Photodetachment doubly excited states

We present the results of experimental studies of photon-negative ion interactions involving the dynamics of two electrons. Resonances associated with doubly excited states of Li and He" have been observed using laser photodetachment spectroscopy. Total and partial photodetachment cross sections have been investigated. In the former case, the residual atoms are detected irrespective of their excitation state, while in the latter case only those atoms in specific states are detected. This was achieved by the use of a state selective detection scheme based on the resonant ionization of the residual atoms. In addition, in the case of Li-photodetachment, the threshold behavior of the Li(2 P)+e-(ks) partial cross section has been used to accurately measure the electron affinity of Li. 

The total photodetachment cross section describes the probability that an electron is detached from a negative ion following the absorption of a photon, regardless the excitation state of the residual atom or the energy or direction of the emitted electron. A total cross section is the sum of partial cross sections for detachment into each of the energetically allowed continua. This is illustrated in Fig. 1. Here we show the three possible channels accessible to a doubly excited state of Li" that lies just below the Li(32P) detachment threshold. The total cross section may be determined by... 

Fig. 1 Partial energy level diagram for the Li /Li systems showing the autodetaching decay of a doubly excited state of Li into different continua. Each of the three continuum channel is characterized by the state that the residual Li atom is left in following photodetachment via the doubly excited state. A particular decay channel is isolated by state selectively detecting the excited residual atom.

The small dip at 4.456 eV is a Wigner cusp that arises from the opening of the Li(32p)+e (ks) photodetachment channel (hereafter labeled 32Pks, for brevity). The other two structures are resonances associated with the decay of doubly excited states. These resonances overlap one another as well as the Wigner cusp, making it... 

Doubly excited states of He of doublet symmetry have been observed in studies of electron impact on He [23]. In contrast, data on quartet states are sparse. Selection mles on photoexcitation from the P° ground state limit excited state production to those of S, P and symmetry. Recently, the He photodetachment cross section... 

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