The Ionization Of Ammonia Molecules According To The Intermediate Rydberg States In Femtosecond Laser Field

The interaction between laser and materials is not only an important filed of physics,but also an important way for human to understand our world and outspace.In recent years,especially with the rapid development of laser technology,the one cycle femtosecond laser pulse is generated.The interaction of laser field with atoms,molecules and clusters under such extreme conditions is a challenge to basic science.Many physical phenomena have been observed by femtosecond laser interact with various forms of materials.Molecular Rydberg state and molecular vibration excitation also play important roles.All of these research topics are important frontiers in atomic and molecular physics.This paper is divided into two parts according to the molecular photoelectron imaging experiment.In the first part,photoelectron velocity imaging was used to study and compare the ionization processes of ammonia molecules on conditions with or without hexapole state selection.We obtained two-dimensional photoelectron images of NH3 molecule under different laser intensities in a linearlypolarized femtosecond laser field with a wavelength of 400 nm.The photoelectron kinetic energy distributions?KEs?and photoelectron Angle distributions?PADs?of NH3 molecule were extracted from the original images.The six photoelectron peaks in the kinetic energy distribution in the presence of a hexapole are divided into slow electron region and fast electron region and they are all labeled.Peak1 is labeled as 1³2³?3+1?multi-photon process reaches the intermediate state of E state.Peak 2,3,4 is labeled as 1²2³?2+2?,1¹2³?2+2?and 1⁰2³?2+2?multi-photon process.Two-photons excited intermediate Rydberg state A¹A₂??v_'2=3?does not change,only the vibrational states of ammonia ions changed,which belongs to REMPI process.At the same time,channel shift was observed.Peak5 and peak 6 are respectively marked as?2+2+1?,?2+2+2?process,and belong to ATI processes.Peak 1 occurs only when there is a hexapole,because the hexapole enables state selection and beam focusing.Through our analysis,it can be seen that peak 1 only appears in the case of hexapole,so that the electrons with low kinetic energy can be detected after flying out of the focused molecular beam.In the second part of this paper,The Above-Threshold ionization processes of ammonia molecules with a laser wavelength of 800 nm femtosecond were studied.There are mainly six ATI peaks in thephotoelectron kinetic energy distributions.Peak 1 can be identified as the6+2+1 multi-photon process passing through B Rydberg intermediate state.Peak 4 is the 6+2+2 multi-photon process passing through B Rydberg intermediate state,peak 5 is the 6+2+3 multi-photon process passing through B Rydberg intermediate state,and peak 6 is the 6+2+4multi-photon process passing through B Rydberg intermediate state.While peak 2 is labeled as the 6+2+1 multi-photon process passing through the intermediate state of A Rydberg,peak 3 is the 6+2+1multi-photon process passing through the intermediate state of C Rydberg.At the same time in this process,the phenomenon of channel shift was also observed.Finally,the vibrational excitation of ammonia molecules in the process of Above-Threshold ionization is further analyzed and the vibrational levels of each peak under different laser intensity are given.