Rotational diffusion of a molecular cat

  Ori Katz-Saporta  ,  Efi Efrati  
Weizmann Institute of Science

We show that a simple isolated system can perform rotational random walk on account of internal excitations alone. We consider the classical dynamics of a "molecular cat": a triatomic “molecule” connected by three harmonic springs with non-zero rest lengths, suspended in free space.  In this system, much like for falling cats, the angular momentum constraint is non-holonomic allowing for rotations with zero overall angular momentum. The geometric nonlinearities arising from the non-zero rest lengths of the springs suffice to break integrability and lead to chaotic dynamics. The coupling of the non-integrability of the system and its non-holonomic nature results in an angular random walk of the molecule. We study the properties and dynamics of this angular motion analytically and numerically. For low energy excitations the system displays normal-mode-like motion, while for high enough excitation energy we observe regular random-walk. In between, at intermediate energies we observe an angular Lévy-walk type motion associated with a fractional diffusion coefficient interpolating between the two regimes.