Photoionization Of Helium In Femtosecond Extreme Ultraviolet Fields

High-order harmonic generation(HHG)in gas is an important technical means to produce coherent radiation in the extreme ultraviolet(EUV)band,which is based on tabletop.It realizes an experimental condition that has been primarily relying on large scientific devices,such as synchrotron radiation and free electron laser.EUV photons can ionize most atoms and molecules singly,doubly,or even multiply,which provides a promising technical solution for investigating ultrafast dynamical processes of atoms and molecules.The thesis studies the photoionization of helium(He)by femtosecond EUV with a photoelectron velocity map imaging(VMI)setup.The EUV pulses were produced using HHG in an argon gas cell.Either harmonic was selected by a homemade monochromator and focused into the He gas beam.The photoelectron angular distribution(PAD)was extracted from experimental data.The comparison of the PAD under different harmonic interacted shows that it does not conform to the distribution characteristics of single photon single ionization,and the discrepancy varies with harmonic orders.Further analysis reveals that the discrepancy arose from the double excited state and the electron correlation of He,in which mechanisms such as "shake-off" make photoelectrons superposition on which produced by single photon single ionization and cause a discrepancy in PAD between the experimental measurements and existing theoretical prediction.Theoretical simulations support experimental measurements and explain the variation of the photoelectron angular distribution with photon energy for He in EUV fields.The thesis demonstrates that photons with different photon energy in the EUV band can modulate the ionization channel of He.