Photoelectron Imaging Study Of Strong Field Ionization Of CH₃I Clusters

In this thesis,using photoelectron image of（CH₃I）_n at different laser intensities are measured in a VMI spectrometer,we perform a experimental study on strong-field ionization of clusters.We provide the explanation for low-kinetic-energy structure and smoothly structureless distribution in the photoelectron energy spectra.In addition,the reason why the height of the peak structure in the electron kinetic energy distribution changes with the light intensity is also explained.In summary,this thesis studies the photoelectron energy spectra,the electron momentum spectrum,electron angular distribution and time-of-flight mass spectrometry generated by（CH₃I）_n ionization,which means that the ionization behavior of（CH₃I）_n in strong laser field is studied in many aspects.The study of the interaction mechanism between clusters and strong laser fields is an important part of strong field physics.It is also a research hotspot in the field of Physics of Intense Field（also known as extreme physics）,and many interesting physical phenomena have been discovered,such as:direct ionization,thermalization,autoionization,etc.However,there are still many physical phenomena that have not yet been explained for the ionization mechanism of clusters is more complicated than atoms and molecules.In this thesis,we study the Van der Waals cluster of（CH₃I）_n.Photoelectron imaging method is an effective and commonly used method to study strong field physics and we can obtain photoelectron imaging original image by photoelectron imaging method.Then,through the inverse Abel transform process and the integration of the three-dimensional slice map and the transformation of coordinates,we can obtain the electron momentum spectrum and the electronic kinetic energy distribution.We use the high pressure gas adiabatic expansion method to obtain Van der Waals clusters,and then（CH₃I）_n interacts with a beam of 800nm lasers that are focused by a focusing lens and whose polarization direction is horizontal in a right-angled intersection.In this paper,the intensity of laser is all 10¹³W/cm² order,and the low-kinetic-energy structure generated by the frustrated recombination appears in the electron kinetic energy distribution.The ionization probability of（CH₃I）_n increases with the increase of light intensity,and the yield of quasi-free electrons also increases with the increase of light intensity.As the light intensity increases,the rate of ultra-slow electrons increases,which is basically consistent with the results observed by Schütte et al.At the same time,the peak width and the isotropy of ultra-low-energy electrons increase with the increase of light intensity,which is because the plasma expansion effect is enhanced under high light intensity.There are equally spaced peak structures in the electron kinetic energy distribution,and the energy difference between the two peaks is equivalent to an 800nm photon energy.This is a direct evidence that of the existence of above-threshold ionization when（CH₃I）_n interacts with a strong laser field at a lower intensity,and the electrons that make up this structure do not collide and escape out directly.This thesis also fits the thermal electron distribution under four different light intensities to obtain the electron temperature of the thermal electrons.This experiment also found that the electron yield perpendicular to the laser direction is higher than the electron yield parallel to the laser polarization direction on the electron momentum,which is inconsistent with our expectations.In addition to the above experimental phenomena,this paper also compares the ionization behavior of（CH₃I）_n and CH₃I molecules in a strong laser field.