Molecular Elimination Reactions And Hydrogen Migration Processes In An Intense Femtosecond Laser Field

With the advent of ultrafast intense laser, the interaction of atomic and molecular systems with intense laser field has attracted considerable attention in the past few decades. The corresponding phenomena, including of high-order harmonic generation, multi-photon ionization,field-assisted ionization, above threshold ionization, dissociative ionization, field-assisted dissociation, and Coulomb explosion, have been intensively and extensively investigated. The generated Br atom in the photodissociation process of hydrocarbon bromide have great damage to stratosphere ozone, where the catalytic rate of Br on depleting stratosphere ozone is100times more rapid than that of Cl. Thus the photoionization and photodissociation dynamics of hydrocarbon bromide in intense laser field has been an active field. In this thesis, we studied the dissociative ionization and Coulomb explosion processes of1,2-dibromoethane and cyclohexane molecules under an intense femtosecond laser field by dc-slice imaging technology. These specific works are presented as follows:1. Coulomb explosion and dissociative ionization of1,2-dibromoethane were experimentally investigated in near-infrared (800nm) femtosecond laser field by dc-slice imaging technology. The sliced images of these fragment ions C2H4Br+, Br+, C2H4+, Br2+and CH2Br+were obtained, and their corresponding kinetic energy releases (KER) and angular distributions were calculated. It was confirmed that the high-KER components come from Coulomb explosion of1,2-C2H4Br22+, while the low-KER components come from dissociative ionization of1,2-C2H4Br2+. Furthermore, the dissociation pathway leading to C2H4+and Br2was theoretically simulated, and the results showed that the singly charged precursor overcomes an energy barrier to dissociate via an asynchronous concerted mechanism after undergoing isomerization.2. Concerted elimination pathway leading to the fragment ions Br2+and C2H4+ from1,2-dibromoethane molecule has been investigated in800nm femtosecond laser field by dc-slice imaging technology. The kinetic energy release and angular distributions of Br2+and C2H4+demonstrated that Br2+resulted from the two-body Coulomb explosion of the doubly charged parent ion. Ab initio calculations showed that the doubly charged precursor overcome a small energy barrier (0.18eV) and then dissociated into Br2+and C2H4+through a synchronous concerted elimination mechanism. Moreover, the relative yield of the Br2+channel was obtained, and it remained about3.4%when the laser intensity exceeded1.0×1014W/cm2.3. The hydrogen migration pathways of cyclohexane in800nm femtosecond laser field have been investigated by dc-slice imaging technology. The observation of fragment ions CH3+, C2H5+and C3H7+could be regarded as the direct evidence that the chemical bond rearrangement processes associated with hydrogen migration occured in the dissociative ionization process of cyclohexane. The sliced images of fragment ions CH3+, C2H5+, C3H7+and their partner fragment ions C5H9+, C4H7+, C3H5+were obtained, and their corresponding kinetic energy release (KER) and angular distributions were calculated. It was confirmed that the high-KER components come from the two-body Coulomb explosion of C6H122+, while the low-KER components come from the dissociative ionization of C6H12+.Moreover, the relative yields of fragment ions CH3+, C2H5+and C3H7+were obtained, which approached to4.1%,2.6%and0.7%with the increase of the laser intensity, respectively. At last, the Coulomb explosion process of C6H122+into C2H4+and was studied with the aid of ab initio calculations.