A time-of-flight technique has been used to measure the radiative lifetime of the
21S0 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
23S1 and the
21S0 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
21S0 state is separated from that of the
23S1 state by illuminating the beam with an rf-discharge helium lamp. The
21S0 state is quenched by resonant absorption of a 20 581 - \AA{}A photon, raising the atom to the
21P1 state, which then decays preferentially to the
1S0 ground state; the
23S1 state remains unaffected because it is the ground state for the triplet system. The time-of-flight distribution for the
21S0 atoms is therefore obtained from the difference between the full beam and the quenched beam. A comparison of the number of
21S0 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 \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.
We present the first experimental determination of the
2 3P1−−1 1S0 transition rate in helium and compare this measurement with theoretical quantum-electrodynamic predictions. The experiment exploits the very long ($\ensuremath{\sim}1\text{ }\text{ }\mathrm{\text{minute}}$) confinement times obtained for atoms magneto-optically trapped in an apparatus used to create a Bose-Einstein condensate of metastable (
2 3S1) helium. The
2 3P1−−1 1S0 transition rate is measured directly from the decay rate of the cold atomic cloud following 1083 nm laser excitation from the
2 3S1 to the
2 3P1 state, and from accurate knowledge of the
2 3P1 population. The value obtained is $177\ifmmode\pm\else\textpm\fi{}8\text{ }\text{ }{\mathrm{s}}^{\ensuremath{-}1}$, which agrees very well with theoretical predictions, and has an accuracy that compares favorably with measurements for the same transition in heliumlike ions higher in the isoelectronic sequence.
Exited atoms may relax to the ground state by radiative decay, a process which is usually very fast (of order nanoseconds). However, quantum-mechanical selection rules can prevent such rapid decay, in which case these "metastable" states can have lifetimes of order seconds or longer. In this Letter, we determine experimentally the lifetime of the longest-lived neutral atomic state-the first excited state of helium (the 2(3)S1 metastable state)-to the highest accuracy yet measured. We use laser cooling and magnetic trapping to isolate a cloud of metastable helium (He*) atoms from their surrounding environment, and measure the decay rate to the ground 1(1)S0 state via extreme ultraviolet (XUV) photon emission. This is the first measurement using a virtually unperturbed ensemble of isolated helium atoms, and yields a value of 7870(510) seconds, in excellent agreement with the predictions of quantum electrodynamic theory.
The radiative lifetime of
1s2s3S1 Li+ ions has been measured by a comparison of the count rate of spontaneous decay photons (210 \AA{}) to a laser-induced fluorescence signal proportional to the total number of metastable ions. The ions were confined in a rf-quadrupole trap, and the fluorescence signal used was the intercombination transition
1s2p3P1 to $1{s}^{2}^{1}S_{0}$ at 202 \AA{}. The
3P1 state was populated from
3S1 by absorption of laser photons at 5485 \AA{}. The result is ${\ensuremath{\tau}}_{r}=58.6\ifmmode\pm\else\textpm\fi{}12.9$ sec (95% confidence level); theory predicts ${\ensuremath{\tau}}_{r}=49.1$ sec.
The cross section for the transfer of excitation from the
23P0 level of helium to the
23P1 and
23P2 levels as a result of collisions with the ground-state helium atom (
11S0) has been measured with a conventional resonance-fluorescence technique. A
He4 source excited the $2^{3}S_{1}\ensuremath{-}2^{3}P_{0}$ transition in a
He3 absorption cell. The scattered light was examined with a high-resolution grating spectrometer and the ratio of the $2^{3}P_{0}\ensuremath{\rightarrow}2^{3}S_{1}$ to the $2^{3}P_{1,2}\ensuremath{\rightarrow}2^{3}S_{1}$ intensities was measured as a function of cell pressure from 0.05 to 10 Torr. The cross section for collision-induced transitions out of the
23P0 level is found to be (68\ifmmode\pm\else\textpm\fi{}3) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$
cm2 at room temperature.