Molecular Clusters Ionization In Femtosecond Laser Field And Its Electron And Ion Imaging Studies

In recent years,theoretical and experimental research on the interaction between ultrashort femtosecond laser and clusters has been a very active field.The interaction of ultrashort pulses and clusters is an important system for studying the multi-electron dynamics and nuclear dynamics of complex systems,and the high-energy ions and electrons generated by the action are also important mediators for triggering the interaction between substances.Ionization of clusters will induce highly complex dynamic processes of electrons and nuclei.The time scale of evolutionary dynamics spans from nanoseconds to attoseconds,and involves the fundamental problem of a large number of interactions between particles.Among them,the interaction of strong near infrared?NIR?femtosecond laser pulses and clusters contains a variety of complex ionization mechanisms in the ultrafast laser field,such as frustrated ionization recombination,direct ionization,electron heating,electron ionization after electron collision excitation,laser driven electron rescattering,quasi-free electrons in the plasma and few body-associated decay processes of multiple excited ions,etc.Since the cluster system itself is much more complicated than atoms and molecules,many physical mechanisms of cluster ionization have not been well explained.This paper takes two different molecular clusters?ammonia clusters and ethanol clusters?as examples,and uses ion velocity imaging?VMI?to measure the time-of-flight mass spectrometry,electron and ion imaging of the clusters generated by the ionization of the clusters in a strong laser field,The physical mechanism of the interaction between molecular clusters and strong femtosecond lasers was studied.First,we use the electronic imaging method to obtain the electron momentum distribution and corresponding electron kinetic energy distribution of ammonia clusters under different light intensity and wavelength conditions.The distribution characteristics of different regions of electron momentum are explained and explained.The unstructured smooth distribution of the higher energy part and the higher peak distribution of the low energy part were studied separately.Combined with the measurement results,the frustration ionization process,the direct ionization process of the light field,the electron thermalization process,and the electron collision re-scattering process are mainly discussed.Studies have shown that the frustrated ionization process corresponds to the low energy distribution on the electron energy spectrum,and direct ionization corresponds to the isotropic electron distribution of the corresponding angular distribution.At the same time,the thermalized electrons appear as quasi-free electrons clustered in the cluster potential undergo multiple collisions leading to rapid thermalization,showing a smooth kinetic energy distribution and conforming to the Boltzmann distribution,through the function P?e??exp?-e/Ta?Curve fitting can be in good agreement with the experiment.For the inelastic collision process,the electron can obtain kinetic energy up to 20Up.In addition,through the measurement of the photoelectron angular distribution,it is observed that the distribution of electrons emitted along the laser polarization direction and perpendicular to the laser polarization direction exhibits different characteristics,and the angle distribution of electron emission in different energy regions depends on different characteristics,indicating that different physical mechanisms emit The electrons satisfy different anisotropies,so the angular distribution of photoelectrons can be clearly used as a criterion for determining the electron emission mechanism of the cluster system,which is of great significance.Another cluster studied in this paper that interacts with the femtosecond laser is the ethanol cluster.In the experiment,we used the time-resolved TPX3CAM camera to measure the time-of-flight mass spectrometry of ethanol ion for the first time,and gave the variation relationship of the cluster signal with the laser intensity and the cluster ion imaging distribution.Two different types of clusters?C₂H₅OH?_nH⁺,?C₂H₅OH?_n?H₂O?H⁺were mainly observed,and the formation reasons and physical mechanisms of protonated ethanol clusters and water and protonated ethanol clusters were analyzed.Using the ion velocity imaging device,the momentum distribution of cluster ions was measured.From the process of the cluster and laser,the main reason for the generation of ions came from the impact ionization of electron ions.The measurement results show that for larger clusters,the momentum distribution is narrower,and the cluster ion energy spectrum is affected by the laser intensity and cluster size and shows a peak structure.