Does nuclear permutation symmetry allow dynamical localization in symmetric double-well achiral molecules?

Area of Science:

• Quantum Chemistry
• Molecular Dynamics
• Spectroscopy

Background:

• Coherent tunneling in symmetric double-well potentials is crucial for understanding molecular dynamics.
• Textbook models often simplify tunneling by focusing solely on the tunneling mode, neglecting other molecular degrees of freedom.

Purpose of the Study:

• To reexamine the conventional picture of coherent tunneling in symmetric double-well potentials.
• To investigate the role of molecular symmetry and quantum statistics in tunneling phenomena.
• To clarify the conditions for dynamical localization in molecular systems.

Main Methods:

• Theoretical analysis of molecular wavefunctions and their symmetry properties.
• Consideration of nuclear permutations and their impact on quantum statistics.
• Integration over non-tunneling molecular degrees of freedom to obtain reduced probability densities.

Main Results:

• The indistinguishability of identical particles, governed by quantum statistics, prevents wavepacket localization in specific molecular configurations.
• Double-well eigenstates must be entangled with other molecular modes to maintain correct symmetry.
• Reduced probability densities are always symmetrically distributed, quenching interference effects.
• Dynamical localization in the space-fixed frame is permissible for aligned molecules due to parity-dependent state combinations.

Conclusions:

• Coherent tunneling dynamics are fundamentally constrained by quantum statistics, preventing localization in indistinguishable molecular sites.
• Dynamical localization in the space-fixed frame is allowed and distinct from localization in molecular configurations.
• This distinction is critical for interpreting ultrafast molecular dynamics experiments.