Ultrafast dynamics in complex molecular solids: Ultrafast imaging of laser ablation of PMMA and theory of mechanical energy flow in ultrahot crystalline naphthalene

Abstract

In a molecular solid, there is an ultrafast transfer of energy between the internal molecular vibrations and the external modes or lattice vibrations. Two interesting examples involving this energy transfer are presented in this thesis. First, an ultrafast imaging apparatus is constructed and used to investigate spatially inhomogeneous dynamics in laser surface ablation of polymers. Second, time dependent mechanical energy redistribution in ultrahot anharmonic solids composed of organic molecules is studied theoretically.Using the ultrafast imaging apparatus, computer-digitized images are acquired over the time range 10\sp{-12}-10\sp0 s. The images are analyzed to obtain the time dependent behavior of the ablated polymer. Near the peak of the ablation pulse, self-focusing begins and produces a small diameter filament lasting for 20 ps. The polymer irradiated by the filament then undergoes explosive thermal decomposition, ejecting fragments from a conical volume into the atmosphere above the surface. These ablated materials generate a hemispherical, supersonic shock wave called a blast wave. A mechanism for the ablation process involving nonlinear absorption is proposed.Vibrational cooling and multiphonon up pumping are simulated using model parameters for crystalline naphthalene. In vibrational cooling, the redistribution of large excitation energy localized in a high frequency vibration is investigated in two limiting cases. In the weak excitation case, a single molecule is excited in a large crystal where the effect of emitted phonons during the vibrational cooling is ignored. In the strong excitation case, every molecule of the crystal system is excited so that the emitted phonons affect subsequent dynamics. In multiphonon up pumping, the redistribution of thermal energy of a 40 kbar shock wave is studied using a supercomputer. The thermal effect of the shock wave is simulated by exciting an enormous number of acoustic phonons. In both vibrational cooling and up pumping processes, the phonons achieve an internal equilibrium in a few ps through efficient intraphonon scattering. New thermal equilibrium is reached within 1 ns for both cases.U of I OnlyETDs are only available to UIUC Users without author permissio