Excitation And Ionization Dynamics Of Small Molecules In Ultrashort Pulse Laser Fields

Interaction of intense laser pulses with atomic and molecular systems has beena subject of a number of experimental and theoretical studies during the past years.From the experimental phenomenology, an adequate description of atomic or molecu-lar processes in intense fields involving freedoms of electrons and nuclei is required. Ingeneral, by directly numerically the time-dependent SchrSdinger equation, the evolu-tion of a wave packet on the corresponding potential energy surface can be employedto described the molecular dynamics process.In this thesis, using several numerical calculation models which are developedby us based on the quantum time-dependent wave packet method, some ultra-fastdynamics processes have been investigated in ultrashort laser fields for the interestedmolecules. The main works are as follows.(1) To accurately and efficiently simulated molecular dynamics, suitable coordi-nates and basis functions which should be matched the molecular topology are neces-sary. By comparing the calculations using the Fourier series and Legendre polynomialsas angular basis sets, it is found that the wave packet model employing Fourier series(FFT) has advantage for triatomic molecules with well bounded potential energy sur-faces while the one using Legendre polynomials is suitable for floppy molecules. TheBessel discrete variable representation (DVR) method is introduced to describe theelectronic wave packet by direct numerical solution of the time-dependent Schrodingerequation. Using the Bessel functions, the singular term, r^-2 or r^-1, in the kineticenergy operators at the origin is analytically solved, which dramatically reduces thenumerical difficulty around r= 0. As illustration examples, the high-order harmonicgeneration (HOHG) spectra in atomic hydrogen and the ionization rate of molecularion H₂⁺ at different intensities of the driving pulse are calculated. These calculatedresults are found to be in excellent agreement with the previous results.(2) The excitation dynamics of diatomic molecules is theoretically studied usingthe one-dimension model. Exposing a three-level Na2 molecule in intense femtosecondlaser fields, the population completely transfers from the initial state to the targetstate. Due to the delta excitation, the excited wave packet will oscillate on the final state. This molecular dynamics can be interpreted by adiabatic population transferpassage via the concept the light-induced potential (LIP). In terms of the adiabaticpopulation transfer passage, the molecular dissociation rate and selective excitationof vibrational level can be successfully controlled by varying the duration time, delaytime and peak intensity of driving laser pulses.(3) The effects of the molecular rotation and alignment on the molecular dynam-ics of Autler-Townes (AT) splitting in a multiphoton resonance ionization spectrumare numerically simulated using a two-dimension wave packet model. The AT split-ting results from rapid Rabi oscillations induced by an ultrashort intense laser pulse inrovibrational coherent states that belongs to different electronic terms. But Rabi oscil-lations will be damped because of the effects of rotation and alignment of the moleculein laser fields. The AT splitting in a photoelectron spectrum could be observed onlyfor a well aligned molecule where sufficiently rapid Rabi oscillations appear. The othereffects including the initial rotational temperature, the FWHM of laser pulses and thepulse shape, on the molecular dynamics are also studied.(4) The absorption spectra of triatomic molecules OClO and NO₂ are numericallycalculated using the three-dimension wave packet model. The simulated results are inexcellent agreement with the corresponding experimental observations. The calculatedresults can be used to explain the molecular structure and character. The vibronicexcitations A²A₂(v₁, v₂, v₃)←X²B₁(0, 0, 0) of the triatomic molecule OClO usingfemtosecond laser pulses of varying intensities are investigated. With an ultrashortlaser pulse of certain FWHM (full width at half maximum), the vibrational level canbe selectively excited. The changes in the vibrational population distributions causedby simple variation of the pulse are remarkable.