Research Of Dissociation And Ionization Of Molecule In Femtosecond Laser Field And Rotational State Selection Of Molecules In Static Electric Hexapole Field

In this thesis, we attend to study the properties of molecules in external field and try to obtain the detailed dynamic information concerning with the interaction of polyatomic molecules with intense femtosecond laser, the energy structure of simple triatomic molecules and manipulating dynamical processes with rotational state selection of molecules.Dissociation and ionization of polyatomic molecules in intense femtosecond laser field due to their complexity of quantum states and many-particle correlation has been a hot topic for years. Especially on the subject at moderate laser intensities, there are still a lot of phenomena which need to be interpreted. For further solving these problems, the development of novel spectroscopic techniques is necessary. We have built a velocity map imaging apparatus with a kinetic energy resolution of <0.1 eV. Using this apparatus, the interaction of CH₃I molecules with intense femtosecond laser has been investigated experimentally. The kinetic energy distribution and the angular distribution of the various atomic fragment ions In+ (n=1-3) have been measured. Several dissociative ionization and coulomb explosion channels were identified for In+ (n=1-3). In the laser field with the intensity of 3.7×10¹⁴ W/cm², five channels can produce the fragment ion I+, including two dissociative ionization channels and three Coulomb explosion channels, one dissociative ionization and three Coulomb explosion channels can lead to the fragment ion I2+, and there are only two Coulomb explosion channels contributing to the generation of the fragment ion I3+. As expected for a geometric alignment dominated the interaction process, these atomic ion fragments show the anisotropic angular recoil distributions peaked in the laser polarization direction. Also these results exhibited very different angular distribution of the fragment ions from dissociative ionization and Coulomb explosion channels. Furthermore, we have also studied the dependence of the kinetic energy release (KER) of In+ (n=1-3) upon the laser intensity. The relative weight of the various contributions from the identified dissociative ionization and Coulomb explosion channels was found to depend on the laser intensity obviously. We found that the interpretation of this experimental observation invoked a sequential ionization process of the molecules.Ultrafast dynamics and dissociative ionization of CS₂ and SO2 were studied by means of femtosecond time resolved pump-probe method combined with a time-of- flight mass spectrometer. By carefully controlling the intensities of the 266 nm and 400 nm lasers as pump or probe light, the transients of both parent ion (CS₂⁺) and fragment ions (S+ and CS+) were observed. It was found that all ion signals decay exponentially, but different for the cases with 266 nm or 400 nm light as pumping. We attributed these differences to the evolution of different Rydberg states 6s and 4f pumped by two 266 nm photons or three 400 nm photons, respectively. The lifetimes of these two Rydberg states were obtained simultaneously from one pump-probe experiment by the best fitting of the transients. It is the first time to study the transient single of fragment ions S+ and CS+ produced by dissociation of CS₂⁺. Our observation suggested that the final state of parent ionic molecules CS₂⁺ in the laser field is its C 2?g+ state, based on the measured S+/CS+ branching ratio. Another observation in the present experiment is that the S+/CS+ ratio is dependent on the delay time of the two lasers, indicating that the dissociation process of CS₂⁺ is related to the evolution of the intermediate Rydberg state of the neutral molecules. This femtosecond pump-probe method was also employed to study the dissociation dynamics of SO2. The results clearly showed that the F state of SO2 dissociates along the S-O bond. The transients of S+ and O+, however, have different behavior, which consist a fast growth and a long decay component. Possible mechanism of the fragment formation was discussed for understanding the dissociation dynamics of the F state of SO2. Our experimental results extend the previous studies on ultrafast dynamics of these molecules on excited states and also provide better understanding for structures, energies, and dissociation processes of these molecules.A molecular rotational state-selecting hexapole machine has been designed and built in our lab. We studied the focusing curves of the two symmetry top molecules, CH₃I and CHCl3, on the hexapole machine. Comparing to the theoretical simulation, we observed different rotational states could be focused at different voltage applied on the hexapole via the first order Stark effect. During the rising part of the focusing curve for CH₃I, there was only one single rotational state at some particular voltage. As for CHCl3 there were two rotational states contributing to the rising part of its focusing curve. It is expected that if the rotational temperature of the molecular beam can be further decreased or the beam velocity can be increased, more pure population on a single rotational state in molecular beam could be achieved. This in turn will be important for producing an ideal initial sample beam in the precision study of structure and dynamics of molecules on a unique quantum state.