Dyes which function as molecular optical heaters and optical thermometers can be doped 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 = 10I2 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 spectrosoopy and theoretically using molecular dynamics simulations. Specific examples presented here include (1) the generation of 750 O 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 AT > 100 O C in liquids and >500 O 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.
The recombination after flash photolysis of carbon monoxide (CO) to protoheme (PH) in glycerol: water is studied over ten decades in time (1 ps to 10 ms). The rebinding consists of an initial nonexponential geminate phase followed by a slower exponential bimolecular phase. The entire time course of this reaction between 260 and 300 K can be explained in a unified way using a simple, analytically tractable diffusion model involving just three parameters: the relative diffusion constant, the contact radius, and the intrinsic rate of reaction at contact.
Molecular mechanical energy transfer in energetic materials is investigated because of the likely possibility of a relationship between energy transfer rates and impact sensitivities. Energy transfer in the liquid high explosive nitromethane (NM) is studied by picosecond infrared pumping of C-H stretching vibrations (-3000 cm-') and picosecond incoherent anti-Stokes Raman probing of six lower energy Raman-active vibrations in the 1400-480 cm-I range. Vibrational cooling of C-H excited NM is shown to require at least 200 ps.During vibrational cooling, substantial transient overheating is observed in the higher energy vibrations in the 1400-900 cm-' range. Overheating refers to instantaneous vibrational quasitemperatures which are temporarily greater than the final temperature of the bulk liquid. The overheating and the increasing delay in the rise of excitation in certain vibrations is used to infer that ladder (cascade) type vibrational cooling processes are important in ambient temperature NM. Molecular thermometry is used to estimate the absolute efficiencies of energy transfer between some of the pumped and probed vibrations. This detailed study of energy transfer in a high explosive presents a more complete picture than the relatively simplified theoretical models for energetic material initiation presently in use.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.