Single- and multiphoton studies of atomic negative ions: Electron affinities, threshold laws, and near-threshold structure

Abstract

<p>While neutral atoms and positively charged ions are bound in a Coulomb potential, the excess electron in an atomic negative ion is bound in an induced dipole potential (falling off as r-4 ). One consequence is that only a small number of states are bound in a negative ion, typically only the fine structure of one term, in contrast to the infinite number of states bound in a neutral or positively charged atomic system. The novel structure and binding potential of negative ions have attracted much interest on a fundamental level. In addition, negative ions play an important role in such areas as accelerator mass spectroscopy (AMS), atmospheric physics, and astrophysics. Despite considerable ongoing theoretical and experimental efforts, a number of atomic negative ion states remain poorly known or even completely unknown. Recently, tunable pulsed laser techniques have led the way in high precision measurements in atomic negative ions. This thesis presents results obtained with the tunable infrared laser source at McMaster University using single-photon threshold detachment and multiphoton resonant detachment spectroscopy, as well as non-resonant multiphoton threshold studies. Experiments investigating the structure of 13 elemental negative ions are detailed: Cs - , Cr- , Mo- , W - , Ru- , Os- , Ir - , Pt- , Cu- , Ag - , B- , Al- , and Bi - . Of particular note are the results of the first measurements on the negative ion of osmium which have provided the first and only known example of an atomic negative ion having both even and odd parity bound states. Strong continuum structures are also observed in Cs- , Os- , and W- for the first time, and appear as near-threshold Feshbach and shape resonances. Investigations of the behaviour of single- and multiphoton threshold laws are also presented. Specifically, single photon s -wave and p -wave threshold behaviours are measured to energies well above threshold and are compared to the models proposed by O'Malley and Farley. It is found that better results may be obtained if the two models are combined. Finally, two-photon experiments in Au- have demonstrated the extreme sensitivity of the near-threshold multiphoton detachment cross sections and the multiphoton threshold law to the polarisation state of the laser. These studies have led to the first observation of a Wigner d -wave law, and the technique is argued to have valuable applications to studies of negative ions which have structure lying near a photodetachment threshold.</p>