Photoionization of Lithium in the Inelastic Scattering Region

Abstract

The photoionization of lithium to energies between the Li 1 1s2s 3 S and 1s2p 1 P thresholds is studied with accurate ground and target state wave functions. Saddle-point full-core plus correlation wave functions are used for the closed channel. The result agrees with recent photoion measurements. Thirty nine resonances are found in the observed spectra and the predicted sharp peaks agree with experiment to meV levels. The predicted line profile agrees closely with experiment. Several earlier identifications are reassigned. Strong mixing of different resonance series is also found in a few instances. [S0031-9007(97)02417-4] PACS numbers: 31.15. Ar, 32.30.Rj, 32.80.Fb Lithium exhibits many-electron atomic properties that are not found in smaller systems. In a two-electron system, the thresholds are one-electron states with exact degeneracy in the nonrelativistic approximation. For a three-electron system, this degeneracy is no longer present. However, the two-electron thresholds could be close in energy and the Feshbach resonances associated with different thresholds may lie in the same energy region. Depending on the energy positions and the angular coupling strength of the resonances, they may or may not mix strongly. Consequently, the resulting spectra could be very complex and irregular. The lithium spectrum between the Li 1 1s2s 3 S and 1s2p 3 P thresholds is a classic example. It is unusual in that it has extended regions of strong absorption and a highly irregular resonance pattern. Ederer, Lucatorto, and Madden [1] pointed out that, "detailed theoretical calculations will be required for positive identification of the structure in this region." This mystery has remained for over a quarter of a century.The first high-resolution absorption spectrum of singly core-excited lithium is reported by Ederer et al.[1] using synchrotron radiation. In this reference, the energy positions of the ͑1s2s͒ 3 S, np 2 P 0 , ͑1s2p͒ 1 P, ns 2 P 0 , and ͑1s2p͒ 1 P, nd 2 P 0 states are measured to an accuracy of a few meV. This precision experiment has stimulated much experimental and theoretical interest [2][3][4][5][6][7]. The absolute photoabsorption cross section of lithium is measured by Mehlmam et al. [4] A comprehensive review of earlier works on lithium is given by Sonntag and Zimmermann [8].Recently, great advances have been made in the synchrotron-related experiments. In particular, Kiernan et al.[9] reinvestigated the photoabsorption of singly 1s-excited lithium using photoion measurement techniques. Impressive well-resolved spectra were obtained. Although R-matrix calculations are carried out in Kiernan et al. [9], their spectra are mainly analyzed and identified using a quantum defect method. In the energy region of interest, there should be five resonance series, ͑1s2s͒ 1 S, np, ͑1s2p͒ 3 P, ns, ͑1s2p͒ 3 P, nd, ͑1s2p͒ 1 P, ns, and ͑1s2p͒ 1 P, nd. Kiernan et al. have identified four with only two members in the ͑1s2p͒ 3 P, ns series; the ͑1s2p͒ 3 P, nd resonance series is missing. Since lithium is one of the sim...

K-shell photoionization of Be-like carbon ions: experiment and theory for C2+

“…Both experimental investigations using synchrotron radiation [22,23] and theoretical studies within the single and multiconfiguration Hartree-Fock [22,24] and the R-matrix methods [25] have been performed for the 1s 2 2s 2 1 S ground-state of the neutral Be atom. These investigations on Be-like systems clearly indicate that the most prominent decay channel is to the 1s 2 2p 2 P o excited state of the resulting ion, as confirmed by a recent theoretical study [11]: the saddle-point complex rotation method [26] illustrated, for the case of Be-like carbon, that production of the C 3+ 1s 2 2p 2 P o excited state from the decay channels of 1s2s 2 2p and 1s2s2p 2 states to 1s 2 2p + s, d, gives a branching ratio of 99.8%. This is comparable to the case of approximately 98.3% in neutral beryllium [22] and 99.4% in Be-like boron [11].…”

The use of basis splines and non-orthogonal orbitals inR-matrix calculations: application to Li photoionization

“…n max defines the active set of orbitals (all orbitals with n n max are included). The results should be compared with the 'exact' energy of −7.478 060 au, obtained by McKenzie and Drake (1991) and Chung (1991). Because the computational effort for exploring the resonance photoionization at many energy points depends crucially on the number of CSFs, and because we are not interested in extremely accurate calculations, but rather only in obtaining an estimate of the efficiency of the present approach, in the final calculations we include only CSFs with expansion coefficients greater than 0.0001.…”

Multiconfigurational Hartree-Fock close-coupling ansatz: Application to the argon photoionization cross section and delays

“…An alternative solution is to use complex scaling, where radial coordinates are rotated in the complex plane. In this case, the photoabsorption cross section can be directly computed as the imaginary part of the frequency-dependent polarizability [15][16][17]. Complex scaling methods present some advantages as well as some disadvantages with respect to scattering methods.…”