Excitation Cross Sections for Li-like Ions of Beryllium and Boron

Abstract

We report on calculation of electron-impact excitation cross sections for Li-like ions of boron and beryllium. The data were produced with a number of modern methods in atomic collision theory, such as convergent close-coupling, K-matrix and Coulomb-Born-exchange. The results obtained are compared with other calculations and available expermental data, and the recommended cross sections for all transitions between atomic terms with principal quantum numbers n 4 are presented as tables of fitting parameters.Due to the importance of beryllium and boron for fusion and astrophysics, the collisional data for ions of these elements have often been a subject of research, including calculations of excitation cross sections. The most recent compilation of the recommended data for electron-impact excitation of the Be qþ and B qþ ions [1] was principally based on the R-matrix (RM) and distorted-wave (DW) methods, and included transitions from the ground into the n ¼ 2 and n ¼ 3 excited states for all ions. It was found later [2,3] that the accuracy of the calculated RM and DW cross sections, which were used for deriving the recommended data sets for the Li-like ions of beryllium and boron, may be inferior to that of the recently developed methods, such as the convergent close-coupling (CCC) and the R-matrix with pseudostates (RMPS). The disagreement was found to be especially noticeable for the intermediate energy region, where the coupling with the continuum states is important, and hence the applicability of the recommended data [1] may be limited. In addition, as has been noted above, transitions between excited states were not considered in Ref. [1]. It has to be mentioned that the cross sections for the transitions between excited states can only rarely be found in the literature, although they may be very important for applications. This can be exemplified by the experiment on laser pumping of specific atomic levels [4], where such transitions crucially affect the measured light emission. Thus, it seems appropriate to produce a new set of highaccuracy excitation data for the Li-like ions of boron and beryllium, including transitions between excited states.The non-perturbative close-coupling (CC) type methods, which are known to provide the highest accuracy among the existing theoretical approaches, normally require significant computational efforts, in terms of both hardware requirements and computation times. This justifies development of simpler and faster, yet sufficiently accurate, methods which could be of practical use with widely available computational tools. As for the highly-charged H-and Li-like ions, it was already shown [5,6] that the first-order Coulomb-Bornexchange method (CBE) with normalization on one channel can be reliably applied to the calculation of the excitation cross sections for ions with a large spectroscopic charge (Z ! 3). For the low-charge ions, however, the channel interaction effects, which are ignored within the CBE approximation, become more important and thus require applic...

Electron-impact broadening of the 3s–3p lines in low-Z Li-like ions

“…For higher-Z ions (Z ≥ 4), a detailed comparison of all excitation cross sections between the states with the principal quantum number n ≤ 4 has shown that the CCC data practically coincide with the perturbative Coulomb-Born-exchange calculations [18]. Furthermore, a recent paper on electron-impact excitation in Be II and B III (also including the transitions between the excited states up to n ≤ 4) again demonstrated a high level of agreement between the CCC and the K-matrix cross sections [17]. Thus, the available data comparisons indicate that the CCC method seems to provide high accuracy for the excitation cross sections of Li-like ions and, correspondingly, for the inelastic contributions to the collisional line broadening.…”

“…Indeed, it was confirmed in a number of publications that the CCC excitation cross sections agree well with both the experimental data and other theoretical non-perturbative results for the ground state excitations. Such comparisons were done, e.g., for the R-matrix with pseudostates and K-matrix method results [15,16,17]. For higher-Z ions (Z ≥ 4), a detailed comparison of all excitation cross sections between the states with the principal quantum number n ≤ 4 has shown that the CCC data practically coincide with the perturbative Coulomb-Born-exchange calculations [18].…”

Atomic data and collisional–radiative model for beryllium and its ions

“…The energies of the levels and (if available) the oscillator strengths were taken from the NIST database. A new improved set of CCC excitation and ionization cross sections for neutral beryllium as well as CCC data [14] for Be + were used. For selected transitions in Be, Be + and for ions Be 2+ , Be 3+ the cross sections were computed by the ATOM code [6] (the K-matrix for excitation and the normalized Coulomb-Born-exchange for ionization).…”