Fast-beam laser spectroscopy of helium-like silicon and hydrogen-like nitrogen-towards improved precision

Using Doppler-tuned fast-beam laser spectroscopy and a high finesse build-up cavity excited by a 1319 nm Nd:YAG laser we previously measured the inter-combination 1$s$2$s \quad ^{1}$S$_{0}$-1$s$2$p \quad ^{3}$P$_{1}$ interval in Si$^{12+}$ to be 7230.5(2) cm$^{-1 }$[1]. The precision was limited by uncertainty in the velocity of the \textit{$\beta $ }$\sim $ 5{\%} ion beam. An order of magnitude higher precision would provide a clear test of calculations of QED contributions in two-electron ions. We aim to attain this by employing co- and counter-propagating beams and a dual wavelength high-finesse cavity. Work towards developing the necessary 1450 nm narrow-band laser will be presented. Work is also in progress on an improved measurement of the 2S$_{1/2}$-2P$_{3/2}$ fine structure - Lamb shift transition in N$^{6+}$[2]. Our aim is to test QED calculations relevant to the interpretation of high-precision spectroscopy of atomic hydrogen. Our new set-up uses two $^{13}$CO$_{2}$ lasers and a 5$^{o}$ interaction geometry. [1] M. Redshaw and E.G. Myers, PRL \textbf{88 }023002 (2002) [2] E.G. Myers and M. R. Tarbutt, in \textit{Hydrogen Atom, }edited by S.G. Karshenboim et al., Springer 2002.