  Atomic negative ions are fragile quantum systems in which correlation among
the valence electrons plays a very important role. For doubly excited states
correlation becomes dominant. Photodetachment is the process where a negative
ion absorbs a photon and as a consquence ejects the outermost electron. A new
detection scheme has been developed. The residual atom of a photodetachment is
state selectively ionised by resonance ionisation spectroscopy. Investigations
of the photodetachment threshold of tellurium with neutral particle detection
yielded an electron affinity of 1589618(5) m^-1. With state selective detection
the Li(2p) photodetachment threshold was investigated and the electron affinity
was determined to be 498490(17) m^-1. Resonance structure in the Li^-
photodetachment cross section near the Li(3p) threshold has been investigated.
According to an assisting calculation this structure is analogous to the
symmetrically excited intrashell (_3{0}_3^+)^1P^o state in H^-. In He^- the
three doubly excited states, 1s3s4s ^4S, 1s3p^2 ^4P and 1s3p4p ^4P were
investigated. The energy positions of all these resonances [1s3s4s ^4S:
E_0=(2386803.1+-4.2) m^-1, Gamma=160(16) m^-1; 1s3p^2 ^4P: E_0=2.4782(55)x10^6
m^-1, Gamma=40(3)x10^3 m^-1; 1s3p4p ^4P: E_0=2633297(40) m^-1, Gamma=492(35)
m^-1] agree with a recent calculation. The width of the 1s3s4s ^4S resonance
agrees with this calculation, but for the 1s3p^2 ^4P resonance there is a
slight discrepancy and the 1s3p4p ^4P resonance is only half as broad as
predicted.
