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Physical Review

Lifetime of the 2S State of Atomic Hydrogen

Wade L. Fite, R. T. Brackmann, David G. Hummer, R. F. Stebbings

Published Dec 15, 1961• 11 citations
Publisher

Abstract

A sensitive test for any mixing of the $2{S}_{\frac{1}{2}}$ and $2{P}_{\frac{1}{2}}$ state of atomic hydrogen is the measurement of the rate for single-quantum decay of the $2S$ atom to the ground state. A new upper limit of this decay rate has been determined. A section along a beam of $2S$ atoms, produced by electron excitation of a ground-state atom beam, was viewed by an iodine-vapor-filled ultraviolet photon counter, which responds to the Lymanalpha radiation of the single-quantum decay process. From the counts observed when an electrostatic quenching field was superposed on the counter's field of view, the necessary experimental parameter (product of $2S$ atom current and counter efficiency) was determined. With the field removed, a portion of the remaining counts could be ascribed to quenching on collision of the $2S$ atoms with residual gases in the vacuum chamber, the quenching cross sections for which were measured. The decay rate not ascribable to known quenching effects was 420 ${\mathrm{sec}}^{\ensuremath{-}1}$. Since unknown quenching effects may have been operative, this figure must be considered only as an upper limit for the natural single-quantum decay rate.

DOI
10.1103/physrev.124.2051.2

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Lifetime of the 2S State of Atomic Hydrogen

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A sensitive test for any mixing of the $2{S}_{\frac{1}{2}}$ and $2{P}_{\frac{1}{2}}$ state of atomic hydrogen is the measurement of the rate for single-quantum decay of the $2S$ atom to the ground state. A new upper limit of this decay rate has been determined. A section along a beam of $2S$ atoms, produced by electron excitation of a ground-state atom beam, was viewed by an iodine-vapor-filled ultraviolet photon counter, which responds to the Lymanalpha radiation of the single-quantum decay process. From the counts observed when an electrostatic quenching field was superposed on the counter's field of view, the necessary experimental parameter (product of $2S$ atom current and counter efficiency) was determined. With the field removed, a portion of the remaining counts could be ascribed to quenching on collision of the $2S$ atoms with residual gases in the vacuum chamber, the quenching cross sections for which were measured. The decay rate not ascribable to known quenching effects was 420 ${\mathrm{sec}}^{\ensuremath{-}1}$. Since unknown quenching effects may have been operative, this figure must be considered only as an upper limit for the natural single-quantum decay rate.

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Inelastic collision rates of trapped metastable hydrogen

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We report the first detailed decay studies of trapped metastable $(2S)$ hydrogen. By two-photon excitation of ultracold H samples, we have produced clouds of at least $5\ifmmode\times\else\texttimes\fi{}{10}^{7}$ magnetically trapped $2S$ atoms at densities greater than $4\ifmmode\times\else\texttimes\fi{}{10}^{10}{\mathrm{cm}}^{\ensuremath{-}3}$ and temperatures below $100\ensuremath{\mu}\mathrm{K}.$ At these densities and temperatures, two-body inelastic collisions of metastable atoms are evident. Experimental values for the total two-body loss rate constant are ${K}_{2}{=1.8}_{\ensuremath{-}0.7}^{+1.8}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}{\mathrm{s}}^{\ensuremath{-}1}$ at $87\ensuremath{\mu}\mathrm{K}$ and ${K}_{2}{=1.0}_{\ensuremath{-}0.5}^{+0.9}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}{\mathrm{s}}^{\ensuremath{-}1}$ at $230\ensuremath{\mu}\mathrm{K}.$ These results are in the range of recent theoretical calculations for the total $2S\ensuremath{-}2S$ inelastic rate constant. The metastable clouds were excited in a gas of ground-state $(1S)$ hydrogen with peak densities reaching $7\ifmmode\times\else\texttimes\fi{}{10}^{13}{\mathrm{cm}}^{\ensuremath{-}3}.$ From the one-body component of the metastable decay, we derive experimental upper limits for ${K}_{12},$ the rate constant for loss due to inelastic $1S\ensuremath{-}2S$ collisions.

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Radiative Lifetime of the 2 S01 Metastable State of Helium

Robert S. Van Dyck, C. E. Johnson, Howard A. Shugart

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 A•Oct 1, 1971•99 citations
A sensitive test for any mixing of the 2S122{S}_{\frac{1}{2}}2S21​​ and 2P122{P}_{\frac{1}{2}}2P21​​ state of atomic hydrogen is the measurement of the rate for single-quantum decay of the 2S2S2S atom to the ground state. A new upper limit of this decay rate has been determined. A section along a beam of 2S2S2S atoms, produced by electron excitation of a ground-state atom beam, was viewed by an iodine-vapor-filled ultraviolet photon counter, which responds to the Lymanalpha radiation of the single-quantum decay process. From the counts observed when an electrostatic quenching field was superposed on the counter's field of view, the necessary experimental parameter (product of 2S2S2S atom current and counter efficiency) was determined. With the field removed, a portion of the remaining counts could be ascribed to quenching on collision of the 2S2S2S atoms with residual gases in the vacuum chamber, the quenching cross sections for which were measured. The decay rate not ascribable to known quenching effects was 420 sec\ensuremath−1{\mathrm{sec}}^{\ensuremath{-}1}sec\ensuremath−1. Since unknown quenching effects may have been operative, this figure must be considered only as an upper limit for the natural single-quantum decay rate.
A sensitive test for any mixing of the 2S122{S}_{\frac{1}{2}}2S21​​ and 2P122{P}_{\frac{1}{2}}2P21​​ state of atomic hydrogen is the measurement of the rate for single-quantum decay of the 2S2S2S atom to the ground state. A new upper limit of this decay rate has been determined. A section along a beam of 2S2S2S atoms, produced by electron excitation of a ground-state atom beam, was viewed by an iodine-vapor-filled ultraviolet photon counter, which responds to the Lymanalpha radiation of the single-quantum decay process. From the counts observed when an electrostatic quenching field was superposed on the counter's field of view, the necessary experimental parameter (product of 2S2S2S atom current and counter efficiency) was determined. With the field removed, a portion of the remaining counts could be ascribed to quenching on collision of the 2S2S2S atoms with residual gases in the vacuum chamber, the quenching cross sections for which were measured. The decay rate not ascribable to known quenching effects was 420 ${\mathrm{sec}}^{\ensuremath{-}1}$. Since unknown quenching effects may have been operative, this figure must be considered only as an upper limit for the natural single-quantum decay rate.
We have observed the crossing of the atomic hydrogen levels ${\ensuremath{\beta}}_{B}(2{S}_{\frac{1}{2}}, {m}_{J}=\ensuremath{-}\frac{1}{2}, {m}_{I}=\ensuremath{-}\frac{1}{2})$ and ${e}_{B}(2{P}_{\frac{1}{2}}, {m}_{J}=+\frac{1}{2}, {m}_{I}=\ensuremath{-}\frac{1}{2})$ near 605 G. A new atomic-beam method is used. H atoms are excited to the metastable 2S2S2S state by electron bombardment. The $2S({m}_{J}=\ensuremath{-}\frac{1}{2})$ state is quenched in the bombardment region. The 2S(mJ=+12)2S({m}_{J}=+\frac{1}{2})2S(mJ​=+21​) beam passes through a zero-field region where 13\frac{1}{3}31​ of it is converted to the single hyperfine level ${\ensuremath{\beta}}_{B}$. The ${\ensuremath{\beta}}_{B}$ atoms enter a uniform magnetic field parallel to the beam. With no externally applied electric field, 92% of them reach the detector. When we apply an electrostatic field of about 0.7 V/cm perpendicular to the beam, the ${\ensuremath{\beta}}_{B}$ atoms are strongly quenched near the ${\ensuremath{\beta}}_{B}\ensuremath{-}{e}_{B}$ crossing point. Observed quenching agrees with the Bethe-Lamb theory of the 2S2S2S lifetime in external fields. We measure magnetic field by the nuclear magnetic resonance frequency of protons in water. Natural asymmetry and minor experimental corrections reduce the observed center of the ${\ensuremath{\beta}}_{B}\ensuremath{-}{e}_{B}$ quenching resonance by 1 part in 104{10}^{4}104. The corrected ${\ensuremath{\beta}}_{B}\ensuremath{-}{e}_{B}$ crossing occurs at 2577.57\ifmmode\pm\else\textpm\fi{}0.25 kc/sec, proton nmr in water. From this, we calculate the Lamb shift S\mathcal{S}S in H, n=2n=2n=2. With a diamagnetic shielding correction, and the accurately known 2S2S2S hyperfine interval, we get $\mathcal{S}=1058.07\ifmmode\pm\else\textpm\fi{}0.10$ Mc/sec. This disagrees with $\mathcal{S}=1057.77\ifmmode\pm\else\textpm\fi{}0.10$ Mc/sec from the Lamb experiments.
A determination of all the state multipoles of the n=2 excited state of atomic hydrogen has been made. The kinematical conditions were a crossed electron-atom-beam geometry in which the incident electron energy was 350 eV and the scattering angle 3'. The experimental technique measures the scattered 10.2 eV energy loss electrons in coincidence with the 10.2 eV photons from either the 2P state or the delayed Stark-quenched 2S-state decay. An external electrostatic field mixes the S and P states, permitting the observation of their coherence by measuring the linear and circular photon polarisations, while in-plane angular correlations for a zero electric field determine the P-state multipoles. The state multipoles were deduced from a least-squares fit of the measured data to the quantum mechanical expressions determined for a model of an S-P (m j =1/2) superposition excited state with averages over the singlet and triplet spin states.
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A time-of-flight technique has been used to measure the radiative lifetime of the 21S02 ^{1}S_{0}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 23S12 ^{3}S_{1}23S1​ and the 21S02 ^{1}S_{0}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 21S02 ^{1}S_{0}21S0​ state is separated from that of the 23S12 ^{3}S_{1}23S1​ state by illuminating the beam with an rf-discharge helium lamp. The 21S02 ^{1}S_{0}21S0​ state is quenched by resonant absorption of a 20 581 - \AA{}A photon, raising the atom to the 21P12 ^{1}P_{1}21P1​ state, which then decays preferentially to the 1S0^{1}S_{0}1S0​ ground state; the 23S12 ^{3}S_{1}23S1​ state remains unaffected because it is the ground state for the triplet system. The time-of-flight distribution for the 21S02 ^{1}S_{0}21S0​ atoms is therefore obtained from the difference between the full beam and the quenched beam. A comparison of the number of 21S02 ^{1}S_{0}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{\mathrm{He}}^{3}He3 and He4{\mathrm{He}}^{4}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.