Experimental electron-impact excitation rate coefficients for beryllium-like Si XI

Absolute excitation rate coefficients for a large number of levels (up to n = 4) of the singlet and triplet system of Si XI have been experimentally determined in a well-diagnosed theta-pinch plasma with the plasma spectroscopic method. The population of the metastable triplet ground state has been derived from the intensity of the intercombination line. A theoretical population model has been developed to investigate whether collisional coupling of the levels and cascading affected the measurements. The model has also been used to calculate line intensity ratios of strongly density or temperature dependent line pairs which can be used for plasma diagnostic purposes. The measured effective excitation rate coefficients are accurate to about +/-50%. All of them, apart from the value for the 2s4s S-3(1) level, agreed within the error margins with existing theoretical data.