Rotor in a Cage: Infrared Spectroscopy of an Endohedral Hydrogen-Fullerene Complex

TL;DR

This study observes and models the quantum translational and rotational motions of hydrogen molecules confined inside C60 fullerenes using infrared spectroscopy, revealing detailed vibrational and coupling phenomena.

Contribution

It provides the first detailed experimental observation and theoretical modeling of quantized motions of H2 inside C60, including vibrational, translational, and rotational coupling.

Findings

Infrared lines correspond to vibrational, translational, and rotational excitations.

Coupling between translational and rotational modes causes observable splittings.

The theoretical model accurately predicts frequencies and intensities of transitions.

Abstract

We report the observation of quantized translational and rotational motion of molecular hydrogen inside the cages of C60. Narrow infrared absorption lines at the temperature of 6K correspond to vibrational excitations in combination with translational and rotational excitations and show well resolved splittings due to the coupling between translational and rotational modes of the endohedral H2 molecule. A theoretical model shows that H2 inside C60 is a three-dimensional quantum rotor moving in a nearly spherical potential. The theory provides both the frequencies and the intensities of the observed infrared transitions. Good agreement with the experimental results is obtained by fitting a small number of empirical parameters to describe the confining potential, as well as the ortho to para ratio.