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Physical Review A

Radiative Lifetime of the 2 S01 Metastable State of Helium

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

Published Oct 1, 1971• 99 citations
Publisher

Abstract

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.

DOI
10.1103/physreva.4.1327

Related papers

Radiative Lifetime of the Metastable 2S01 State of Helium

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

Two fixed, spatially separated detectors are used to measure the decay in flight of a cooled, thermal beam of pulsed metastable helium atoms. The preliminary value obtained for the radiative lifetime of the metastable $2^{1}S_{0}$ state of helium is 20 \ifmmode\pm\else\textpm\fi{} 2 msec.

Physical Review Letters•Nov 16, 1970•38 citations

Quenching of the 2S01 Metastable State of Helium by an Electric Field

Charles E. Johnson

A time-of-flight technique has been used to measure the quenching of the $2^{1}S_{0}$ 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 $2^{1}S_{0}$ and $2^{3}S_{1}$ 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 $2^{1}S_{0}$ state is separated from that of the $2^{3}S_{1}$ state by illuminating the beam before the electric field region with an rf-discharge helium lamp. The $2^{1}S_{0}$ state is quenched by resonant absorption of a 20 581-\AA{} photon, raising the atom to the $2^{1}P_{1}$ state, which then decays 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 $2^{1}S_{0}$ time-of-flight distribution is therefore obtained from the difference between the full beam and the quenched beam. The number of $2^{1}S_{0}$ 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 ($=k{E}^{2}$). The result for the quenching constant $k$ for both ${\mathrm{He}}^{3}$ and ${\mathrm{He}}^{4}$ 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.

Physical Review A•Mar 1, 1973•10 citations

Lifetime of the 2P11 and 3P11 States of Atomic Helium

James M. Burger, Allen Lurio

The radiative lifetime of the $2^{1}P_{1}$ and $3^{1}P_{1}$ state of atomic helium has been measured by the level-crossing technique. In the experiment, a beam of $2^{1}S_{0}$ metastable atoms was first produced and then excited to the $2^{1}P_{1}$ or $3^{1}P_{1}$ state with resonance radiation. The lifetimes were measured by observing the zero-field level-crossing signals in the detected 584- or 537-\AA{} fluorescent radiation when these $^{1}P_{1}$ states decayed to the $1^{1}S_{0}$ ground state. The results are $\ensuremath{\tau} (2^{1}P_{1})=(5.57\ifmmode\pm\else\textpm\fi{}0.15)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec and $\ensuremath{\tau} (3^{1}P_{1})=(1.73\ifmmode\pm\else\textpm\fi{}0.11)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ sec. These values are in excellent agreement with theory.

Physical Review A•Jan 1, 1971•21 citations

Dipole polarizability of 2S13 and 2S01 metastable helium measured by the electric deflection time-of-flight method

David A. Crosby, Jens C. Zorn

The static dipole polarizability of helium atoms in the metastable $2^{3}S_{1}$ and $2^{1}S_{0}$ states has been determined by measuring the deflection of a beam of excited helium atoms that is caused by an inhomogeneous electric field of known properties. The necessary velocity distribution information is obtained from time-of-flight measurements, and a resonance quenching technique made it possible to distinguish the singlet and triplet components of the beam. The results, $\ensuremath{\alpha}(2^{3}S_{1})=(44.6\ifmmode\pm\else\textpm\fi{}3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{3}$ and $\ensuremath{\alpha}(2^{1}S_{0})=(108\ifmmode\pm\else\textpm\fi{}13)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}$ ${\mathrm{cm}}^{3}$, agree with the theoretical value that has been used to calibrate earlier polarizability measurements of the alkali metals and the heavier noble gases.

Physical Review A•Aug 1, 1977•25 citations

Metastable Helium: A New Determination of the Longest Atomic Excited-State Lifetime

Sean S. Hodgman, R. G. Dall, L. J. Byron +3 more

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.

Physical Review Letters•Jul 30, 2009•121 citations

Hanle Effect in 2P1 Helium

Edward S. Fry, W. Williams

The oscillator strength for the resonance ($1^{1}S\ensuremath{-}2^{1}P$) transition in atomic helium has been determined from a measurement of the lifetime of the $2^{1}P$ state. This lifetime was obtained using a modified zero-field level crossing (Hanle) method. The scattering sample was a thermal velocity beam of $2^{1}S$ metastable atoms. Resonant 2-\ensuremath{\mu} ($2^{1}S\ensuremath{-}2^{1}P$) radiation is absorbed by the $2^{1}S$ atoms, which are excited to the $2^{1}P$ state. This state then primarily decays to the ground $1^{1}S$ state with the emission of resonance (584 \AA{}) radiation. The lifetime of the $2^{1}P$ state was determined from measurements of the magnetic-field dependence of the angular distribution of the 584 \AA{} intensity. This new technique eliminates the problem of constructing a vacuum ultraviolet resonance lamp. The measured lifetime $\ensuremath{\tau} (2^{1}P)$ is (5.63 \ifmmode\pm\else\textpm\fi{} 0.22) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ sec. The resulting absorption oscillator strength for the resonance transition is 0.273 \ifmmode\pm\else\textpm\fi{} 0.011. This oscillator strength agrees with the theoretical value.

Physical Review•Jul 5, 1969•14 citations
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.
Two fixed, spatially separated detectors are used to measure the decay in flight of a cooled, thermal beam of pulsed metastable helium atoms. The preliminary value obtained for the radiative lifetime of the metastable 21S02^{1}S_{0}21S0​ state of helium is 20 \ifmmode\pm\else\textpm\fi{} 2 msec.
A time-of-flight technique has been used to measure the quenching of the 21S02^{1}S_{0}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 21S02^{1}S_{0}21S0​ and 23S12^{3}S_{1}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 21S02^{1}S_{0}21S0​ state is separated from that of the 23S12^{3}S_{1}23S1​ state by illuminating the beam before the electric field region with an rf-discharge helium lamp. The 21S02^{1}S_{0}21S0​ state is quenched by resonant absorption of a 20 581-\AA{} photon, raising the atom to the 21P12^{1}P_{1}21P1​ state, which then decays 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 21S02^{1}S_{0}21S0​ time-of-flight distribution is therefore obtained from the difference between the full beam and the quenched beam. The number of 21S02^{1}S_{0}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=k{E}^{2}=kE2). The result for the quenching constant kkk for both He3{\mathrm{He}}^{3}He3 and He4{\mathrm{He}}^{4}He4 with EEE 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.
The radiative lifetime of the 21P12^{1}P_{1}21P1​ and 31P13^{1}P_{1}31P1​ state of atomic helium has been measured by the level-crossing technique. In the experiment, a beam of 21S02^{1}S_{0}21S0​ metastable atoms was first produced and then excited to the 21P12^{1}P_{1}21P1​ or 31P13^{1}P_{1}31P1​ state with resonance radiation. The lifetimes were measured by observing the zero-field level-crossing signals in the detected 584- or 537-\AA{} fluorescent radiation when these 1P1^{1}P_{1}1P1​ states decayed to the 11S01^{1}S_{0}11S0​ ground state. The results are $\ensuremath{\tau} (2^{1}P_{1})=(5.57\ifmmode\pm\else\textpm\fi{}0.15)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}$ sec and $\ensuremath{\tau} (3^{1}P_{1})=(1.73\ifmmode\pm\else\textpm\fi{}0.11)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}$ sec. These values are in excellent agreement with theory.
The static dipole polarizability of helium atoms in the metastable 23S12^{3}S_{1}23S1​ and 21S02^{1}S_{0}21S0​ states has been determined by measuring the deflection of a beam of excited helium atoms that is caused by an inhomogeneous electric field of known properties. The necessary velocity distribution information is obtained from time-of-flight measurements, and a resonance quenching technique made it possible to distinguish the singlet and triplet components of the beam. The results, $\ensuremath{\alpha}(2^{3}S_{1})=(44.6\ifmmode\pm\else\textpm\fi{}3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}$ cm3{\mathrm{cm}}^{3}cm3 and $\ensuremath{\alpha}(2^{1}S_{0})=(108\ifmmode\pm\else\textpm\fi{}13)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}$ cm3{\mathrm{cm}}^{3}cm3, agree with the theoretical value that has been used to calibrate earlier polarizability measurements of the alkali metals and the heavier noble gases.
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 oscillator strength for the resonance ($1^{1}S\ensuremath{-}2^{1}P$) transition in atomic helium has been determined from a measurement of the lifetime of the 21P2^{1}P21P state. This lifetime was obtained using a modified zero-field level crossing (Hanle) method. The scattering sample was a thermal velocity beam of 21S2^{1}S21S metastable atoms. Resonant 2-\ensuremath{\mu} ($2^{1}S\ensuremath{-}2^{1}P$) radiation is absorbed by the 21S2^{1}S21S atoms, which are excited to the 21P2^{1}P21P state. This state then primarily decays to the ground 11S1^{1}S11S state with the emission of resonance (584 \AA{}) radiation. The lifetime of the 21P2^{1}P21P state was determined from measurements of the magnetic-field dependence of the angular distribution of the 584 \AA{} intensity. This new technique eliminates the problem of constructing a vacuum ultraviolet resonance lamp. The measured lifetime $\ensuremath{\tau} (2^{1}P)$ is (5.63 \ifmmode\pm\else\textpm\fi{} 0.22) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}10}$ sec. The resulting absorption oscillator strength for the resonance transition is 0.273 \ifmmode\pm\else\textpm\fi{} 0.011. This oscillator strength agrees with the theoretical value.