Radiative Lifetime of the2 S01Metastable State of Helium

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Abstract
A time-of-flight technique has been used to measure the radiative lifetime of the 2 S01 state of helium. After passing from a cooled source, helium atoms are excited by a pulsed anti-parallel electron beam. The atomic beam then contains not only ground-state helium atoms but also metastable atoms in both the 2 S13 and the 2 S01 states. The metastable atoms are detected at two positions, 1.9 and 6.7 m from the electron gun, by Auger ejection of an electron from a copper target, the first of which is a 60% transmitting mesh. Ground-state helium atoms cannot cause an electron to be ejected and hence are not detected. The time-of-flight distribution for the metastable 2 S01 state is separated from that of the 2 S13 state by illuminating the beam with an rf-discharge helium lamp. The 2 S01 state is quenched by resonant absorption of a 20 581 - ÅA photon, raising the atom to the 2 P11 state, which then decays preferentially to the S01 ground state; the 2 S13 state remains unaffected because it is the ground state for the triplet system. The time-of-flight distribution for the 2 S01 atoms is therefore obtained from the difference between the full beam and the quenched beam. A comparison of the number of 2 S01 metastables within a given velocity interval at the two detectors determines the number which decay in flight and yields a value for the two-photon radiative lifetime. The value of the singlet lifetime for both He3 and He4 is 19.7 ± 1.0 msec, agreeing with the theoretical value of 19.5 msec, but disagreeing with the value 38 ± 8 msec measured by Pearl using a movable detector. The 1.0-msec error is an estimate of the remaining systematic errors in the experiment.