A time-of-flight technique has been used to measure the quenching of the
21S0 metastable state of helium by a static electric field. Neutral ground-state helium atoms effusing from a cooled source slit are immediately excited to the
21S0 and
23S1 metastable states by a pulse of antiparallel magnetically focused electrons. The metastable beam is collimated before passing through a uniform electric field region 0.5 m long and is then preferentially detected at the end of the time-of-flight region, 1.825 m from the electron gun. The time-of-flight distribution for the
21S0 state is separated from that of the
23S1 state by illuminating the beam before the electric field region with an rf-discharge helium lamp. The
21S0 state is quenched by resonant absorption of a 20 581-\AA{} photon, raising the atom to the
21P1 state, which then decays to the
1S0 ground state; the
23S1 state remains unaffected because it is the ground state for the triplet system. The
21S0 time-of-flight distribution is therefore obtained from the difference between the full beam and the quenched beam. The number of
21S0 atoms arriving at the detector in specific velocity intervals with the electric field off is compared to the number with the field on to determine the quenching rate (
=kE2). The result for the quenching constant
k for both
He3 and
He4 with
E in kV/cm is $k=0.933\ifmmode\pm\else\textpm\fi{}0.005$; this value is in good agreement with theory and with an earlier less accurate experiment. The error in the present experiment arises from a 0.5% uncertainty in the effective length of the electric field region.
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.
The atomic beam $E\ensuremath{-}H$ gradient-balance method has been employed to measure the
z-component ${\ensuremath{\alpha}}_{z}({m}_{J})$ of the electric polarizability tensors of the
mJ=+2 and
mJ=+1 magnetic substates of
3P2 metastable argon. The measurements were normalized to the polarizability of
3S1 metastable helium, which is calculable to within an accuracy of 1%. In this method, a metastable beam is passed through a region containing transverse, congruent, inhomogeneous electric and magnetic fields. The condition that atoms in a particular magnetic substate experience no deflection in such a field region is ${\ensuremath{\alpha}}_{z}({m}_{J})E(\frac{\ensuremath{\delta}E}{\ensuremath{\delta}z})={\ensuremath{\mu}}_{\mathrm{eff}}(\frac{\ensuremath{\delta}H}{\ensuremath{\delta}z})$, where ${\ensuremath{\mu}}_{\mathrm{eff}}$ is the magnetic moment in the field
H, and $\frac{\ensuremath{\delta}E}{\ensuremath{\delta}z}$, $\frac{\ensuremath{\delta}H}{\ensuremath{\delta}z}$ are the
z components of the gradients of the magnitudes of the electric and magnetic fields, respectively. Because of the congruency of the
E and
H fields $\frac{(\frac{\ensuremath{\delta}E}{\ensuremath{\delta}z})}{E}=\frac{(\frac{\ensuremath{\delta}H}{\ensuremath{\delta}z})}{H}$, and therefore ${\ensuremath{\alpha}}_{z}({m}_{J})=\frac{{\ensuremath{\mu}}_{\mathrm{eff}}H}{{E}^{2}}$. Because of the normalization, it was unnecessary to find either
E or
H. The results, in units of ${10}^{\ensuremath{-}24}$
cm3, are ${\ensuremath{\alpha}}_{z}(\ifmmode\pm\else\textpm\fi{}1)=50.4\ifmmode\pm\else\textpm\fi{}3.5$ and ${\ensuremath{\alpha}}_{z}(\ifmmode\pm\else\textpm\fi{}2)=44.5\ifmmode\pm\else\textpm\fi{}3.1$. The measured values of ${\ensuremath{\alpha}}_{z}(+1)$ and ${\ensuremath{\alpha}}_{z}(\ifmmode\pm\else\textpm\fi{}2)$ can be used to determine the polarizability tensors of each of the magnetic substates. The other components of the diagonalized tensors, with ${\ensuremath{\alpha}}_{x}({m}_{J})={\ensuremath{\alpha}}_{y}({m}_{J})$, in units of ${10}^{\ensuremath{-}24}$
cm3 are ${\ensuremath{\alpha}}_{x}(0)=46.4\ifmmode\pm\else\textpm\fi{}2.4$, ${\ensuremath{\alpha}}_{x}(\ifmmode\pm\else\textpm\fi{}1)=47.2\ifmmode\pm\else\textpm\fi{}2.3$, ${\ensuremath{\alpha}}_{x}(\ifmmode\pm\else\textpm\fi{}2)=50.4\ifmmode\pm\else\textpm\fi{}3.5$, and ${\ensuremath{\alpha}}_{z}(0)=52.4\ifmmode\pm\else\textpm\fi{}4.8$.
The electric dipole polarizability of the alkali-metal atoms Na, K, Rb, and Cs are determined by measuring the deflection of a velocity-selected beam by an inhomogeneous electric field. It is shown experimentally that scattering of slower beam atoms by background gas in the apparatus is an important systematic error in measurements of this kind if a beam-velocity selector is not used, and that this scattering is responsible for the discrepancies between different, earlier beam-deflection measurements of polarizability. The polarizability values obtained in this work are $\ensuremath{\alpha}(\mathrm{Na})=24.4\ifmmode\pm\else\textpm\fi{}1.7$
A˚3, $\ensuremath{\alpha}(\mathrm{K})=45.2\ifmmode\pm\else\textpm\fi{}3.2$
A˚3, $\ensuremath{\alpha}(\mathrm{Rb})=48.7\ifmmode\pm\else\textpm\fi{}3.4$
A˚3, and $\ensuremath{\alpha}(\mathrm{Cs})=63.3\ifmmode\pm\else\textpm\fi{}4.6$
A˚3.
The static electric dipole polarizability of the
4s2(1S0) ground state of the calcium atom has been measured using an electric deflection technique. The result is normalized to the polarizability of lithium by comparing deflections of calcium and lithium beams passing through an inhomogeneous electrostatic field. We find (25 \ifmmode\pm\else\textpm\fi{} 2.5) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}24}$
cm3 for the polarizability of calcium.
The static electric dipole polarizabilities of barium and strontium atoms have been measured using an electric deflection technique. The results are normalized to the polarizability of lithium by comparing deflections of the alkaline-earth beams to deflections of a lithium beam passing through an inhomogeneous electrostatic field. The measured polarizabilities are (39.7 \ifmmode\pm\else\textpm\fi{} 3.2) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}24}$
cm3 for barium and (27.6 \ifmmode\pm\else\textpm\fi{} 2.2) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}24}$
cm3 for strontium.