Negative Ion Excited States

This dissertation describes high-resolution tunable laser photodetachment studies of both valence and dipole -bound excited states of negative ions. Also discussed is a merged laser-ion beam photodetachment spectrometer. The photodetachment cross section of C(,2)('-) displays many sharp resonances, due to excitation of autodetaching levels of the B('2)(SIGMA)(,u)('+) valence state lying above the onset of the (C(,2) + e) continuum. The positions of approximately 1,000 resonances were measured, allowing the spectroscopic constants of the ground X('2)(SIGMA)(,g)('+) and excited B('2)(SIGMA)(,u)('+) states to be determined, including spin-rotation constants. The previously unobserved A('2)(PI)(,u) state has been characterized by deperturbation analysis. Strong A-X transitions are predicted near 2.5 microns. The widths of C(,2)('-) resonances with v = 6 -10 and J = 1-60 have also been measured, providing autodetachment rates as a function of both rotation and vibration, ranging from 10('8) to 10('11) s('-1). The first conclusive observation of an entirely new class of states, called dipole-bound or dipole-supported states, is discussed for the case of acetaldehyde enolate (vinyl oxide) negative ion. In these novel states, the extra electron is weakly bound ((TURN)5 cm('-1)) in a very diffuse orbital ((TURN)100 (ANGSTROM)) by the dipole moment of the neutral core. Weak electric fields (< 70 V/cm) are found to rapidly detach the dipole-bound electron. These dipole-bound states resemble Rydberg states of neutrals, but differ from Rydbergs in many important respects. They should be present for all neutrals with dipole moments > 2 Debye. The merged laser-ion beam (also known as coaxial -beams) spectrometer has 0.0005 cm('-1) resolution in the visible region, simultaneously achieving very high sensitivity. It employs quadrupole beam deflectors to merge and laser and negative ion beams, and can scan spectra either by scanning the laser frequency or the Doppler shift of the ion beam. Both electrons and neutral photodetachment products are detected. Even very low energy electrons (< 20 meV) are collected and counted, and the collector may be adjusted to collect either all electrons or only low energy electrons as needed.