Infrared emission spectra of fullerene C60 thin films

Infrared (IR) spectra of thin films of fullerene ${\mathrm{C}}_{60}$ molecules of icosahedral (${I}_{h}$) symmetry are recorded as thermal emission of radiation at elevated temperatures in the range of $27{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$--$88{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. Temperature dependence is analyzed for relative intensities of four IR-active emission bands of vibrational ${T}_{1u}$ modes of ${\mathrm{C}}_{60}$. Theoretical simulations are performed on the basis of the temperature-dependent thermal population of vibrationally excited states in a ${\mathrm{C}}_{60}$ 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 ${T}_{1u}(i)$, with $i=1--4$, at 526, 575, 1182, and 1427 ${\mathrm{cm}}^{\ensuremath{-}1}$, whereas the intensity of higher-frequency modes, ${T}_{1u}(3)$ and ${T}_{1u}(4)$, increases more drastically than that of ${T}_{1u}(1)$ and ${T}_{1u}(2)$. The analyses show clearly how and why the IR emission spectrum of ${\mathrm{C}}_{60}$ changes with temperature. For the thick sample of ${\mathrm{C}}_{60}$, saturation of the IR emission bands is observed due to self-absorption. The molecular IR emission of ${\mathrm{C}}_{60}$ is shown to be an ideal system for consideration of the vibrational temperature and an appropriate probe for astrophysical conditions of cosmic ${\mathrm{C}}_{60}$ molecules.