Theory of Ultrahot Molecular Solids: Vibrational Cooling and Shock-InducedMultiphonon Up Pumping in Crystalline Naphthalene

摘要

A new method is presented for calculating ultrafast vibrational energyredistribution in anhamonic solids composed of large molecules. An improvement over weak coupling model allows the emitted phonons to act back on the excited vibrations. The model is used to investigate the dynamics of ultrahot molecular solids, materials with enormous levels of vibrational or phonon excitation. Ultrahot solids are produced in laser ablation and shock induced detonation. Model parameters for crystalline naphthanlene are used to study multiphonon up pumping after a 40 kbar shock and vibrational cooling after strong excitation of a high frequency vibrational fundamental. The phonons attain a state of internal equilibrium characterized by a time dependent phonon quasitemperature Theta sub p(t) within a few ps. Energy redistribution among the phonons is efficient as phonons are more anharmonic than molecular vibrations. In up pumping, a large excess of phonons at T=0 decreases as vibrations are pumped by phonons. Under these conditions, the rates of anharmonic scattering processes are maximum at t=0 and the lower levels of molecular vibrations are pumped before the higher levels. The vibrational population distribution then rapidly attains an approximate state of quasiequilibrium, characterized by a vibrational quasitemperature Theta sub nu(t) is achieved in about 100 ps. In vibrational cooling, there is initially a large excess of high frequency vibrations an few phonons. Because photons accumulate as the vibrations cool, the rates an anharmonic scattering processes are a minimum at t=O. Under these conditions, the vibrations are far from a state of quasiequilibrium until thermal equilibrium is attained at about 1 ns. Reprints. (JHD)