Development Of Velocity Map Imaging Spectrometer And Its Application In Strong Field Ionization Of Atoms And Molecules

Velocity map imaging(VMI)spectrometer can obtain the strong field products'three-dimentional momentum distribution,through which both the energy distribution and the angular distribution can be extracted at the same time.Due to this excellent ability,the VMI spectromiter has been a powerful tool in researches of strong field physics.This dissertation introduces the construction process of a VMI spectrometer in detail and related researches with the spectrometer.We investigate the influence of the laser-gas interaction volume to the imaging resolution.Furthermore,with dispersion-compensated method and aplanatic-lens method,we improve the energy resolution largely.Then,combining the spectrometer with the ultrafast laser,we selectively enhance the resonant multiphoton ionization through certain atomic Rydberg states.Firstly,the principle and the setup of the VMI spectrometer are introduced detailedly according to its structrue:flight electric field,vaccum system,gas inlet system,laser system,detector and experimental results analysis.We stress the introduction of flight electric field and the design of the electrostatic lens.Furthermore,with imaging experiments of the multiphoton ionization of Xe atoms,we test the detection ability of the VMI spectrometer.Moreover,at the same time of the spectrometer's construction,we made some reseach and improvement about the imaging performance of the VMI spectrometer.With different sizes of laser-gas interaction area,the energy resolution is detailedly investigated along the aixial and radial directions respectively.The simulation results show that the axial interaction mainly affects the electron/ions with high energy.The radial interaction size has a minor influence on the energy resolution for the electron or ion with medium energy,but it is crucial for the resolution of the electron or ion with low kinetic energy.By tracing the flight trajectories we show how the electron or ion energy resolution is influenced by the interaction size.With the electrostatic lens,the focuses of ions/electrons with different kinetic energy located different position along the spectrometer axis,which exerts a large influence on the resolution.Thus,with some methods to reduce this effect of the electrostatic lens,the resolution can be improved obviously.A novel design called disperstion-compensated VMI is presented to improve the kinetic energy resolution of a velocity map imaging spectrometer.The main modifications,compared to the original design of Eppink and Parker(1997 Rev.Sci.Instrum.68 3477),are two additional grid electrodes.The deflection field constructed by the two electrodes is to compensate the dispersion of the 'v'-shaped energy resolution.Simulations of the imaging process show that the kinetic energy resolution can be improved drastically by the dispersion-compensated method.Quantitatively,the value of the energy resolution is reduced to half of the traditional VMI.Furthermore,we propose another method,which is more simple and feasible,to improve the energy resolution of photoions called aplanatic-lens VMI.Building on the traditional VMI spectrometer with three aperture electrodes,we apply a stepped voltage on the extractor electrode of the spectrometer to reduce the spherical aberration effect of the electrostatic lens.With precisely controlling the moment of the voltage,we suppress the spherical aberration effect induced by the converging electrostatic lens in the traditional VMI spectrometer.Both simulated and experimental results demonstrate that the energy resolution is significantly improved using this aplanatic-lens VMI spectrometer.Quantitatively,the value of the energy resolution is reduced to 1/3 of the traditional VMI.High-resolution photoelectron momentum distributions of Xe atoms ionized by 800-nm linearly polarized laser fields have been traced at different laser intensities using the VMI spectrometer.At certain laser intensities,the momentum spectrum of Xe exhibits a distinct double-ring structure,which appears to be absent at lower or higher laser intensities.By investigating the intensity-resolved photoelectron energy spectrum,we find that this double-ring structure originates from resonant multiphoton ionization involving multiple Rydberg states of atoms.Varying the laser intensity,we can selectively enhance the resonant multiphoton ionization through certain atomic Rydberg states.The photoelectron angular distributions of multiphoton resonance are also investigated for the low-order above-threshold ionization.We find that the photoelectron angular distribution depends not only on the angular momentum quantum number,but also on the principal quantum number of the Rydberg states.