Oscillator Strengths for Ultraviolet Transitions in P II - The Multiplet at 1308 Å*

Singly-ionized phosphorus, the element's dominant ion in neutral diffuse interstellar gas, is used to probe depletion onto grains. Previous experimental results and recent theoretical calculations of the oscillator strengths and lifetimes for the P II multiplet at λ1308 reveal discrepancies. We seek a resolution to the question of the most appropriate ƒ-values to use for interstellar studies by extending our earlier work on the multiplet containing the line at 1153 Å through measurements of the pertinent data for the multiplet λ1308. Through beam-foil spectroscopic techniques, we present experimental lifetimes and branching fractions to derive oscillator strengths for transitions within the P II multiplet (3s²3p^2 3P - 3s3p^{3 3}P^o) at 1308 Å. These comprehensive beam-foil measurements, which are the most precise set currently available experimentally, resolve discrepancies involving the earlier experimental and theoretical results. Interstellar phosphorus abundances derived from λ1308 can now be interpreted with greater confidence. We also obtained an experimental lifetime for the 3p4s ³P₀^o level of P IV; it agrees well with the available theoretical calculation.