A time-of-flight technique has been used to measure the radiative lifetime of the $2 ^{1}S_{0}$ 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 $2 ^{3}S_{1}$ and the $2 ^{1}S_{0}$ 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 $2 ^{1}S_{0}$ state is separated from that of the $2 ^{3}S_{1}$ state by illuminating the beam with an rf-discharge helium lamp. The $2 ^{1}S_{0}$ state is quenched by resonant absorption of a 20 581 - \AA{}A photon, raising the atom to the $2 ^{1}P_{1}$ state, which then decays preferentially to the $^{1}S_{0}$ ground state; the $2 ^{3}S_{1}$ state remains unaffected because it is the ground state for the triplet system. The time-of-flight distribution for the $2 ^{1}S_{0}$ atoms is therefore obtained from the difference between the full beam and the quenched beam. A comparison of the number of $2 ^{1}S_{0}$ 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 ${\mathrm{He}}^{3}$ and ${\mathrm{He}}^{4}$ is 19.7 \ifmmode\pm\else\textpm\fi{} 1.0 msec, agreeing with the theoretical value of 19.5 msec, but disagreeing with the value 38 \ifmmode\pm\else\textpm\fi{} 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.A time-of-flight technique has been used to measure the radiative lifetime of the $2 ^{1}S_{0}$ 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 $2 ^{3}S_{1}$ and the $2 ^{1}S_{0}$ 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 $2 ^{1}S_{0}$ state is separated from that of the $2 ^{3}S_{1}$ state by illuminating the beam with an rf-discharge helium lamp. The $2 ^{1}S_{0}$ state is quenched by resonant absorption of a 20 581 - \AA{}A photon, raising the atom to the $2 ^{1}P_{1}$ state, which then decays preferentially to the $^{1}S_{0}$ ground state; the $2 ^{3}S_{1}$ state remains unaffected because it is the ground state for the triplet system. The time-of-flight distribution for the $2 ^{1}S_{0}$ atoms is therefore obtained from the difference between the full beam and the quenched beam. A comparison of the number of $2 ^{1}S_{0}$ 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 ${\mathrm{He}}^{3}$ and ${\mathrm{He}}^{4}$ is 19.7 \ifmmode\pm\else\textpm\fi{} 1.0 msec, agreeing with the theoretical value of 19.5 msec, but disagreeing with the value 38 \ifmmode\pm\else\textpm\fi{} 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.

Physical Review AOct 1, 197199 citations

A time-of-flight technique has been used to measure the radiative lifetime of the

2 ^{1}S_{0}

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 ^{3}S_{1}

and the

2 ^{1}S_{0}

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 ^{1}S_{0}

state is separated from that of the

2 ^{3}S_{1}

state by illuminating the beam with an rf-discharge helium lamp. The

2 ^{1}S_{0}

state is quenched by resonant absorption of a 20 581 - \AA{}A photon, raising the atom to the

2 ^{1}P_{1}

state, which then decays preferentially to the

^{1}S_{0}

ground state; the

2 ^{3}S_{1}

state remains unaffected because it is the ground state for the triplet system. The time-of-flight distribution for the

2 ^{1}S_{0}

atoms is therefore obtained from the difference between the full beam and the quenched beam. A comparison of the number of

2 ^{1}S_{0}

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

{\mathrm{He}}^{3}

and

{\mathrm{He}}^{4}

is 19.7 \ifmmode\pm\else\textpm\fi{} 1.0 msec, agreeing with the theoretical value of 19.5 msec, but disagreeing with the value 38 \ifmmode\pm\else\textpm\fi{} 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.