TRANSITION-PROBABILITIES FOR THE 3S(2)3P(P-2(0))-3S3P(2)(P-4) INTERSYSTEM LINES OF SI-II

Intensity ratios of lines of the spin-changing ''intersystem'' multipl et of Si II (4P --> 2P(o)) at 234 nm have been used to determine elect ron densities and temperatures in a variety of astrophysical environme nts. However, the accuracy of these diagnostic calculations have been limited by uncertainties associated with the available atomic data. We report the first laboratory measurement, using an ion-trapping techni que, of the radiative lifetimes of the three metastable levels of the 3s3p2 4P term of Si II. Our results are 104 +/- 16, 406 +/- 33, and 81 1 +/- 77 mus for lifetimes of the J = 1/2, 5/2, and 3/2 levels, respec tively. A-values were derived from our life-times by use of measured b ranching fractions. Our A-values, which differ from calculated values by 30% or more, should give better agreement between modeled and obser ved Si II line ratios.