| Excited states play an important role in the processes of molecular reactions. Investigation of the structure, characteristic and dynamics of excited states is an important frontier in atom and molecule physics. These investigations are significant to elaborate the mechanism, regularity of chemical reaction and the characteristic of the products. In these aspects, a very interesting subject in recent years is to study the high lying Rydberg states spectroscopy, photodissociation and photoionization processes by using the method of laser multiphoton ionization. The related contents studied in literature include the determination of the ion product species, the internal energy distribution and dynamic mechaniam of the product formation.In this thesis, by use of femtosecond two-color pump-probe technique and resonance-enhanced multiphoton ionization spectroscopy, we studied the Rydberg state dynamics of some small polyatomic molecules, such as OCS, CS2, NH3, CF3I and C6H6. Our main results including the lifetimes and de-excitation mechanism of these Rydberg states are described as follows:1. For OCS molecules, by use of femtosecond (2+1') resonance enhanced multiphoton ionization with the pump pulse at 268nm and probe pulse at 402nm, the lifetime of [2 п 1/2]4pπ(1∑+) v 1=1 Rydberg state is determined to be 1071±llfs which indicates the predissociative character of the Rydberg state. Itspredissociation may happen due to the potential energy surface intersection with another repulsive one.2. The study of predissociative dynamics of 6sσg Rydberg state of CS2 at two different wavelength gives out the lifetimes of 409 6fs at 265nm and 1034 31fs at 267.5nm. Two-photon absorption at 265nm and 267.5nm respectively excites CS2 to [l/2]6sσg Rydberg state and [3/2]6sσg Rydberg states. This research may provide some valuable information about the potential energy surface of 6sσg Rydberg state.3. The pump-probe experiments on NH3 showed that two-photon 266nm absorptionexcited to the molecule E 1A1 Rydberg state and two-photon 400nm absorption to the A 1A2 state. The data analyses of the ion signal intensity changing withthe pump-probe time delay show that, the lifetime of A 1A2 is 68 12fs obtained from (2+1') process when 400nm as pump pulse and 266nm as probe pulse and the lifetime of E1A1 is 1053 156fs obtained from (2+1') process when 266nm as pump pulse and 400nm as probe pulse. The predissociation mechanism ofE1A1 is discussed to being a non-radiative transition to the vibrational excitedA1A2 state Referencing to the previous studies in literature.4. The photoionization-dissociation dynamics of CF3I are studied with femtosecond two-color time resolved mass spectroscopy with a pump pulse of 265nm and a probe pulse of 398nm. Two-photon absorption at 265nm excites CF3I to 5p3 7sσ(2 1/2) Rydberg state which is then ionized and dissociated by absorbing of one or two 398nm photons. CF3+ produced by (2+1') process and I+ by (2+2') process. The lifetime of the 5p37sσ(2 1/2) Rydberg state is about 220fs and thede-excitation of the 5p3 7so( 2 1/2) Rydberg state is due to coupling with some states such as some ion-pair states, the lifetime of this coupling state is more than 30ps determined by the experiments.5. The detection of benzene photoions in pump-probe experiments with femtosecond laser pulse at 400 and 266nm has allowed us to study the dynamics of the internal conversion of the electronic states of benzene. The lifetime of S2 state obtained is about 50fs and the lifetime of S1 state populated by the internal conversion from the S2 state is about 6.5 0.2ps. The deactivation of the S2 and S1 states may be caused by conical intersection of the corresponding potential surfaces with the lower electronic ones.
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