Superposition of Quantum and Classical Rotational Motions in Sc2C2@C84 Fullerite

TL;DR

This paper models the combined quantum and classical rotational motions in Sc2C2@C84 fullerite, deriving Raman spectra predictions that align well with experimental data across various temperatures.

Contribution

It introduces a theoretical framework for superimposing quantum tunneling and classical rotations in encapsulated fullerene systems, linking these motions to observable Raman spectra.

Findings

The theory accurately predicts Raman spectral features across temperatures.

Quantum and classical rotational motions are effectively coupled in the model.

Experimental spectra agree well with theoretical predictions.

Abstract

The superposition of the quantum rotational motion (tunneling) of the encapsulated Sc2C2 complex with the classical rotational motion of the surrounding C84 molecule in a powder crystal of Sc2C2@C84 fullerite is investigated by theory. Since the quantum rotor is dragged along by the C84 molecule, any detection method which couples to the quantum rotor (in casu the C2 bond of the Sc2C2 complex also probes the thermally excited classical motion (uniaxial rotational diffusion and stochastic meroaxial jumps) of the surrounding fullerene. The dynamic rotation-rotation response functions in frequency space are obtained as convolutions of quantum and classical dynamic correlation functions. The corresponding Raman scattering laws are derived, the overall shape of the spectra and the width of the resonance lines are studied as functions of temperature. The results of the theory are confronted with experimental low-frequency Raman spectra on powder crystals of Sc2C2@C84 [M. Krause et al., Phys. Rev. Lett. 93, 137403 (2004)]. The agreement of theory with experiment is very satisfactory in a broad temperature range.