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Quantum Dynamics at Novel Types of Conical Intersections

Subject Area Theoretical Chemistry: Electronic Structure, Dynamics, Simulation
Optics, Quantum Optics and Physics of Atoms, Molecules and Plasmas
Term from 2016 to 2024
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 324648380
 
Final Report Year 2025

Final Report Abstract

In this project, we investigated the effects of the couplings between electronic, vibrational, and rotational degrees of freedom on quantum dynamics of various molecular processes. The proposal mainly involves three lines of research, namely, (i) non-adiabatic quantum dynamics at lightinduced conical intersections (LICIs), (ii) effect of nuclear permutation symmetry on dynamical localization in symmetric molecules, and (iii) phonon-driven vibrational relaxation of adsorbates on surfaces. In the first subproject, we developed a topological approach to fully unwind the molecular wavefunction at a LICI, allowing to disentangle the effects of the geometric phase (GP) and nonadiabatic electronic transitions. Leaning on the fingerprints of the GP in molecular observables, this technique provides a clear recipe for the search of LICIs in molecules. To corroborate our theoretical/numerical findings, a simple corresponding experimental scheme, dealing with laser dissociation of D2+ molecules, has been proposed. The extension of the developed approach to other types of electronic degeneracies, such as glancing intersections, has also been explored. In the second subproject, we scrutinized the effect of nuclear permutation symmetry on dynamical localization of vibrational or electronic densities in highly symmetric molecules, viz. those involving identical (indistinguishable) molecular structures or configurations. Using rigorous group-theoretic analysis, we show that nuclear permutation symmetry engenders entanglement between different molecular degrees of freedom, which subsequently leads to complete quenching of (dynamical) localization on identical molecular configurations−an observation that is in sharp contradiction with the textbook view of iconic quantum processes and the interpretation of well known experiments. These include umbrella inversion of ammonia, electronic Kekulé dynamics in benzene, and electron-localization following laser dissociation of H2+ Molecules. The third subproject is devoted to the development of approximate wavefunction approaches for phonon-induced vibrational relaxation of adsorbates at surfaces. A process which cannot be handled with standard wavefunction methods, due to the curse of dimensionality. To alleviate the exponential scaling of the latter, we developed two different methods, namely, ‘the hierarchical effective mode approach’ and ‘ the quantum tier model’. Both lead to a drastic reduction of the dimension of the adsorbatesurface vibrational space. These approaches have been successfully applied to the decay of H-Si (D-Si) vibrations on a fully H(D)-covered silicon surface, which is modeled with a phonon bath of more than two thousand oscillators. These approximate yet accurate numerical schemes embody most dissipative and (de)coherent effects, which cannot be accounted for with the widely used ‘reduced density matrix formalism’, and thus provide more reliable estimates of the adsorbate vibrational lifetimes.

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