Markmann, Andreas; (2004) Theoretical study of photoinduced reactions in ionic crystals mechanisms, energy dissipation and coherence effects. Doctoral thesis (Ph.D), UCL (University College London).

Text Theoretical_study_of_photoindu.pdf Download (37MB)

Abstract

Recent advances in laser technology have allowed driving coherent microscopic dynamics of reactive species in some chemical reactions. This technique is called coherent control or quantum control. The possibility of coherent control in reactive centres in direct contact with ionic crystals was studied theoretically on the example reactions: (1) Desorption of hydrogen from a hydroxyl ion formed due to HCl adsorption at the MgO(001) surface and (2) dissociation of the Self-Trapped Exciton (STE) in alkali halides into a pair of point defects (F-H pair). The second takes place inside a crystal lattice. A complete self-contained theoretical study of the coherent control reaction (1) is presented, involving DFT calculations and classical and quantum dynamics simulations. Reaction schemes for (2) are studied using an empirical model for the STE in a generic alkali halide. To perform the tasks required by the aims of this project, a simple criterion for the spatial localisation of normal modes, a molecular dynamics initialisation scheme allowing dissipation dynamics at non-zero temperature and an extension of the optimal control method that greatly enhances its usability were developed. All of these techniques are applicable to a large class of theoretical problems. Comparative studies of phonon modes localised at surfaces and surface steps in three ionic crystals lead to the conclusion that the order of these crystals with increasing localisation of surface modes is CaF₂(111), MgO(001), KBr(001). For MgO and KBr, step modes were found that appear to be capable of funnelling vibrational energy to the step and transport it along the step. Vibrational modes localised at the corners of a large finite cube of MgO were identified that may play an important role in the dissipation from admolecule modes of the appropriate frequency. Reaction (1) is justified by the comparatively low localisation in MgO(001) in conjunction with a low anharmonicity, promising small dissipation from the admolecule. Density functional simulations of HCl on the MgO (001) surface at low coverage predict dissociative adsorption with chemisorption of the proton to a surface oxygen. High simulated coverage was found to be the reason for a previous contradictive study. Activation energies for hydroxyl and chlorine rotation and libration were found to be accessible at room temperature. The first dynamical study of vibrational dissipation from a molecule adsorbed to an insulator surface was performed. This was done at a thermal population of all normal modes rather than at zero Kelvin. The stretch vibration of the hydroxyl was found to interact most strongly with hydroxyl rotation and libration. Vibrational energy transfer proceeds via the latter two into the crystal lattice. Dissipation starts only a considerable time after excitation. A non-symmetric collision of the proton with the chlorine ion appears to play a role in igniting the dissipation process. A coherent control mechanism for the dissociation of OH⁻ formed at the MgO surface due to HCl adsorption was suggested and its experimental feasibility was demonstrated. Double excitations are used in the proposed excitation process. This allows a short time scale for the excitation process, which in turn diminishes the adverse effects of dissipation. A static electric field can be used to desorb atomic hydrogen from the excited molecule. An ultrafast reaction for the dissociation of the STE was proposed. This is the first prediction of a coherent control reaction for a specific reactive centre within a crystal. The use of a bound excited state of the STE to deposit a wave packet on the dissociative flank of the STE electronic state represents a qualitatively new reaction scheme. It involves a subtle interplay between electronic and vibrational dynamics of the wave function.

Download activity - last month

Export as JSONXMLCSV

Download activity - last 12 months

Export as XMLCSVJSON

Downloads by country - last 12 months

Export as JSONXMLCSV

Archive Staff Only

View Item