The radiative lifetime of the
21P1 and
31P1 state of atomic helium has been measured by the level-crossing technique. In the experiment, a beam of
21S0 metastable atoms was first produced and then excited to the
21P1 or
31P1 state with resonance radiation. The lifetimes were measured by observing the zero-field level-crossing signals in the detected 584- or 537-\AA{} fluorescent radiation when these
1P1 states decayed to the
11S0 ground state. The results are $\ensuremath{\tau} (2^{1}P_{1})=(5.57\ifmmode\pm\else\textpm\fi{}0.15)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec and $\ensuremath{\tau} (3^{1}P_{1})=(1.73\ifmmode\pm\else\textpm\fi{}0.11)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ sec. These values are in excellent agreement with theory.
The authors report on the two-photon excitation of 1 S 0 and 1 D 2 states in mercury by pulsed ultraviolet dye laser radiation. A time-resolved registration of two-photon-excited fluorescence provided an accurate determination of the radiative lifetimes of the levels 7 1 S 0 to 9 1 S 0 and 6 1 D 2 to 8 1 D 2 . In an external magnetic field Zeeman quantum beats and the zero-field level crossing signal have been observed. Furthermore the authors describe conditions which cause strong emission of amplified spontaneous radiation in transitions between highly excited mercury levels.