Electron Imaging Of Alkaline Earth Metal Calcium Atoms In Femtosecond Laser Fields

The study of atomic multiphoton ionization process is an effective method to explore the structure and dynamic properties of quantum states.As one of the direct products of ionization,photoelectrons carry a wealth of information,including the population of the quantum states,the structure of atoms,the ionization dynamics and the dynamics in the laser field after ionization.The analysis of photoelectron energy distribution and angular distribution is important to the study of atomic ionization.The energy distribution can be used to analyze the excitation and ionization process of atoms,while the angular distributions encoding the wave packet characteristics of the outgoing electron under the action of the laser field and further verify the ionization excitation channel of the photoelectron.The main work of this paper is divided into two following aspects:The first one is the ionization of calcium atom in the monochromatic femtosecond laser pulse.In this paper,multiphoton ionization of calcium atom in 400 nm(60 fs)or800 nm(50 fs)femtosecond laser is studied with energy-and angle-resolved photoelectron imaging technology.The resonant ionization channels including two resonant states(4s4p and 4s5p)and the non-resonant ionization channels is identified by photoelectron energy spectrum,and there are three cation states(4s,4p and 3d)are envolved.The angular distributions further confirm the identification of the excitation and ionization process.It is worth noting that the populations of 4p and 3d cation states involves multielectron effect,which is obviously reflected in the photoelectron angular distribution.At the same time,we observe something related to Fano's propensity rule.Since the Fano's propensity rule is only applicable to single-photon transitions,we compare the observed pathways from multiphoton ionization of calcium atoms,and find it agree with the one and two photons Fano's propensity rule.Based on this we have discussed the applicability of Fano's propensity rule in the process of multiphoton transitions.The second research is studying the ionization of calcium atoms in two-color laser field.The asymmetry of polarization direction of the relative phase-controlled twocolor laser field has an obvious effect on the ionization yield of calcium atoms,which is directly reflected in the asymmetry of the upper and lower parts of the twodimensional momentum of photoelectrons.By introducing asymmetric parameters and fitting the yields of each peak in photoelectron energy spectrum,the sine-modulation depth and relative phase between each characteristic peak are obtained,and the different modulation depth of each characteristic peak are observed,and the possible reason was analyzed for the different intensity ratio between two lasers.In addition,the relative phase difference between the characteristic peaks decreases with the increase of ATI order,and the intrinsic phase information of the electron disappears gradually,that is,the influence of the laser fields on the measured phase increases with the increase of ATI order.Then,we apply the asymmetric parameter to the images of photoelectron,and analyze the phase dependent asymmetry of photoelectron angular distribution.For electrons with the same energy,there is phase delay between the electrons emitted from different emission angles,which may be caused by the different action of laser fields and the electrons emitted from different emission angles.