Theoretical Investigation of C_60 IR Spectrum

TL;DR

This paper presents a theoretical model explaining the weak IR-active modes of C60 as combination modes caused by anharmonicity, with electrical anharmonicity aligning well with experimental observations.

Contribution

It introduces a semi-empirical model distinguishing mechanical and electrical anharmonicity origins of IR modes in C60, favoring electrical anharmonicity for experimental consistency.

Findings

Electrical anharmonicity reproduces experimental IR spectrum features.

Mechanical anharmonicity predicts a different spectral pattern.

Weak IR-active modes are likely combination modes due to anharmonicity.

Abstract

A semi-empirical model of the infrared (IR) spectrum of the C$_{60}$ molecule is proposed. The weak IR-active modes seen experimentally in a C$_{60}$ crystalline sample are argued to be combination modes caused by anharmonicity. The origin of these 2-mode excitations can be either mechanical (anharmonic interatomic forces) or electrical (nonlinear dipole-moment expansion in normal modes coordinates). It is shown that the electrical anharmonicity model exhibits basic features of the experimental spectrum while nonlinear dynamics would lead to a qualitatively different overall picture.