We report resolution of the 1% discrepancy between theory and experiment for the lifetime of the first excited state of Na. Determining the atomic lifetime by a novel method, we have measured the natural linewidth of the $3{}^{2}{S}_{1/2}\ensuremath{\rightarrow}3{}^{2}{P}_{3/2}$ transition with
<0.25 accuracy through precision spectroscopy on an optically prepared sample of ultracold, two-level atoms. The resulting
32P3/2 lifetime of 16.237(35) ns disagrees with previous time-domain measurements but is in good agreement with recent calculations and a new fast beam result.
The oscillator strength for the resonance ($1^{1}S\ensuremath{-}2^{1}P$) transition in atomic helium has been determined from a measurement of the lifetime of the
21P state. This lifetime was obtained using a modified zero-field level crossing (Hanle) method. The scattering sample was a thermal velocity beam of
21S metastable atoms. Resonant 2-\ensuremath{\mu} ($2^{1}S\ensuremath{-}2^{1}P$) radiation is absorbed by the
21S atoms, which are excited to the
21P state. This state then primarily decays to the ground
11S state with the emission of resonance (584 \AA{}) radiation. The lifetime of the
21P state was determined from measurements of the magnetic-field dependence of the angular distribution of the 584 \AA{} intensity. This new technique eliminates the problem of constructing a vacuum ultraviolet resonance lamp. The measured lifetime $\ensuremath{\tau} (2^{1}P)$ is (5.63 \ifmmode\pm\else\textpm\fi{} 0.22) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ sec. The resulting absorption oscillator strength for the resonance transition is 0.273 \ifmmode\pm\else\textpm\fi{} 0.011. This oscillator strength agrees with the theoretical value.