Atomic Physics with the Goddard High-Resolution Spectrograph on the Hubble Space Telescope. II. Oscillator Strengths for Singly Ionized Iron

Moderate-to-high signal-to-noise interstellar absorption line data, obtained with the Goddard High-Resolution Spectrograph aboard the Hubble Space Telescope toward β¹ Sco, HD 18100, and μ Col, are used to derive absolute oscillator strengths (f-values) for a number of weak and strong ultraviolet resonance transitions of Fe II spanning a range of line absorption strengths (i.e., the product off-value and wavelength, fλ) of over a factor 4000. Included in the list of lines seen in the spectrum of β¹ Sco are 4s ⁶D9/2-y ⁴F⁰7/2 λ1611.2 (Wλ = 12.8±1.1 mÅ) and the first known interstellar detection of the weak transition 4s ⁶D9/2-Z ⁶F⁰7/2 λ2367.6 (Wλ = 2.0±0.3 mÅ). The new f-values, which carry absolute 1 σ uncertainties of 7%-18%, are referenced to the results obtained from a detailed analysis of the moderately weak transitions 4s ⁶D₉₁₂-z ⁴D⁰₇₁₂ 2249.9 and 4s ⁶D9/2-Z ⁴F⁰9/2 λ2260.8 using the recent well-measured (±8%) laboratory f-values of Bergeson, Mullman, & Lawler (1994). For the ultraviolet transitions studied here, the large range of absorption strengths provides the opportunity for self-consistent analyses of ISM Fe II abundances over a large dynamic range (e.g., between different sight lines sampling a large range of column densities or in single sight lines with multiple and widely varying component structure). For example, application of these new f-values to the Fe II transitions observed toward β¹ Sco enables us to trace variations of the column density of Fe II with velocity to a sensitivity of better than 1 part in 10,000.