Ionization Yield And Photoelectron Imaging Of Nonadiabatic Aligned Methyl Halide Molecules

In this thesis, using methyl halide molecules as examples, we perform a combined experimental and theoretical study on the properties of molecular alignment by the interactions with both hexapole static electric field and femtosecond laser field. The method which can extract the molecular alignment parameter?P₂?cos???the expectation value of second order Legendre polynomial) from velocity map imaging experimental data is discussed. A clear and systematic recognition on molecular rotational dynamics and its physical mechanism is obtained. With the prepared aligned and anti-aligned molecules, we perform a study on the ionization process of methyl halide molecules through measuring the ionization yield and photoelectron imaging to acquire a deeper understanding of methyl halide molecules' ionization behavior.The study on the ionization of polyatomic molecules in intense femtosecond laser fields has been a hot topic for years in strong field physics due to their complexity of quantum states and many-particle correlation. There are still a lot of phenomena which need to be interpreted, especially when laser power is right between the multiphoton ionization regime and tunneling ionization regime. Molecular alignment technique can relate the space-fixed frame with the molecule-fixed frame, which makes the study on strong field physics under molecular coordinate possible. The development of novel experimental technique is necessary. We have built up an experimental apparatus combined Hexapole with velocity map imaging system. Hexapole state selection is used to prepare the symmetric top molecules in |1±1???1? rotational state. The molecules are aligned by a strong 800 nm laser field, which is linearly polarized perpendicular to the weak static extraction field of the time of flight setup. The molecules are subsequently ionized by a second time delayed probe laser pulse. The period of the revivals in the ion signal is reproduced by the calculation without fitting any other parameter except an amplitude scaling factor. It is shown that with the experimentally favourable perpendicular geometry of the time-of-flight setup, concomitant deviations from the axial symmetry are sufficiently small to be neglected. The calculated alignment parameters at the maximum alignment and anti-alignment correspond well to the values derived from the velocity map images. The laser induced controllable alignment was found to have the upper and lower extreme values of ?P₂?cos??? =0.7 for the aligned molecule and-0.1 for the anti-aligned molecule.The PPT model is programmed. By comparing the simulated results with the ionization yield data of Krypton and Xenon in the laser intensity regime from 1.0×1013 W/cm2 to 1.0×1015 W/cm2 by the time of flight mass spectrometer, the validaty of the programme is assured and the laser intensity used is calibrated. In this regime the ionization of Xenon is very well described by PPT model. The simplistic barrier suppression ionization model yields saturation intensity which is in good agreement with PPT theory for Xenon. For Krypton at the low part of the intensity regime a contribution of resonance enhanced multiphoton absorption has been observed. The alignment dependent ionization yield of CH3 Br and CH3 I molecules has been measured experimentally. By comparing a modified PPT theory simulation to the experimental results we analyze the contribution from the HOMO orbital and the low-lying otbital to ionization process. The ionization yield of CH3 Br in the lower part of the intensity regime also deviates from the simulation of PPT model. We attribute this deviation to the contribution from ionization through the orbital with A1 symmetry, where resonant excitation process may be involved. At higher laser intensities ionization takes place via the e orbital?HOMO? which is aligned perpendicular to the symmetry axis of the molecule. For CH3 I the ionization process is well described by PPT model and the ionization takes predominately place via the e orbitals of the HOMO. Hexapole state-selection in combination with laser alignment of the molecule gives detailed information about the different photon absorption pathways involved in the ionization process and the contributions of the HOMO and low-lying orbital to the ion signal. The photoelectron imaging of aligned and anti-aligned CH3 I molecule has been measured through angular resolved photoelectron spectroscopy method. The peaks in the photoelectron kinetic energy spectrum is assigned by comparing the photoelectron kinetic energy distribution changes under different laser intensity. We see a branching ratio change of 2 peaks between aligned and anti-aligned CH3 I molecules measured under the same laser intensity which is 9.6×1012 W/cm2. We attribute it to the change of the effective ionization potential between the aligned and anti-aligned CH3 I molecules.