Applications of Ultrafast Temperature Jump Spectroscopy to Condensed PhaseMolecular Dynamics

摘要

Dyes which function as molecular optical heaters and optical thermometers can bedoped into a wide variety of molecular materials. Here we show how picosecond light pulses can be used to deposit a known amount of heat and to measure the temperature, i.e. to perform accurate optical calorimetry, at heating rates up to dT/dt = 10(exp.12) deg/s. Nonequilibrium mechanical energy transfer processes, including mechanical energy flow into or out of molecules by vibrational cooling or multiphonon up pumping, and the physical and chemical properties of superheated liquids and solids are investigated experimentally by ultrafast temperature jump spectroscopy and theoretically using molecular dynamics simulations. Specific examples presented here include (1) the generation of 750 deg C molecular hot spots lasting a few picoseconds, produced at near-IR dye molecule centers which sequentially absorb tens of photons during a picosecond pulse, (2) the production and measurement of bulk temperature jumps delta T > 100 deg C in liquids and >500 deg C in polymers, (3) the investigation of multiphonon up pumping processes in energetic materials by picosecond Raman spectroscopy, and (4) direct solid-state temperature measurements made during laser photothermal surface ablation of polymers using optical calorimetry.