Ionization And Electron Imaging Of Ammonia And Methyl Bromide Molecules In Femtosecond Laser Field

Photoelectron imaging detection technique has gotten better and deeper developments and applications.Images of photoelectrons generated in molecular ionization process induced by ultrashort laser pulses is a good way to analyze the ionization mechanisms,involved quantum state,molecular structure and ultra-fast dynamics.Ionized photoelectrons carry the information of the involved molecular quantum state and structure.Thus,through studies on the photoelectron kinetic energy distribution(KED)and/or photoelectron angular distribution(PAD),one can access the detailed information on molecular orbitals,structure and dynamics,etc.The main research part of this thesis is the following two areas:In the first part,a monochromatic laser with a wavelength of 800 nm and a pulse width of 50 fs is mainly used to interact with the ammonia molecule,and the ionization process of the ammonia molecule is analyzed in detail after obtaining the photoelectron velocity imaging of the ammonia molecule.The above-threshold ionization process of ammonia molecules induced by a femtosecond laser field at 800 nm is studied in the intensity range from 1.6×10¹³ W/cm²to 5.7×10¹³ W/cm².Channel switching under different laser intensities is observed and identified in the photoelectron kinetic energy spectra of ammonia.Based on the photoelectron kinetic energy distributions and the photoelectron angular distributions,the characteristic peaks observed are exclusively assigned to the multiphoton resonance through certain intermediate states,followed by multiphoton above-threshold ionization.Three more pronounced resonance peaks exist in the resonance region,and multiple high-order above-threshold ionization peaks exist above the single photon energy,and the characteristic peaks observed all originate from six photon resonances,which in turn absorb different numbers of photons for ionization.The ionization pathway of the ammonia molecule was cross confirmed from a combined analysis of energy calculations and photoelectron angular distribution.In the second part,the ionization paths of methyl bromide molecules were probed by using a single 800 nm and 400 nm laser to induce the ionization paths,respectively.Under the single 400 nm laser pulse,the critical laser intensity at which the dissociation of the methyl bromide molecule does not occur is found in combination with the photoelectron imaging using the laser intensity dependence property,and the mass spectra of methyl bromide molecules and the dissociated fragments after raising the laser intensity are confirmed.The laser intensities selected for the study of the single ionization process of methyl bromide range from 1.7×10¹² W/cm² to 4.4×10¹² W/cm²,and the corresponding photoelectron imaging and kinetic energy distributions are obtained.The ionization paths and above threshold ionization processes of methyl bromide molecules in a single 400 nm laser pulse were confirmed by the combination of energy levels as well as photoelectron angular distributions.When the laser pulse wavelength was 800 nm,the laser intensity selected for the laser-induced ionization of methyl bromide molecules ranged from 6.2×10¹² W/cm² to 10.1×10¹² W/cm².Kinetic energy spectroscopy was also extracted from photoelectron imaging to analyze the ionization paths of methyl bromide molecules given the combination of pondermotive energy and non-resonance effect.