At the level of individual molecules, familiar concepts of heat transport no longer apply. When large amounts of heat are transported through a molecule, a crucial process in molecular electronic devices, energy is carried by discrete molecular vibrational excitations. We studied heat transport through self-assembled monolayers of long-chain hydrocarbon molecules anchored to a gold substrate by ultrafast heating of the gold with a femtosecond laser pulse. When the heat reached the methyl groups at the chain ends, a nonlinear coherent vibrational spectroscopy technique detected the resulting thermally induced disorder. The flow of heat into the chains was limited by the interface conductance. The leading edge of the heat burst traveled ballistically along the chains at a velocity of 1 kilometer per second. The molecular conductance per chain was 50 picowatts per kelvin.

Ultrafast spectroscopy was used to study vibrational energy transfer between vibrational reporter groups on different parts of a molecule in a liquid. When OH stretching vibrations of different alcohols were excited by mid-infrared laser pulses, vibrational energy was observed to move through intervening CH ₂ or CH groups, taking steps up and down in energy, ending up at terminal CH ₃ groups. For each additional CH ₂ group in the path between OH and CH ₃ , the time for vibrational energy transfer increased by about 0.4 picosecond.

In a suspension of reverse micelles, in which the surfactant sodium dioctyl sulfosuccinate (AOT) separates a water nanodroplet from a bulk nonpolar CCl ₄ phase, ultrafast vibrational spectroscopy was used to study vibrational energy transfer from the nanodroplet through the AOT interfacial monolayer to the surrounding CCl ₄ . Most of the vibrational energy from the nanodroplet was transferred to the polar AOT head group within 1.8 picoseconds and then out to the CCl ₄ within 10 picoseconds. Vibrational energy pumped directly into the AOT tail resulted in a slower 20- to 40-picosecond transfer of energy to the CCl ₄ .