Asymmetry parameter and total cross section for the photodetachment of the metastableBe−1s2

Abstract

We report the results of calculations of the asymmetry parameter and of the cross section for the photodetachment of the metastable state Be 1s 2s2p P, for photoelectron energies above the Be 1s 2s2p P' threshold of 0.74 -2.74 eV. The calculations were based on configuration-interaction-type wave functions, where the final scattering state is obtained via the solution to all orders of multichannel reaction (K) matrices. Very good agreement is observed with the recent measurement of Pegg et al. [Phys. Rev. A 50, 3861 (1994)]. PACS number(s): 32.80.Fb In a recent publication, Pegg et al. [1]reported the results of measurements of the single-photon detachment cross section o. and of the asymmetry parameter P at photon energy A, cu=2.076 eV for the process Be 1s 2s2p P+Acu+Be 1s 2s2p Po+ configuration-interaction (CI) methods with appropriate boundary conditions. The two quantities tr(s) and P(c) were computed from the definitions of Dill and Fano [10] (for reviews on the definition and calculation of photoionization observables see Refs. [11] -[13]): (ss and sd), (1)~( e) = 2~(it), (2) where a = 1.815 eV, according to the experimental value for the detachment threshold [2]. The o was measured to be 31 3 Mb, in serious disagreement with the only previously reported measurement, by Bae and Peterson [3], who found o.=9~5 Mb. The P was measured to be 0.49~0.02. No other measurements or theoretical results exist for these properties. The observed discrepancy between the two measurements and the lack of any theoretical predictions on the cross section and angular distribution of process (1) have led us to undertaking the calculation of o. (s) and P(s) in the photon energy range 1.0 -3.0 eV. This range includes the one previously examined [3] (1.65 -2.34), and corresponds to photoelectron energies above the Be 1s 2s2p P' threshold of 0.74 -2.74 eV. The Be ls 2s 2p P state belongs tp the class of excited states of atomic negative ions, which are bound. (For a recent review see Ref. [4]. ) As with many other such states,in the Hartree-Fock (HF) approximation this state is unbound. However, the fact that symmetry does not allow any hole-filling pair correlations (which would cause decay), and that the correlation energy of the three Be L-shell electrons (2s2p P) is larger than that of the Be (2s2p P') pair, turns the Be 1s 2s2p P state into a discrete one [5,6]. Its first observation and identification were reported in [3,7]. Its existence was also verified from the observation [8] of the Be 1s 2p 5 -1s 2s2p P transition, whpse, possibility, wavelength, and probability had been predicted theoreticallyThe present calculations were carried out in the framework of the state-specific theory of photoionization processes [9], which uses separately optimized wave functions that are expansions over symmetry-adapted bound and scattering configurations, whose coefficients are obtained by 1 P(s)=, 2~(it)P(i ),