Physical Review A

Detection of nitrogen atoms in the 2s(2p)$sup 3$3s $sup 6$S/sub 5/2/ metastable autoionizing state

Clifford E. Fairchild, Hari Prasad Garg, C. E. Johnson

Published Aug 1, 1973• 0 citations

Abstract

Metastable nitrogen atoms, produced by electron-impact dissociation of N/ sub 2/, have been detected in time-of-flight apparatuses locatcd at Oregon State University and at University of California at Berkeley. Two different types of metastable atoms are detected; one type resembies Rydberg hydrogen atoms, while the other does not. The measured natural lifetime for the non-Rydberg atoms is 100 plus or minus 25 mu see. Additional infonnation regarding the identity of both types of metastable atoms is obtained by observing their quenching in a static electric field. The field-dependent decay constant for the Rydberg atoms is essentially a step function, and these atoms are entirely quenched by a field of 1.5 kV/cm. Both results are consistent with theory if the Rydberg atoms are in levels with principal quantum number greater than 25. The field-dependent decay constant for the non-Rydherg atoms varies quadratically with the strength of the applied electric field. In a field of 4 kV/cm applied along a 6-cm length of flight path, the value of this decay constant is 0.3 x 10/sup 6/ sec/sup -1/. The quenching results, together with the lifetime measurement, are a strong more » indication that the non-Rydberg atoms detected in the time-of-flight spectra are nitrogen atoms in the 2s(2p)/sup 3/3s /sup 6/S/sub 5/2/ metastable autoionizing state. (a uth) « less

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Physical Review A

Detection of nitrogen atoms in the 2s(2p)$sup 3$3s $sup 6$ S/sub 5/2/ metastable autoionizing stateDetection of nitrogen atoms in the 2s(2p)$sup 3$3s $sup 6$S/sub 5/2/ metastable autoionizing state

Clifford E. Fairchild, Hari Prasad Garg, C. E. Johnson

Published Aug 1, 1973• 0 citations

OpenAlex

Abstract

Related papers

Detection of Nitrogen Atoms in the 2s(2p)33sS526 Metastable Autoionizing State

Clifford E. Fairchild, Hari Prasad Garg, C. E. Johnson

Metastable nitrogen atoms, produced by electron-impact dissociation of ${\mathrm{N}}_{2}$ , have been detected in time-of-flight apparatuses located at Oregon State University and at University of California at Berkeley. Two different types of metastable atoms are detected; one type resembles Rydberg hydrogen atoms, while the other does not. The measured natural lifetime for the non-Rydberg atoms is 100 \ifmmode\pm\else\textpm\fi{} 25 \ensuremath{\mu}sec. Additional information regarding the identity of both types of metastable atoms is obtained by observing their quenching in a static electric field. The field-dependent decay constant for the Rydberg atoms is essentially a step function, and these atoms are entirely quenched by a field of 1.5 kV/cm. Both results are consistent with theory if the Rydberg atoms are in levels with principal quantum number greater than 25. The field-dependent decay constant for the non-Rydberg atoms varies quadratically with the strength of the applied electric field. In a field of 4 kV/cm applied along a 6-cm length of flight path, the value of this decay constant is 0.3 \ifmmode\times\else\texttimes\fi{} ${10}^{6}$ ${\mathrm{sec}}^{\ensuremath{-}1}$. The quenching results, together with the lifetime measurement, are a strong indication that the non-Rydberg atoms detected in the time-of-flight spectra are nitrogen atoms in the $2s{(2p)}^{3}3s^{6}S_{\frac{5}{2}}$ metastable autoionizing state.Metastable nitrogen atoms, produced by electron-impact dissociation of ${\mathrm{N}}_{2}$, have been detected in time-of-flight apparatuses located at Oregon State University and at University of California at Berkeley. Two different types of metastable atoms are detected; one type resembles Rydberg hydrogen atoms, while the other does not. The measured natural lifetime for the non-Rydberg atoms is 100 \ifmmode\pm\else\textpm\fi{} 25 \ensuremath{\mu}sec. Additional information regarding the identity of both types of metastable atoms is obtained by observing their quenching in a static electric field. The field-dependent decay constant for the Rydberg atoms is essentially a step function, and these atoms are entirely quenched by a field of 1.5 kV/cm. Both results are consistent with theory if the Rydberg atoms are in levels with principal quantum number greater than 25. The field-dependent decay constant for the non-Rydberg atoms varies quadratically with the strength of the applied electric field. In a field of 4 kV/cm applied along a 6-cm length of flight path, the value of this decay constant is 0.3 \ifmmode\times\else\texttimes\fi{} ${10}^{6}$ ${\mathrm{sec}}^{\ensuremath{-}1}$. The quenching results, together with the lifetime measurement, are a strong indication that the non-Rydberg atoms detected in the time-of-flight spectra are nitrogen atoms in the $2s{(2p)}^{3}3s^{6}S_{\frac{5}{2}}$ metastable autoionizing state.

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Auto-Ionizing States in the Alkali Atoms with Microsecond Lifetimes

P. D. Feldman, Robert Novick

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Physical ReviewAug 5, 1967170 citations

Detection of nitrogen atoms in the 2s(2p)$sup 3$3s

sup 6

S/sub 5/2/ metastable autoionizing state

Metastable nitrogen atoms, produced by electron-impact dissociation of

{\mathrm{N}}_{2}

, have been detected in time-of-flight apparatuses located at Oregon State University and at University of California at Berkeley. Two different types of metastable atoms are detected; one type resembles Rydberg hydrogen atoms, while the other does not. The measured natural lifetime for the non-Rydberg atoms is 100 \ifmmode\pm\else\textpm\fi{} 25 \ensuremath{\mu}sec. Additional information regarding the identity of both types of metastable atoms is obtained by observing their quenching in a static electric field. The field-dependent decay constant for the Rydberg atoms is essentially a step function, and these atoms are entirely quenched by a field of 1.5 kV/cm. Both results are consistent with theory if the Rydberg atoms are in levels with principal quantum number greater than 25. The field-dependent decay constant for the non-Rydberg atoms varies quadratically with the strength of the applied electric field. In a field of 4 kV/cm applied along a 6-cm length of flight path, the value of this decay constant is 0.3 \ifmmode\times\else\texttimes\fi{}

{10}^{6}

${\mathrm{sec}}^{\ensuremath{-}1}$. The quenching results, together with the lifetime measurement, are a strong indication that the non-Rydberg atoms detected in the time-of-flight spectra are nitrogen atoms in the

2s{(2p)}^{3}3s^{6}S_{\frac{5}{2}}

metastable autoionizing state.

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.

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.

{\mathrm{I}}^{b}

spectral terms. Magnetic and electric fields have been found to be effective in reducing the lifetime of the metastable atoms. The Zeeman quenching is discussed in a separate paper. The Stark quenching rate has been found to be proportional to the square of the electric field and to increase rapidly with atomic number

Z

. It is suggested that the Stark quenching results from a second-order interaction which involves the product of the electric field and the spin-orbit or other magnetic operators.