2003A&A...397.1143H

We report on experimental transition probabilities for thirteen forbidden [Fe II] lines originating from three different metastable FeII levels. Radiative lifetimes have been measured of two metastable states by applying a laser probing technique on a stored ion beam. Branching ratios for the radiative decay channels, i.e. M1 and E2 transitions, are derived from observed intensity ratios of forbidden lines in astrophysical spectra and compared with theoretical data. The lifetimes and branching ratios are combined to derive absolute transition probabilities, A-values. We present the first experimental lifetime values for the two Fe II levels a⁴G_9/2 and b²H_11/2 and A-values for 13 forbidden transitions from a⁶S_5/2, a⁴G_9/2 and b⁴D_7/2 in the optical region. A discrepancy between the measured and calculated values of the lifetime for the b²H_11/2 level is discussed in terms of level mixing. We have used the code CIV3 to calculate transition probabilities of the a⁶D-a⁶S transitions. We have also studied observational branching ratios for lines from 5 other metastable Fe II levels and compared them to calculated values. A consistency in the deviation between calibrated observational intensity ratios and theoretical branching ratios for lines in a wider wavelength region supports the use of [Fe II] lines for determination of reddening.