Photoelectron spectroscopy of negative ions : energy- and angle-resolved measurements

A crossed laser-negative ion beams apparatus has been used to investigate both Ca^- and He^- ions. The fast negative ion beam was perpendicularly crossed with a photon beam from a flashlamp-pumped dye laser. Photoelectrons ejected in the direction of the ion beam were collected and energy analyzed by an electrostatic electron spectrometer. The resolved electron peaks in the spectra corresponded to photodetachment processes that left the residual atoms in either their ground state or excited states.

Experimental evidence is presented to show that Ca^- is a stable ion and is formed in the (4s²4p) ²P state. This result represents the first report of a stable negative ion for a Group IIA element. The structure of Ca^- was determined by means of photoelectron detachment spectroscopy. Kinematic effects associated with a fast moving ion beam were exploited, in a novel way, in these experiments. The electron affinity of Ca was measured to be 0.043 ± 0.007 eV, which is in good agreement with recent calculations.

Energy- and angle-resolved spectroscopy measurements have been used to study, for the first time, the spectral dependence of the angular distributions of electrons photodetached from a beam of He^- ions. Photoelectron angular distributions were measured by varying the angle between the electric field vector of the linearly polarized laser beam and the fixed collection direction. In the spectral range of the measurements, 1.775 - 2.456 eV, photodetachment into either the He(1s2s) ³S + e or He(1s2p) ³P + e continua is allowed. These competing channels were resolved and the spectral dependence of the angular distributions were measured for each case. The He(1s2s) ³S exit channel produced a spectral dependence that could be explained, using an independent electron description, by interferences between the outgoing s- and d-waves. The shape of the spectral dependence for the He(1s2p) ³P exit channel could not, however, be explained by an independent electron model. Correlation is seen to affect the electron emission processes. Various final state interaction models are discussed to account for the observed spectral dependence.