Femtochemistry Of Small Molecules Using Quantum Wavepacket Calculations

Along with the development of the techniques of ultra-short laser pulses, it inter-ests more and more people to study the molecular reaction processes using real-timestrategy. The emergence of the new nomenclature "femtochemistry"indicates that thefield is becoming a brand-new individual subject.The molecular reaction process, induced by an ultra-short laser pulse, usually canbe described by the evolution of a wavepacket on the corresponding potential energysurface (curve). From the view point of the steady state, physical insight cannot ob-tained in common cases because the time-dependent wavepacket results from the su-perposition of many excited states evolving with time. It is known that the molecularstructures are complex and usually there are no simple analytic forms for the molecularpotential energy surface (curve). As a result, quantum numerical simulation on the po-tential energy surface (curve) are essential for understanding the ultra-fast experimentalresults.In the last decades, numerical simulation using quantum mechanical principles inHilbert polynomial space has witnessed a signi?cant evolution in the past tens years.Accuracy, simpleness and easy implementation have been known as the characteristicsof the quantum numerical calculation since the DVR (discrete variable representation)technique was introduced. Quantum numerical simulation becomes more importanttoday than ever for the cheap but high-quality computation resources.In the present thesis, using the time-dependent wavepacket models developed byus which are aimed at the interested molecules, ultra-fast processes of several smallmolecules induced by femtosecond laser pulses are studied. Autler-Townes splitting onelectronic potential energy curves was found and some experimental results were giventheir underlying mechanisms. Some accompanying experimental work has also beensucceeded.On theoretical work, three-dimensional wavepacket model using fast Fouriertransform technique was developed. By comparing with the same numerical model butusing Legendre polynomials as the angular basis set, it was found that the wavepacketmodel using fast Fourier transform has advantages for the nonlinear tri-atomic moleculewhich has a deep well potential energy surface. The Hermiticity of the resulted Hamil-tonian matrix using fast Fourier transform or DVR technique was found to be impor-tant for keeping the agreement of the numerical results using different coordinates, bycomparing the numerical results obtained from the wavepacket models using Radau,bond-bond angle, Jacobi and hyperspherical coordinates. On numerical simulations,using the developed one-dimensional numerical model, ac-Stark shift and Stark split-ting of diatomic molecule, NO and Na2, was investigated. The absorption and Ramanspectra of the OClO molecule were calculated using the three-dimensional wavepacketmodel. By comparing with the related experimental results, it was found that the abinito potential energy surface is a good model for describing the OClO molecule. Thephotoelectron spectra from the ground X2B1 and excited A2A2 electronic state werecalculated which further were used to analyze the simulated ultra-fast pump-probe re-sults. Based upon the simulated pump-probe results of the OClO molecule, the reportedexperimental results were reinterpreted. Some related confusing experimental resultswere clari?ed. Upon our numerical work, we reach the conclusion that accidental res-onance in a femtosecond pump-probe experiment is often unavoidable because of thebroad bandwidth and high peak-intensity of the laser pules, and we must pay enoughattention to it. Or the experimental results may not lead us to the real process. On experimental work, with the Ti:sapphire femtosecond laser system made in ourlaboratory, the resonance ionization paths of NO were successfully selected by usingthe obvious ac-Stark shift of the high Rydberg state of the NO molecule. The work ofthis part will not be detailed in the present thesis.