Vibrational Relaxation and Vibrational Cooling in Molecular Crystals

272 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1988.The processes of vibrational relaxation (VR) and vibrational cooling (VC) are investigated both experimentally and theoretically in low temperature crystals of complex molecules, specifically benzene, naphthalene, anthracene and durene. In addition to ps CARS data previously obtained on these pure crystals, the VR of dilute impurity molecules (naphthalene, anthracene) in crystalline host matrices (durene, naphthalene) is studied with the ps photon echo technique. The results obtained by echoes on vibrations in the electronically excited state are compared to previous ps time delayed coherent Raman studies of ground state vibrations of the pure host matrix. The relaxation channels for guest and host, and the effects of molecular and crystal structure on VR rates are determined.In the VR process, a vibration is deexcited, while VC consists of many sequential and parallel VR steps which return the crystal to thermal equilibrium. A theoretical model is developed to describe vibrational cooling (VC) in crystals of large molecules at zero and finite temperatures. An initially excited vibration relaxes via anharmonic coupling by sequential emission of many lattice phonons until all vibrational energy is destroyed. The time evolution of vibrational excitation probability is described with a master equation. Various models for the phonon density of states, which exerts primary control over the VR process, are considered. It is found that VC occurs on a much slower time scale than VR, and that the rate of VC is only weakly dependent on temperature, even in systems where VR is highly temperature dependent. These VC calculations are applied to naphthalene as well as model system in order to predict the decay from the initial state, the time dependent populations of transient vibrational excitations, and the return to the vibrationless ground state. All these quantities are directly related to experimental observables such as incoherent anti-Stokes Raman scattering and hot luminescence.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD