Lifetime Measurements in Si II, Si III, and Si IV

We have measured radiative decay times in Si II, Si III, and Si IV in the wavelength region 700-6 000 Å using the beam-foil technique. The lifetimes and transition probabilities have been evaluated by alternative methods of curve-fitting and cascade analysis. These results are compared with theoretical transition probabilities, and values in other members of the isoelectronic sequences. A recently introduced cascade analysis technique is shown to extract reliable lifetimes from heavily cascaded decay curves which are unresolvable by normal curve-fitting techniques. It can also yield information on the relative populations of the excited levels produced in the foil interaction. The present estimates of silicon in astrophysical objects are not changed by our measurements of transition probabilities in Si II and Si III. The solar photospheric and coronal abundance estimates of silicon relative to hydrogen thus still differ by a factor of three. We have measured the transition probabilities of most of the silicon lines observed in the red-shifted quasar spectra.