchi, K. M. T. Yamada, and O. Asvany
  
• “Spectroscopy of the lowfrequency vibrational modes of CH+3 isotopologues,” J. Mol. Spectrosc., vol. 347, pp. 1–6, 2018
  O. Asvany, S. Thorwirth, B. Redlich, and S. Schlemmer
  
• “Fingerprints of microscopic superfluidity in HHe+ clusters,” Mol. Phys., vol. 117, p. 1559, 2019
  A. G. Császár, T. Szidarovszky, O. Asvany, and S. Schlemmer
  
• “Infrared photodissociation of cold CH+3 -He2 complexes,” Mol. Phys., vol. 117, no. 9-12, pp. 1481–1485, 2019
  M. Töpfer, P. C. Schmid, H. Kohguchi, K. M. T. Yamada, S. Schlemmer, and O. Asvany
  
• “Infrared Signatures of the HHe+n and DHe+n (n = 3 − 6) Complexes,” J. Phys. Chem. Lett., vol. 10, no. 18, pp. 5325–5330, 2019
  O. Asvany, S. Schlemmer, T. Szidarovszky, and A. G. Császár
  
• “Spectroscopic signatures of HHe+2 and HHe+3,” Phys. Chem. Chem. Phys., vol. 22, p. 22885, 2020
  M. Töpfer, A. Jensen, K. N. Anders, H. Kohguchi, T. Szidarovszky, A. G. Császár, S. Schlemmer, and O. Asvany
  
• “Rovibrational spectroscopy of the CH+ -He and CH+ -He4 complexes,” J. Mol. Spectrosc., vol. 377, p. 111421, 2021
  T. Salomon, J. L. Dom´nech, P. C. Schmid, E. A. Michael, S. Schlemmer, and O. Asvany
  
• “Vibrational spectroscopy of H2He+ and D2He+,” J. Mol. Spectrosc., vol. 377, p. 111423, 2021
  O. Asvany, S. Schlemmer, A. van der Avoird, T. Szidarovszky, and A. G. Császár