Local heating and Raman thermometry in a single molecule

Because of the nonequilibrium nature of thermal effects at the nanoscale, the characterization of local thermal effects within a single molecule is highly challenging. Here, we demonstrate a way to characterize the local thermal properties of a single fullerene (C₆₀) molecule during current-induced heating processes through tip-enhanced anti-Stokes Raman spectroscopy. Although the measured vibron populations are far from equilibrium with the environment, we can still define an "effective temperature (T^eff)" statistically via a Bose-Einstein distribution, suggesting a local equilibrium within the molecule. With increased current heating, T^eff is found to rise up to about 1150 K until the C₆₀ cage is decomposed. Such a decomposition temperature is similar to that reported for ensemble C₆₀ samples, thus justifying the validity of our methodology. Moreover, the possible reaction pathway and product can be identified because of the chemical sensitivity of Raman spectroscopy. Our findings provide a practical method for noninvasively detecting the local heating effect inside a single molecule under nonequilibrium conditions.