Temperature dependence of the ballistic energy transport in perfluoroalkanes.

Temperature dependence of intramolecular energy transport in perfluoroalkane oligomers with a chain length of 3-11 carbon atoms terminated by a carboxylic acid moiety on one end and a -CF2H group on another end was studied in solution experimentally and theoretically. Experiments were performed using a dual-frequency relaxation-assisted two-dimensional infrared spectroscopy method. The energy transport was initiated by exciting the C═O stretching mode of the acid and recorded by measuring a cross-peak amplitude between the C═O stretching and the C-H bending modes as a function of the waiting time between the excitation and probing. An efficient transport regime with a mean free path of 16.4 ± 2 Å is observed at 35 °C. The energy transport speed decreases at elevated temperatures, indicating a switch from the ballistic transport regime to diffusive. The modeling of the energy transport involving both ballistic and diffusive mechanisms is performed. It explains the temperature dependence of the energy transport speed and confirms a switch of the transport regime from ballistic at lower temperatures to diffusive at higher temperatures.