The Correlation Between Strong Field Ionization And Excitation Studied By Covariance Imaging Method

The interaction of laser and matter has always been one of the hotspots and frontier areas in the research of atomic and molecular optical physics.Many strong field physical phenomena are produce by atoms/molecules in the femtosecond laser field,such as high-order above threshold ionization,non-sequence double ionization,higher harmonic emission,Rydberg state excitation,and so on.Tunneling ionization as the most important and fundamental step is closely related to these physical processes according to the three-step re-scattering model.At present,there are a lot of researches about the mechanism of these strong field phenomena have been published,but the correlation between different physical processes is still to be resolved.It is helpful for us to understand the essence of the interaction of atom/molecule and femtosecond intense laser by studying the correlations between different strong field physical processes.Covariance imaging method has many advantages such as high event rate,fast response,little error,and is suitable for different type of signals(such as photons,ions,electrons and neutral excited states).In this paper,we studied the correlation between strong field ionization and Rydberg state excitation of atom by covariance imaging method,and the following research results are obtained:1)we experimentally measured the correlation between different fragments of OCS molecules irradiated by 800 nm,50 fs intense laser field using covariance imaging method.The covariance images are measure under different sample pressures and voltages of the plate,and the influence of event rate and collection efficiency on the signal-to-noise ratio of the covariance images are studied.Finally,we obtained the optimal experiment conditions for covariance imaging method.2)Using time-of-flight mass spectrometry with two detectors to achieve simultaneous measurement of strong field ionization and Rydberg state excitation of Kr atoms,and the correlations between different physical processes is obtained.By measuring the covariance images at different laser field(laser intensity and ellipticity),it is found that the Rydberg state excitation have the positive correlation with tunneling ionization and non-sequence double ionization,but the negative correlation is found to be with the sequence double ionization.By normalizing the covariance matrix,it is found that the correlation between different physical processes varies with the change of laser parameters.Based on the existing knowledge,we analyzed the inner reason of the correlation between different physical processes.These results are helpful to understand the physical mechanism of strong field ionization and excitation,provide reference and guidance for the future study of the relationship between different strong field physical processes,and promote the development of corresponding theoretical methods.