Theoretical spectroscopy

One of the major developments of quantum chemistry in the last years has been the increased accuracy of the first principle eigenenergy and transition moment calculations of small polyatomic molecules based on accurate global potentials and exact nuclear motion Hamiltonians. These calculations not only support the experimental spectroscopy, they allow to obtain qualitative results regarding the internal dynamics of molecules when they are excited to energies corresponding to transition states. At such excitations "new physics" appears, the eigenstates cannot be modeled through normal mode vibrations.

The existence of an ab initio eigenenergy spectrum seems not to be helpful at first sight: the eigenenergies for example of the [H,C,N] molecular system, labelled only by the rigorously good quantum numbers (J and parity), do not permit direct extraction of any physical insight from these data. To search for new classes of patterns in the spectra of vibrationally highly excited small polyatomic molecules and to investigate the validity of the vibrational model (the organization of rotational levels into vibrational levels) used at low internal excitation, the ab initio eigenenergy spectrum must be analyzed in a way similar to how high resolution spectra are assigned and modeled. I developed spectroscopic methods to assign these eigenenergy levels up to excitations corresponding to the transition state. For the [H,C,N] system the complete analysis of the rovibrational spectrum of a polyatomic molecule has been achieved (ca. 5000 vibrational and rotational constants for each possible state of the molecule), including all rovibrational states below and 1500 cm^-1 above the isomerization barrier.  Based on this data set it was already possible to predict and later experimentally confirm new effects regarded to the internal dynamics of linear HAB molecules, some unpublished effects found in the eigenenergy spectrum still have to be explained/modeled.   

 I intend to continue this research and to develop completely automated assignment procedures that combine the eigenenergy based spectroscopic assignment with the quantum chemistry related methods based on state vectors.