Studies On Ionization And Dissociation Of Small Molecules In Intense Femtosecnd Laser Fields

With the development of ultra-short and super-intense laser technology, the interaction between intense femtosecond laser fields and molecules has attracted widespread interest in the past two decades. When the focused laser intensity is beyond 1014W/cm2, the process of multiple ionization occurs and the produced molecular ions dissociate into a range of ion fragments with kinetic energy release(KER). The information on the ionization and dissociation of the molecules in intense laser fields can be obtained by a detailed study of the time-of-flight(TOF) mass spectra and the KER of the various dissociation fragments. Further, the information can help people control the dissociation channels of molecules by femtosecond laser pulses.In this dissertation, the ionization and dissociation of small molecules in intense femtosecond laser fields is studied by using a Ti: sapphire chirped-pulse amplification laser system and a home-made TOF mass spectrometer. The commercial legend-UPS laser system is provided by Coherent Inc. with output parameters of a central wavelength of 800 nm, a 1 kHz repetition rate, 40-fs pulse duration and one -pulse maximum energy of about 2.6 mJ.1. The appearance intensities of N2, O2 and CO molecules are calculated based on the"filed ionization model"of molecules in intense laser fields. The computed critical distance is 5.2 a.u. for N2, 4.2 a.u. for O2 and 5.2 a.u. for CO molecules, respectively. The sequentially dissociative trajectories are also shown when N2, O2 and CO molecules are irratiated by a 45 fs, 1x1016 W/cm2 laser pulse.2. Using neutral molecular beam as target, the dissociation of N2+, O2+ and CO+ molecular ions in intense femtosecond laser fields is studied. The KER and the angular distribution of the atomic fragments show that the dissociation mechanisms of N2+ molecular ions mainly proceed through the couplings between the doublet metastable states( A 2Πu, B 2Σu+ and C 2Σu+) and the upper excited states in the diabatic Floquet representation. The result is different from the conventional conclusion: the dissociation mechanisms of N2+ molecular ions mainly proceed through the couplings between the ground state and the upper states. But for O2+, a dissociation channel a 4Πu→f 4Πg ? 1ω→4Σu+? 2ωwith a KER of 1.3 eV is first observed in our experiment, and all the dissociation channels are the couplings occur between the quartet state a 4Πuand the upper excited states. For the C(1,0) channel and O(1,0) channel of CO+ molecular ions, the former is dominant. The experimental result suggests that the dissociation of CO+ molecular ions is also through the couplings between the excited states.3. The dissociation of multi-charged molecular ions of N2, O2 and CO in intense femtosecond laser fields is investigated. For the low-charged state N22+ molecular ion produced by sequential ionization, its dissociation is through the one-photon transition from the ground state X 3Πuto the excited state D 3Πg in linearly polarized laser fields by analyzing the intensity-difference-spectra of N+ ions. However, in circularly polarized laser fields, the KER of N+ ions can be interpreted by the"filed ionization and Coulomb explosion model". The KER of the various dissociation fragments is measured and the calculated critical distances of three molecules in the experiments are consistent with the theoretical studies. The angular distribution anisotropy of various atomic fragments indicates that the dynamic alignment is predominant. The ions fragments distribution nearly has the same full wave at half maximum that show CO molecular ions are first aligned and further ionized. CO molecular ions are more easily aligned along the laser polarization than N2 molecular ions because the angular distribution of C+ ions has the narrower full width at half maximum than N+ ions.4. The structural deformations of CO2 and H2O molecules and the rearrangement phenomena of CH3OH and C2H5OH molecules in intense femtosecond laser fields are stusied. The angular distribution of CO+ molecular ions from CO22+ molecular ions has a maxium at 30o that is the angle between the laser polarization and the TOF spectrometer axis. The experimental result first reported shows that the structural deformation occurs in the charge state of CO22+, consistently with the theoretical studies. However, the peak of the angular distribution of H+ ions, appearing around 0o, indicates that the H2O3+ and H2O4+ molecular ions have the linear structure different from H2O molecules. The H2+ molecular ions from C2H5OH molecular ions are observed in the experiment. The result suggests that C2H5OH molecular ions rearrange in intense laser fields before the dissociation occurs.At the end of the dissertation, a brief summary is given and some suggestions for the further study are presented.