Improved Measurement of the 1s2s $^{1}$S$_{0}$ -- 1s2p $^{3}$P$_{1}$ Interval in Helium-like Silicon*

Using co-linear fast-beam laser spectroscopy with co- and counter-propagating beams we have measured the 1s2s $^{1}$S$_{0}$ -- 1s2p $^{3}$P$_{1}$ intercombination interval in Si$^{12+}$ with the result 7230.585(6) cm$^{-1}$. The experiment made use of a dual-wavelength, high-finesse, power build-up cavity excited by single-frequency lasers at 1319 nm and 1450 nm. The result is a factor of 30 more precise than previous work and will provide a precision test of relativistic atomic theory.