Meisam Farzalipour Tabriz KoƧ University Title: Quantum mechanical study of time-resolved optical spectra in photochemical dissociation of simple peroxide molecules Abstract: Organic peroxides play an important role in industry as sources of free radicals and initiators in the polymerization reactions. The dissociation of these molecules can be achieved by applying heat as well as photochemical means. Depending on the molecule and/or the solvent, secondary dissociation reactions are also possible. However, the mechanisms of the photochemical dissociation generally have not been clarified. Ultrafast ultraviolet/visible pump-probe absorption spectroscopy is one of the promising methods for "real-time" observation of photochemical reactions as they precede on the initially populated excited-state potential energy surfaces. But, interpretation of spectra for identifying the unstable chemical species and free radicals produced during the photochemical dissociation is not trivial. We will discuss the possibility of using quantum chemical calculations in interpreting these spectra and specific challenges associated with this kind of calculations. We tried to identify the possible products and activation energy of each reaction in photochemical dissociation of simple peroxide molecules (diethyl peroxide, di-tert-butyl peroxide, and dicumyl peroxide) based on transition state energies. The equilibrium geometries and energy of all stationary points, and transition states were calculated by density functional method. Time-dependent density functional theory was used for comparing the results of our calculations with the experimental UV-visible absorption spectra.