Electron Velocity Imaging Of Ultrafast Laser Induced Ionization Of Carbon Monoxide And Methyl Bromide Molecules

Photoionization is one of the most fundamental light-matter interaction processes that triggers many photophysics and photochemistry reactions.The understanding of mechanism and dynamics for photoionization processes is the foremost problem in atomic and molecular physics.With the development of laser technology and photoelectron imaging technology,the ionization process of atoms and molecules has been studied deeply.As one of the direct products of ionization,electron carries abundant physical information.The internal structure of atoms and molecules,the ionization process and the corresponding dynamics can be analyzed in detail by extracting the photoelectron kinetic energy spectrum and the photoelectron angular distributions.The main research of this thesis is divided into the following two parts:The first part is the multi-photon ionization process of heteronuclear diatomics carbon monoxide molecules in the monochromatic laser field with two different pulse durations.In the experiment,the pulse width of 50 fs,the central wavelength of 800 nm and the frequency doubling laser of 400 nm generated by the nonlinear crystal BBO were used as the laser source to interact with carbon monoxide molecules.The ionization pathways for the characteristic channels and the corresponding multiphoton ionization process of carbon monoxide molecules at two different wavelengths was studied by detecting photo-electrons with velocity map imaging spectrometer and their photoelectron kinetic spectrum and photoelectron angular distributions of the observed characteristic peaks are extracted.By analyzing the kinetic energy of each characteristic peak observed in the photoelectron spectroscopy of carbon monoxide molecules at 400 nm laser and its variation with laser intensity,and combining with the existing data of carbon monoxide molecule energy level distributions in the literature,the ionization pathways of the three observed characteristic peaks can be identified in terms of energy.That is,the resonant-enhanced multiphoton ionization process derived from different intermediate states and the non-resonant multiphoton ionization process.The FrankCondon factors of each ionization path is calculated and the most probable ionization path of each characteristic peak is discussed and obtained.Secondly,to explore the effect of laser pulse width on the ionization mechanism of molecules,the ionization process of carbon monoxide molecules was studied using a 35 fs monochromatic 400 nm laser.The comparison of the two photoelectron kinetic spectra showed that the center of the characteristic peaks did not change,indicating that the ionization mechanism of the molecule does not change in the current pulse width variation range.The photoelectron angular distributions of molecular is closely related to the amplitude and phase of the photoionization path,which carries the information of the photoionization path and dynamics of the system.Therefore,the Legendre polynomial fitting analysis of the photoelectron angular distribution of each characteristic peak is carried out,and the fitting parameters of each level of Legendre polynomial are obtained.The second part is the ionization process of methyl bromide molecule in the monochromatic 50 fs laser field.The ionization process was determined to be single ionization by time-of-flight mass spectrometry.When the laser wavelength is 800 nm,five characteristic peaks appear on the photoelectron kinetic energy spectrum of methyl bromide molecule with the increase of laser intensity.Combined with the coordinate positions of the characteristic peaks on the photoelectron kinetic energy spectrum and the available energy level data of methyl bromide molecule,the possible ionization pathways of these five characteristic peaks can be preliminarily identified,including two characteristic peaks of resonance enhanced multi-photon ionization and three above-threshold-ionization ATI of the first characteristic peak.When the laser wavelength is 400 nm,the photoelectron kinetic energy spectrum of methyl bromide molecule has three characteristic peaks.After analysis,all of them are products of multiphoton ionization mechanism involving resonance,and the most possible ionization pathways are discussed.