Infrared emission spectra of fullerene C60 thin films

Infrared (IR) spectra of thin films of fullerene C60 molecules of icosahedral (Ih) symmetry are recorded as thermal emission of radiation at elevated temperatures in the range of 27°C-88°C. Temperature dependence is analyzed for relative intensities of four IR-active emission bands of vibrational T1u modes of C60. Theoretical simulations are performed on the basis of the temperature-dependent thermal population of vibrationally excited states in a C60 molecule, counting explicitly the number of overtones and combinations for all possible energy levels by the excitation of up to seven vibrational quanta. The observed IR emission intensity increases with increasing temperature for all the bands of T1u(i), with i=1-4, at 526, 575, 1182, and 1427 cm-1, whereas the intensity of higher-frequency modes, T1u(3) and T1u(4), increases more drastically than that of T1u(1) and T1u(2). The analyses show clearly how and why the IR emission spectrum of C60 changes with temperature. For the thick sample of C60, saturation of the IR emission bands is observed due to self-absorption. The molecular IR emission of C60 is shown to be an ideal system for consideration of the vibrational temperature and an appropriate probe for astrophysical conditions of cosmic C60 molecules.