Investigation On Atomic Neutral Excitation In Femtosecond Strong Laser Fields Using Nanosecond-laser Ionization

As one of the basic processes of the interaction between ultrafast and intense laser fields and atoms and molecules,Rydberg State Excitation（RSE）is considered to be an important supplement to the physical processes induced by intense field tunneling and ionization,and has received extensive attention from researchers.Over the past few decades,excellent theoretical and experimental work on the excitation of Rydberg states in ultra-fast and intense laser fields has emerged,making this research topic gradually develop into an important research direction in the field of intense field atomic and molecular physics.In this paper,we first reviewed in detail the current research situation of strong field RSE physical processes,and analyzed the future development trends.On this basis,in order to solve the problems of synchronous measurement of strong field RSE and ionization,as well as the quantum state distribution of Rydberg states,we proposed a new experimental measurement method for strong field RSE,namely,nanosecond pulsed laser ionization time of flight mass spectrometry/photoelectron imaging technology,The ionization and excitation processes of Kr atoms in an 800nm femtosecond intense laser field were studied,and the main achievements were as follows:1)An experimental measurement method for studying strong field RSE,nanosecond pulsed laser ionization time of flight mass spectrometry/photoelectron imaging technology,has been established.By controlling the pulse delay time of a femtosecond intense laser field and a nanosecond ionized laser,the ion yields of different valence ions directly ionized by a strong field and the Rydberg excited states ionized by a nanosecond light field are obtained in the same time of flight mass spectrometry,thereby achieving simultaneous measurement of different physical processes of strong field ionization and RSE;Utilizing the narrow linewidth characteristics of nanosecond pulsed lasers and optoelectronic imaging technology,the quantum state distribution of the intense field RSE process can be obtained,and experimental measurements of quantum state resolution can be achieved.2)Using the established experimental techniques,the strong field ionization and excitation processes of Kr atoms in an 800 nm femtosecond intense laser field were studied.The relationship between strong field RSE processes and ionization/double ionization was analyzed by simultaneously measuring the yields of different ions(Kr⁺,Kr²⁺,and（Kr*）⁺);The photoelectron imaging,photoelectron kinetic energy E_K,and corresponding principal quantum number n distribution of the excited state of Kr atom in nanosecond pulsed photoionization have been obtained.The results show that the Rydberg states produced by the strong field RSE are mainly distributed in n=4-9,reproducing the theoretical calculation results in the literature;The relationship between the principal quantum number n distribution of the strong field RSE and the variation of the intensity and ellipticity of the strong laser field is studied.It is shown that with the increase of the intensity of the strong laser,the Rydberg states produced by the strong field RSE tend to occupy a high principal quantum number n,which is qualitatively consistent with the expectations under the semiclassical theoretical framework.