Dynamics Of Ultraviolet Photodissociation And Ionization Of Halogenated Compounds

The ultraviolet photodissociation and ultrafast dynamics of compounds are one of the most important and primary processes in physical chemistry, photochemical reactions, life science and environmental science. The studies on the photodissociation and ultrafast dynamics are helpful not only to obtain the information on the excited states of the molecules but also to understand the nature of the fundamental reactions. For instance, photosynthesis of plants, failure mechanism of ozonosphere, ultraviolate damaged mechanism of cells and vision process are all closely related to ultraviolet photodissociation and ultrafast dynamics of compounds. Cycloalkyl and aromatic halides compounds are the specific system to study the photochemical reactions. Identifying the dynamic mechanism of excited state is not only of great importance in terms of physical chemistry and environmental protection, but also helps us understand the mechanism of many other photochemical processes.This dissertation is mainly focused on the excited state dissociation dynamics of cycloalkyl halides (cyclopentyl bromide) with nanosecond laser and the ultrafast dynamics of the excited state of aromatic halides (2-chloropyridine) with femtosecond laser.In the first part, the photodissociation dynamics of cyclopentyl bromide at234and266nm has been investigated using velocity ion imaging. Translational energy distributions of Br and Br*have been fitted by two Gaussian functions. It is possible that they originated from different conformational structures. Three Gaussian functions are required to fit the distributions of Br*at266nm, which is attributed to the multiphoton dissociative ionization. The rigid radical limits of the impulsive model have been applied to the related energy partitioning. The branching ratios and the relative quantum yields were determined; the results indicated that ground-state bromine was the major dissociation product.In the second part of this dissertation, the ultrafast internal conversion dynamics of 2-chloropyridine has been studied by femtosecond time-resolved photoelectron imaging spectroscopy coupled with time-resolved mass spectroscopy. The ultrafast internal conversion from S2to S1via a close-by conical intersection within (162±5) fs is clearly observed from the time-dependence of the photoelectron spectra. The following deactivations involve the coupling of S2/S0and S1/S0conical intersections, which occur on a timescale of about (5.5±0.3) ps, and lead to the internal conversion to the ground state from the S2and S1state.