Photodetachment partial cross sections

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. 5 Yield of He ions vs. photon energy. The He" signal is proportional to the partial cross section for the photodetachment of He via the 2 Sks,d channel (Fig. 5a) and the 2 Pkp channel (Fig. 5b). The solid line represents a fit of the Fano function (Eq. 1) to the experimental data (dots).

Fig. 7. Yield of Li ions vs. photon energy in the ranges 5.04-5.16 eV (Fig. 7a) and 5.39-5.46 cV (Fig. 7b). The Li" " signal is proportional to the partial cross section for photodetachment of Li via the 3 Skp channel see Fig. 6). The experimental data (dots) has been normalized to theory (solid line with scale at the right). The thick solid lines indicate fits of the double Fano formula (Eq. 3) to the data.

Fig. 9 Yield of Li ions vs. photon energy in the region of the 2 P threshold. The Li signal is proportional to the partial cross section for photodetachement of Li via the 2 Pks channel (See Fig. 1). The solid line is a fit of the Wigner law (Eq. 4) to the data in the range shown. The error bars on selected data points represent the shot noise.

In Fig. 8 we show an extended 3 Skp partial photodetachment cross section which includes measurements below the LiCS S) threshold in addition to the data already shown in Fig. 7b. The data in the range 5.29-5.39 eV represents a relatively low statistics survey scan. It is normalized to the calculation of Pan et a/. [28] using the same factor as for the data between 5.39-5.46 eV. It is clear, even from this relatively low quality data, that the two resonances labeled f and g are observed at approximately the calculated energies but their measured strengths appear to be weaker than predicted. Presumably, the lowest lying resonance, labeled f, is the intrashell resonance representing symmetric excitation of the two valence electrons. 

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