Precision Measurement Of The Electron Affinities Of Transition Metals

Negative ions are commonly formed by the extra electrons bound to the neutral parts through the complicated electron-electron correlation effects.Unlike the electrons in neutral substances or positive ions bound by the Coulomb potential,those in negative ions are bound by the short-range potential which decreases more dramatically.The short-range potential results in only a few bound states of negative ions,which typically include the sole ground state or some fine structures with the lowest energies.In view of these novel properties,negative ions have played a great role in both experimental and theroretical studies of electron correlation effects.As a fundeamental parameter of atoms and molucules,electron affinity?EA?plays an important role in astrophysics,plasma physics and gas-phase ion chemistry.Although the accuracies of the EA measurements for main group elements have been steadily improved to 0.01-0.05 meV,those for transition elements still remain at around 10 meV.EAs of many lanthanides are even unknown.The reasons mainly lie in the low cross sections of p-wave detachments and the extremely complicated electronic structures.Compared with the tranditional threshold photodetachment spectroscopy,the slow electron velocity-map imaging?SEVI?method used in this work has a higher count rate close to the threshold.More important,SEVI has a very high energy resolution.The measurement accuracy is better than 0.1 meV.With the SEVI method,the atomic electron affinities of a few transition elements were precisely determined.In addition,the fine strutures of anions were also well resolved.The experimental results are EA?Nb?=7399.35?50?cm^-1,EA?Fe?=1235.93?28?cm^-1,EA?Co?=5341.45?37?cm^-1,EA(²⁰⁸Pb)=2877.33?13?cm^-1,EA?Re?=487.13?51?cm^-1,and EA?Re?=2652.25?47?cm^-1.The accuracies of EA measuremnts of Nb,Fe,²⁰⁸Pb and Ta have been imporved by two orders of magnitude compared with the previous experiments.The EA value of Re and fine structures of Nb,Fe,Co and Ta anions were experimentally obtained for the first time.The EA value of Hf has also been measured tentatively for the first time to be 1733?72?cm^-1.The EA values with high accuracies and the fine structures of negative ions acquired in this work will provide a test standard for the developments of new theoretical calculation methods.The energy resolutions of photoelectron spectra of molecules at room temperature are usually around 10 meV due to the thermal broadening.Combined with the cold ion trap,the SEVI method shows a good performance in resolving electronic structures and vibrational structures of molecular ions.The high-resolution photoelectron spectra of CoO^-and TaO^-are showed in the last chapter of this thesis.The combination of cold ion trap technique and the SEVI method will be a powerful tool for studying complex systems in the near future.