Quantum Dynamics of a Hydrogen Molecule Confined in a Cylindrical Potential

TL;DR

This paper investigates the quantum rotational and vibrational behavior of hydrogen molecules confined in cylindrical potentials, such as inside carbon nanotubes, revealing how confinement shape affects molecular dynamics and energy levels.

Contribution

It introduces a formalism that couples rotation and translation in confined hydrogen molecules and applies it to nanotubes, providing new insights into their energy spectra and dynamics.

Findings

Energy level degeneracies depend on rotation-translation coupling.

Confinement shape influences molecular off-centering and dynamics.

Predictions for neutron scattering transitions in confined hydrogen molecules.

Abstract

We study the coupled rotation-vibration levels of a hydrogen molecule in a confining potential with cylindrical symmetry. We include the coupling between rotations and translations and show how this interaction is essential to obtain the correct degeneracies of the energy level scheme. We applied our formalism to study the dynamics of H$_{2}$ molecules inside a "smooth" carbon nanotube as a function of tube radius. The results are obtained both by numerical solution of the ($2J+1$)-component radial Schrodinger equation and by developing an effective Hamiltonian to describe the splitting of a manifold of states of fixed angular momentum $J$ and number of phonons, $N$. For nanotube radius smaller than $\approx 3.5$ \AA, the confining potential has a parabolic shape and the results can be understood in terms of a simple toy model. For larger radius, the potential has the "Mexican hat" shape and therefore the H$_{2}$ molecule is off-centered, yielding radial and tangential translational dynamics in addition to rotational dynamics of H$_{2}$ molecule which we also describe by a simple model. Finally, we make several predictions for the the neutron scattering observation of various transitions between these levels.