Electron Imaging Of Alkali Metal Atoms In Femtosecond Laser Fields

The multiphoton ionization process of atoms is an effective method to study the interaction between atoms and laser fields because it reflects the characteristics of laser pulses and reveals the dynamics information of atoms perturbed by laser field.Resonance-enhanced multiphoton ionization with photoelectron spectroscopy is a powerful tool to study the dynamics of photoionization of neutral atoms.The kinetic energy distribution describes the information of the internal states of the remaining parent nucleus,and the photoelectron Angle distribution summarizes the characteristics of the partial free electron wave packet of the emitted electrons and the arrangement of the resonant intermediate states.The experimental research of this paper is mainly divided into the following two aspects:First,the first part is the interaction of monochromatic laser pulses with potassium atoms.In the experiment,a laser with half height and full width of 50 fs and central wavelength of 800 nm interacting with potassium atom beam combined with velocity mapping imaging system is used to detect ions and electrons in the ionization process.The effect of resonant coupled excited states on the final measured electron-electron momentum spectrum is discussed.Consider the dressed states theory,the Autler-Townes effect on the 4s and 4p energy levels by a single 800 nm photon leads to the splitting of the energy spectrum peaks,splitting energy interval as the Rabi frequency,an unexpected electron peak structure is observed in the energy spectrum which violated the dipole transition rule,a preliminary explanation of the observed anomalous electron peaks is given by considering the spin-orbit coupling effect.By changing the single laser pulse energy,we obtain the laser intensity dependence of potassium atoms photoelectron spectroscopy,the peak energy interval with the increase of laser intensity splitting evident as reaching 0.35 e V,the effective Rabi frequency is introduced to correct the measured Rabi frequency,and the effective Rabi frequency displacement considering the influence of ionization probability is in good agreement with the calculated results.As the laser intensity increases,the ionization approaches saturation,so the width of the splitting peak does not increase.As a contrast,no splitting electron peak is observed in the photoionization experiment of potassium atom at 400nm.Two possible explanations are given.By extracting the photoelectric angle distribution and fitting it with spherical harmonic functions,we find that the two splitting electron peaks have the characteristics of(4₃⁰(,)and the abnormal electron peaks have the characteristics of(4₃^±1(,).In order to describe the characteristics of electron wave packet quantitatively,the photon electron angular distribution is expanded by wave division,and the quantitative description of the directly ionized free electron wave packet is obtained.p and f wave between the phase of?/2,the peak splitting of components are exactly the same,this indicates that the phase delay between the partial waves will not be introduced within a certain range of laser intensity.At the same time,the momentum diagram of the interaction between sodium atom and monochromatic800nm laser pulse is given.The results are similar to those obtained in the experiment of potassium atom.However,since sodium atom is a 3s-4s two-photon resonance,there is no spin-orbit coupling effect in S state,meanwhile,we also give the results of the partial wave expansion of the electron peak.The second part is the two-color laser field with potassium atoms interaction,two-color field of potassium atoms emergent electronic wave packet has obvious modulation function,energy spectrum of relative phase dependence can be clearly observed periodic oscillation and imaging of the upper and lower part of the ionization of asymmetry,the introduction of asymmetric parameter fitting oscillation of each peak modulation depth and the corresponding peak phase information,the results of asymmetric parameter fitting are in good agreement with the experiment.We further will be introduced to the asymmetry parameter photoelectron angular distribution image with phase modulation,photoelectron angular phase asymmetry parameter extraction can exit points clearly distinguish different angles electronic has natural phase delay,laser field coupling Angle has an obvious dependence electronic state,some angles of electron emission.Electronic peak production rate asymmetry parameter extraction different angles can be precisely for electronic information of the disturbance of laser field phase,at the same time as the ATI order increase electronic inner shell excited states of initial phase began to disappear,the electrons carry the mother nucleus coulomb potential information began to gradually disappear,the higher the electronic energy is affected by the laser field amplitude,the more intense.The angular asymmetry parameter presents a new method for processing experimental data,which has obvious visual advantages for dealing with two-color field experiment.The periodic oscillation patterns of the related phases are fully represented in the asymmetric relative phase diagram.