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Astron. Astrophys. 328, 752-755 (1997)

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1. Introduction

Weak inter-system lines in low atomic number atoms and ions have received considerable attention in recent years. Inter-system refers to lines that arise from transitions between different multiplet systems so that the selection rule [FORMULA] is violated. In the limit of pure LS-coupling, such transitions are forbidden to all orders. In practice, an electric-dipole transition probability is present because of non-negligible spin-orbit interactions which mix levels of different spin. This mixing is extremely small in low-lying levels of the He atom, but slowly increases in strength with increasing atomic number (and rapidly with charge-state in an iso-electronic sequence) so that for very heavy atoms (or high charge-state ions) these lines can be very strong. One example is the resonance line of Hg I at 253.7nm ([FORMULA] - [FORMULA]).

It is the appearance of very weak inter-system lines in the lighter elements which has attracted so much attention from atomic theorists because the strength of such lines is a sensitive indicator of small deviations from the LS-coupling scheme. For this reason, the associated branching fractions, lifetimes, energy levels, and fine-structure splitting serve as challenging tests for different theoretical approximations (Ellis 1989, Martinson and Ellis 1985). Accurate experimental data are needed for these tests.

Weak inter-system lines are also used as diagnostics of astrophysical plasmas (Smith et al. 1984a) and have been observed in many astrophysical sources (See for example The Universe at Ultraviolet Wavelengths 1981 or Advances in Ultraviolet Astronomy 1982). The long-lived nature of the upper level makes the observed intensity ratio of an inter-system line to an allowed line sensitive to the collisionality of the local environment, so that the inter-system line can be used as a probe of electron density and temperature. The weak oscillator strengths of these lines makes them useful for measurements of column densities in the interstellar medium because such transitions are not easily saturated (Hobbs et al. 1982, Cowan et al. 1982). Again, accurate experimental measurements are needed for these applications.

Unfortunately, the properties that make weak inter-system lines useful in astrophysical sources, also make them very difficult to observe in the laboratory (Edlen et al. 1969). Radiative decay is so slow that collisional quenching of the excited state usually dominates. Consequently, these lines are often seen only very weakly or not at all, in the laboratory, and many of the sought-after parameters have not been measured, to any accuracy. Even when experimental data exist, discrepancies often persist. Such a situation has been noted in the CI-like iso-electronic sequence (Brage 1997).

We present measurements of the branching ratio for the [FORMULA] inter-system multiplet in singly-ionized phosphorus, at [FORMULA] nm and 219.556nm, respectively (Martin et al. 1985). The branching ratio is combined with the previously measured lifetime of the [FORMULA] level (Calamai et al. 1992) to obtain the first experimentally determined transition probabilities for this inter-system pair. This ratio, as well as data for the rest of the Si I iso-electronic sequence, has been the subject of several recent theoretical studies (Fritzche and Grant 1994, LaJohn and Luke 1993a& b, Hibbert 1993, Brage et al. 1993, Brage and Fischer 1993, Hibbert 1988, Ellis and Martinson 1984). Some pertinent data for P II are listed in Table 1.


Table 1. Some calculated and experimental data for the [FORMULA] transitions in PII. (The last line of data is produced by combining the present results with the [FORMULA] lifetime measured by Calamai.)

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© European Southern Observatory (ESO) 1997

Online publication: March 26, 1998