Experimental Study On Evolution Of Cold ⁸⁷Rb Rydberg Atoms Into Ultracold Plasma

Strongly coupled plasmas,such as Jupiter core,white dwarf stars and high energy density matter,are supposed to be existed in extreme conditions,in which the Coulomb interactions between charged particles exceed their kinetic energy,and the evolution of strongly coupled plasmas is dominated by Coulomb potential energy.Strongly coupled plasmas are usually characterized with high-temperature and high-density and it is a challenge for researchers to directly study their dynamics because of their short evolution timescale and small spatial size.With the development of laser cooling and trapping,neutral plasmas at temperatures as low as 1 K were created,called ultracold plasmas?UCPs?,showing many interesting properties in the strongly coupled regime.Ultracold plasma has many advantages,such as well-controlled initial conditions,long evolution timescale and tabletop experimental setup,offering an ideal platform for the studies of strong coupling effect both experimentally and theoretically.In this work,we experimentally investigated the spontaneous evolution of cold dense ⁸⁷Rb Rydberg atoms into UCP.The autoionization spectroscopy of cold Rydberg atoms were measured,and the autoionization mechanisms of nP Rydberg gases in the spontaneous evolution were investigated,as well as the9)-changing collisions between hot electrons and low-9)P Rydberg atoms were studied.Moreover,we presented a new method to measure the plasma temperature via ion velocity map imaging?VMI?technique and obtained the two-dimensional images of velocity distributions for ions and electrons in UCPs experimentally.Firstly,we created an ultracold plasma via the spontaneous ionization of cold dense Rydberg atoms of ⁸⁷Rb in a magneto-optical trap?MOT?.By detecting the electrons in the ultracold plasma,we obtained the ionization spectroscopy of cold Rydberg atoms and measured the9)S_1/2?9?=50–80),9)P_1/2?9?=16–23),9)P_3/2?9?=16–98)and9)D_5/2?9?=49–96)Rydberg levels.By fitting the energy levels of Rydberg states,the first ionization potential of 33690.950?11?cm^-1 and the quantum defects of S,P and D orbitals of ⁸⁷Rb atoms were obtained,in which the quantum defects for p_1/2 and p_3/2series,were given for the first time.Next,we measured the spontaneous evolution process of cold Rydberg atoms under three different electric-field conditions and investigated the autoionization mechanisms of dense nP_3/2?n=20-97?Rydberg gases of ⁸⁷Rb atoms systematically.By observing the characteristic time of the electrons generated through autoionization process,the dependence of autoionization mechanisms on the principal quantum number n of initial nP states was demonstrated,that is,the black-body radiation,electron-Rydberg collision,Rydberg-Rydberg collision and Penning ionization dominate the autoionization of Rydberg gases alternately with the decrease of n value.In addition,n-changing population of low nP Rydberg atoms by hot electron-Rydberg collisions was measured experimentally,and the results were in good agreement with the theoretical calculations.Also,we presented the proposal to obtain the velocity distributions of ions and electrons in UCPs using the ion velocity map imaging?VMI?technique,as well as the design and test results of the equipment.A VMI spectrometer based on a magneto-optical trap?MOT?was designed and calibrated by photoionizing atoms cooled in one-dimensional direction.The preliminary results of two-dimensional velocity images of ions and electrons in UCP were presented and discussed.In brief,we measured the autoionization spectra of ⁸⁷Rb Rydberg atoms,investigated the dominant autoionization mechanisms in the spontaneous evolution of cold dense nP Rydberg atoms,and tested the new method of measuring the temperatures of UCP.A further study of the dynamics in the evolution of ultracold plasma will be carried out based on this work.