Search For Atomic Anions For Laser Cooling Using Cryo-SEVI Method

Laser cooling is one of the most important techniques in the field of physics,which has opened up many new research areas.Laser cooling of positive ions was first realized as early as 1978.Since then,physicists have been refreshing the limits of cooling and observing new physical phenomena.However,the laser cooling of negative ions has never been realized.The reason lies in the lack of a closed and fast electric dipole transition cycle,as negative ions usually have no bound excited states,let alone electric dipole transitions.Since the 1970s,many experimental methods have been invented for the study of anions.However,due to limitations of these methods,our understanding of anions is still far from enough.So far,electric dipole transitions have only been observed in three negative ions,namely Os-,Ce-and La-.Unfortunately,none of them are suitable for laser cooling.In 2016,our group introduced the slow electron velocity-map imaging(SEVI)method into the study of atomic negative ions.We have accurately measured the electron affinities of a large number of transition elements and resolved the energy level structures of their corresponding negative ions.This technique overcomes shortcomings of previous experimental methods and brings the study of anions to a higher level.We hope to search through the periodic table of the elements,and finally find excellent negative ions suitable for laser cooling.This is the main line of my research and the context of this thesis.In this work,we combine the cryogenically-cooled ion trap with SEVI method to study the negative ions of Hf,Ti,Ga,U,and Th.Their electron affinities are determined to be 609.29(34),1436(5),2429.07(12),2540.4(7)and 4901.35(48)cm-1,respectively.In particular,we found a naturally closed transition cycle in Th-:4F3/2e ? 2S1/2o.Using the two-photon resonance detachment method,we experimentally observed this electric dipole transition,and its energy is measured to be 4118.0(10)cm-1.Our experiments and calculations suggest that Th-is an excellent candidate for the laser cooling of negative ions.This work includes our new finding of the chemical reaction as well.We noticed that the ground-state Pd-reacts with H2 much faster than the excited-state Pd-.This is the first time such an effect has been observed for the state-specific reaction of anions.