Precision lifetime measurements on alkali atoms and on helium by beam–gas–laser spectroscopy

The results of a new series of precision lifetime measurements on alkali atoms by means of beam–gas–laser spectroscopy are reported and discussed. The lifetimes of the first excited states in lithium, sodium, potassium and rubidium were measured with uncertainties ranging from ±0.14% to ±0.25%. The results are discussed in comparison with other recent experimental and theoretical work. The longstanding discrepancy between ab initio line strength calculations and the measurements of Gaupp et al. Phys. Rev. A26, 3351 (1982) for the resonance transitions in lithium and sodium is resolved by the new measurements in favour of the theoretical calculations. Finally the role of the beam divergence in lifetime measurements on fast particle beams is discussed and a method to correct for this effect is presented. The importance of the beam divergence effect is illustrated by additional recent lifetime measurements on helium and on lithium.