Physical Review

Collision-Induced Mixing in the 2P3 Levels of Helium

L. D. Schearer

Published Aug 5, 1967• 29 citations

Abstract

The cross section for the transfer of excitation from the $2^{3}P_{0}$ level of helium to the $2^{3}P_{1}$ and $2^{3}P_{2}$ levels as a result of collisions with the ground-state helium atom ($1^{1}S_{0}$) has been measured with a conventional resonance-fluorescence technique. A ${\mathrm{He}}^{4}$ source excited the $2^{3}S_{1}\ensuremath{-}2^{3}P_{0}$ transition in a ${\mathrm{He}}^{3}$ 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 $2^{3}P_{0}$ level is found to be (68\ifmmode\pm\else\textpm\fi{}3) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ at room temperature.

DOI: 10.1103/physrev.160.76

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Collision-Induced Mixing in the First Excited States of Sodium and Potassium

J. A. Jordan, P. A. Franken

Physical ReviewFeb 4, 196639 citations

6P13−6P03 Excitation Transfer in Mercury, Induced in Collisions with N2 Molecules

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Physical Review ADec 1, 197239 citations

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

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Physical Review AJan 1, 197121 citations

Depolarization of Light Scattered by Aligned 2S3 and 2P3 Helium Atoms at Resonance

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Physical ReviewFeb 5, 19688 citations

Measurements of the Zeeman mixing cross section of (6)(2)P(1/2)Cs atoms, induced by collision with helium gases.

Ju Gao

Jan 1, 19930 citations

Collision-Induced Transitions within Excited Levels of Neon

Joel H. Parks, Ali Javan

Physical ReviewAug 30, 196544 citations

Physical Review

Collision-Induced Mixing in the 2P3 Levels of Helium

L. D. Schearer

Published Aug 5, 1967• 29 citations

Publisher

Abstract

The cross section for the transfer of excitation from the

2^{3}P_{0}

level of helium to the

2^{3}P_{1}

and

2^{3}P_{2}

levels as a result of collisions with the ground-state helium atom (

1^{1}S_{0}

) has been measured with a conventional resonance-fluorescence technique. A

{\mathrm{He}}^{4}

source excited the

2^{3}S_{1}\ensuremath{-}2^{3}P_{0}

transition in a

{\mathrm{He}}^{3}

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

2^{3}P_{0}

level is found to be (68\ifmmode\pm\else\textpm\fi{}3) \ifmmode\times\else\texttimes\fi{}

{10}^{\ensuremath{-}16}

{\mathrm{cm}}^{2}

at room temperature.

DOI: 10.1103/physrev.160.76

Related papers

Collision-Induced Mixing in the First Excited States of Sodium and Potassium

J. A. Jordan, P. A. Franken

The cross sections for the collision-induced transfer of excitation from the $^{2}P_{\frac{3}{2}}$ level to the $^{2}P_{\frac{1}{2}}$ level of the first excited states of the alkali atoms have been measured for several alkali-noble gas systems. The cross sections were determined with a conventional resonance fluorescence technique in which the radiation from a spectral source was absorbed and re-emitted by an alkali vapor at low pressure (\ensuremath{\sim}${10}^{\ensuremath{-}6}$ Torr), and the scattered light analyzed with a scanning spectrometer. Provision was made for the introduction of various noble gases at controlled pressures into the alkali vapor. The determined cross sections are (in units of ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ ): Na-He, 41.1\ifmmode\pm\else\textpm\fi{}2.9; Na-Ne, 36.1\ifmmode\pm\else\textpm\fi{}2.5; Na-Ar, 64.6\ifmmode\pm\else\textpm\fi{}5.2; Na-Kr, 68.2\ifmmode\pm\else\textpm\fi{}5.5; Na-Xe, 61.8\ifmmode\pm\else\textpm\fi{}4.9; K-He, 52.8\ifmmode\pm\else\textpm\fi{}5.8; K-Ne, 14.0\ifmmode\pm\else\textpm\fi{}1.2; K-Ar, 34.0\ifmmode\pm\else\textpm\fi{}3.1. The quoted errors are estimated 70% confidence intervals. The measurements are compared with other experimental results and with various estimated cross sections.The cross sections for the collision-induced transfer of excitation from the $^{2}P_{\frac{3}{2}}$ level to the $^{2}P_{\frac{1}{2}}$ level of the first excited states of the alkali atoms have been measured for several alkali-noble gas systems. The cross sections were determined with a conventional resonance fluorescence technique in which the radiation from a spectral source was absorbed and re-emitted by an alkali vapor at low pressure (\ensuremath{\sim}${10}^{\ensuremath{-}6}$ Torr), and the scattered light analyzed with a scanning spectrometer. Provision was made for the introduction of various noble gases at controlled pressures into the alkali vapor. The determined cross sections are (in units of ${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$): Na-He, 41.1\ifmmode\pm\else\textpm\fi{}2.9; Na-Ne, 36.1\ifmmode\pm\else\textpm\fi{}2.5; Na-Ar, 64.6\ifmmode\pm\else\textpm\fi{}5.2; Na-Kr, 68.2\ifmmode\pm\else\textpm\fi{}5.5; Na-Xe, 61.8\ifmmode\pm\else\textpm\fi{}4.9; K-He, 52.8\ifmmode\pm\else\textpm\fi{}5.8; K-Ne, 14.0\ifmmode\pm\else\textpm\fi{}1.2; K-Ar, 34.0\ifmmode\pm\else\textpm\fi{}3.1. The quoted errors are estimated 70% confidence intervals. The measurements are compared with other experimental results and with various estimated cross sections.

Physical ReviewFeb 4, 196639 citations

6P13−6P03 Excitation Transfer in Mercury, Induced in Collisions with N2 Molecules

John M. Pitre, K. Hammond, L. Krause

The cross section for the transition $6^{3}P_{0}\ensuremath{\rightarrow}6^{3}P_{1}$ in mercury, induced in collisions with ${\mathrm{N}}_{2}$ molecules, has been determined by the method of delayed coincidences. The Hg $^{3}P_{1}$ state was excited by pulses of 2537-\AA{} Hg resonance radiation and the decay of the long-lived 2537-\AA{} afterglow was studied in relation to ${\mathrm{N}}_{2}$ pressure. The resulting cross section ${Q}_{12}(^{3}P_{0}\ensuremath{\rightarrow}^{3}P_{1})=4.18\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ ${\mathrm{\AA{}}}^{2}$ was found to be good agreement with the prediction of the principle of detailed balancing when compared with the cross section ${Q}_{21}(^{3}P_{1}\ensuremath{\rightarrow}^{3}P_{0})$. It was also found that, within the sensitivity of the method, collisions with ${\mathrm{N}}_{2}$ molecules do not quench $^{3}P_{1}$ or $^{3}P_{0}$ Hg atoms to the ground state.The cross section for the transition $6^{3}P_{0}\ensuremath{\rightarrow}6^{3}P_{1}$ in mercury, induced in collisions with ${\mathrm{N}}_{2}$ molecules, has been determined by the method of delayed coincidences. The Hg $^{3}P_{1}$ state was excited by pulses of 2537-\AA{} Hg resonance radiation and the decay of the long-lived 2537-\AA{} afterglow was studied in relation to ${\mathrm{N}}_{2}$ pressure. The resulting cross section ${Q}_{12}(^{3}P_{0}\ensuremath{\rightarrow}^{3}P_{1})=4.18\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ ${\mathrm{\AA{}}}^{2}$ was found to be good agreement with the prediction of the principle of detailed balancing when compared with the cross section ${Q}_{21}(^{3}P_{1}\ensuremath{\rightarrow}^{3}P_{0})$. It was also found that, within the sensitivity of the method, collisions with ${\mathrm{N}}_{2}$ molecules do not quench $^{3}P_{1}$ or $^{3}P_{0}$ Hg atoms to the ground state.

Physical Review ADec 1, 197239 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.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 AJan 1, 197121 citations

Depolarization of Light Scattered by Aligned 2S3 and 2P3 Helium Atoms at Resonance

L. D. Schearer

The $2^{3}S_{1}$ (metastable) and $2^{3}P_{1,2}$ excited-state atoms of ${\mathrm{He}}^{4}$ are aligned by an unpolarized beam of resonance radiation. The alignment is detected by the depolarization of the resonance fluorescence at 1\ensuremath{\mu}, which occurs when either the $2^{3}S_{1}$ level or the $2^{2}P_{1,2}$ levels are saturated by a rf magnetic field at the Zeeman frequency. The observed signals are several orders of magnitude larger than the corresponding signals observed by monitoring intensity changes in the transmitted beam. Considerably smaller signals are observed even in the absence of the pumping beam. Expressions for the intensity of the $\ensuremath{\pi}$ and $\ensuremath{\sigma}$ components of the scattered radiation are derived in terms of the alignment and density of the metastable atoms. The alignment of the $2^{3}S_{1} (F=\frac{3}{2})$ and the $2^{3}P_{2} (F=\frac{5}{2})$ levels of ${\mathrm{He}}^{3}$ is also observed. The excited-state linewidth as a function of pressure leads to a radiative decay time of $\ensuremath{\tau}\ensuremath{\approx}0.96\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ sec and a mixing cross section of 53\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ . The alignment of the $3^{3}P$ level of ${\mathrm{He}}^{4}$ is also observed by monitoring polarization changes of the 3889 \AA{} line.The $2^{3}S_{1}$ (metastable) and $2^{3}P_{1,2}$ excited-state atoms of ${\mathrm{He}}^{4}$ are aligned by an unpolarized beam of resonance radiation. The alignment is detected by the depolarization of the resonance fluorescence at 1\ensuremath{\mu}, which occurs when either the $2^{3}S_{1}$ level or the $2^{2}P_{1,2}$ levels are saturated by a rf magnetic field at the Zeeman frequency. The observed signals are several orders of magnitude larger than the corresponding signals observed by monitoring intensity changes in the transmitted beam. Considerably smaller signals are observed even in the absence of the pumping beam. Expressions for the intensity of the $\ensuremath{\pi}$ and $\ensuremath{\sigma}$ components of the scattered radiation are derived in terms of the alignment and density of the metastable atoms. The alignment of the $2^{3}S_{1} (F=\frac{3}{2})$ and the $2^{3}P_{2} (F=\frac{5}{2})$ levels of ${\mathrm{He}}^{3}$ is also observed. The excited-state linewidth as a function of pressure leads to a radiative decay time of $\ensuremath{\tau}\ensuremath{\approx}0.96\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ sec and a mixing cross section of 53\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$. The alignment of the $3^{3}P$ level of ${\mathrm{He}}^{4}$ is also observed by monitoring polarization changes of the 3889 \AA{} line.

Physical ReviewFeb 5, 19688 citations

Measurements of the Zeeman mixing cross section of (6)(2)P(1/2)Cs atoms, induced by collision with helium gases.

Ju Gao

Collisional transitions between Zeeman substates of Cs $\\rm 6\\sp2P\\sb{1/2}$ atoms in collisions with He atoms have been investigated by methods of atomic fluorescence spectroscopy. Cs vapour contained in a quartz cell together with helium at low pressure, was selectively excited by pulsed dye-laser radiation to the $\\rm 6\\sp2P\\sb{1/2,-1/2}$ Zeeman substate in a magnetic field of strength ranging from 1.5 T to 7 T. The fluorescence spectrum consisting of components emitted from the directly excited state and from the collisionally populated state, was resolved with a Fabry-Perot interferometer. Measurements of the relative intensities of the fluorescence components yielded the Zeeman mixing cross section for Cs-He collisions. It was found that the enhancement in the cross section varied as B $\\sp2$ in the range 0 $$ 5 T. A theoretical calculation of the magnetic-field enhancement of the cross section was also carried out, using time-dependent perturbation theory. The results of the calculation are in very good agreement with the experimental results at fields in the range 0 $<$ B $<$ 5 T, though not beyond 5 T where further theoretical work remains to be done.Dept. of Physics. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis1993 .G365. Source: Masters Abstracts International, Volume: 32-02, page: 0636. Adviser: L. Krause. Thesis (M.Sc.)--University of Windsor (Canada), 1993.Collisional transitions between Zeeman substates of Cs $\\rm 6\\sp2P\\sb{1/2}$ atoms in collisions with He atoms have been investigated by methods of atomic fluorescence spectroscopy. Cs vapour contained in a quartz cell together with helium at low pressure, was selectively excited by pulsed dye-laser radiation to the $\\rm 6\\sp2P\\sb{1/2,-1/2}$ Zeeman substate in a magnetic field of strength ranging from 1.5 T to 7 T. The fluorescence spectrum consisting of components emitted from the directly excited state and from the collisionally populated state, was resolved with a Fabry-Perot interferometer. Measurements of the relative intensities of the fluorescence components yielded the Zeeman mixing cross section for Cs-He collisions. It was found that the enhancement in the cross section varied as B$\\sp2$ in the range 0 $$ 5 T. A theoretical calculation of the magnetic-field enhancement of the cross section was also carried out, using time-dependent perturbation theory. The results of the calculation are in very good agreement with the experimental results at fields in the range 0 $<$ B $<$ 5 T, though not beyond 5 T where further theoretical work remains to be done.Dept. of Physics. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis1993 .G365. Source: Masters Abstracts International, Volume: 32-02, page: 0636. Adviser: L. Krause. Thesis (M.Sc.)--University of Windsor (Canada), 1993.

Jan 1, 19930 citations

Collision-Induced Transitions within Excited Levels of Neon

Joel H. Parks, Ali Javan

The interaction of an intense optical-maser field with excited atoms in a gas discharge leads to a considerable change in the population of those levels which are resonant at the maser frequency. This will cause a change in the population of other levels that are connected to the maser levels through collision-induced nonradiative as well as radiative transitions. Using the 1.15-\ensuremath{\mu} optical maser, these effects have been studied in detail for the $2s$ levels of neon where the changes are completely due to collisions. By analyzing the intensity changes in spontaneous emission originating from a gas-discharge cell placed within the maser resonator, we have observed the approach to a partial thermalization of two closely spaced levels. In this way we have been able to obtain the atomic-collision cross section for excitation transfer between the $2{s}_{2}$ and $2{s}_{3}$ levels. In pure neon this has been found to be ${\ensuremath{\sigma}}_{23}=(2.3\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ . For a helium-neon mixture the measured cross section is ${\ensuremath{\sigma}}_{23}=(1.8\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$ . The experimental method used to determine these cross sections eliminates the uncertainties caused by the effects of electron-atom collisions and radiative cascade. These techniques also yield the ratio of the Einstein coefficients for the transitions $2{s}_{2}\ensuremath{\rightarrow}2{p}_{10}$ and $2{s}_{3}\ensuremath{\rightarrow}2{p}_{10}$. The measured ratio is $\frac{{A}_{2}}{{A}_{3}}=3.0\ifmmode\pm\else\textpm\fi{}0.4$.The interaction of an intense optical-maser field with excited atoms in a gas discharge leads to a considerable change in the population of those levels which are resonant at the maser frequency. This will cause a change in the population of other levels that are connected to the maser levels through collision-induced nonradiative as well as radiative transitions. Using the 1.15-\ensuremath{\mu} optical maser, these effects have been studied in detail for the $2s$ levels of neon where the changes are completely due to collisions. By analyzing the intensity changes in spontaneous emission originating from a gas-discharge cell placed within the maser resonator, we have observed the approach to a partial thermalization of two closely spaced levels. In this way we have been able to obtain the atomic-collision cross section for excitation transfer between the $2{s}_{2}$ and $2{s}_{3}$ levels. In pure neon this has been found to be ${\ensuremath{\sigma}}_{23}=(2.3\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$. For a helium-neon mixture the measured cross section is ${\ensuremath{\sigma}}_{23}=(1.8\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$ ${\mathrm{cm}}^{2}$. The experimental method used to determine these cross sections eliminates the uncertainties caused by the effects of electron-atom collisions and radiative cascade. These techniques also yield the ratio of the Einstein coefficients for the transitions $2{s}_{2}\ensuremath{\rightarrow}2{p}_{10}$ and $2{s}_{3}\ensuremath{\rightarrow}2{p}_{10}$. The measured ratio is $\frac{{A}_{2}}{{A}_{3}}=3.0\ifmmode\pm\else\textpm\fi{}0.4$.

Physical ReviewAug 30, 196544 citations

The cross section for the transfer of excitation from the

2^{3}P_{0}

level of helium to the

2^{3}P_{1}

and

2^{3}P_{2}

levels as a result of collisions with the ground-state helium atom (

1^{1}S_{0}

) has been measured with a conventional resonance-fluorescence technique. A

{\mathrm{He}}^{4}

source excited the

2^{3}S_{1}\ensuremath{-}2^{3}P_{0}

transition in a

{\mathrm{He}}^{3}

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

2^{3}P_{0}

level is found to be (68\ifmmode\pm\else\textpm\fi{}3) \ifmmode\times\else\texttimes\fi{}

{10}^{\ensuremath{-}16}

{\mathrm{cm}}^{2}

at room temperature.

The cross sections for the collision-induced transfer of excitation from the

^{2}P_{\frac{3}{2}}

level to the

^{2}P_{\frac{1}{2}}

level of the first excited states of the alkali atoms have been measured for several alkali-noble gas systems. The cross sections were determined with a conventional resonance fluorescence technique in which the radiation from a spectral source was absorbed and re-emitted by an alkali vapor at low pressure (\ensuremath{\sim}${10}^{\ensuremath{-}6}$ Torr), and the scattered light analyzed with a scanning spectrometer. Provision was made for the introduction of various noble gases at controlled pressures into the alkali vapor. The determined cross sections are (in units of ${10}^{\ensuremath{-}16}$

{\mathrm{cm}}^{2}

): Na-He, 41.1\ifmmode\pm\else\textpm\fi{}2.9; Na-Ne, 36.1\ifmmode\pm\else\textpm\fi{}2.5; Na-Ar, 64.6\ifmmode\pm\else\textpm\fi{}5.2; Na-Kr, 68.2\ifmmode\pm\else\textpm\fi{}5.5; Na-Xe, 61.8\ifmmode\pm\else\textpm\fi{}4.9; K-He, 52.8\ifmmode\pm\else\textpm\fi{}5.8; K-Ne, 14.0\ifmmode\pm\else\textpm\fi{}1.2; K-Ar, 34.0\ifmmode\pm\else\textpm\fi{}3.1. The quoted errors are estimated 70% confidence intervals. The measurements are compared with other experimental results and with various estimated cross sections.

The cross section for the transition $6^{3}P_{0}\ensuremath{\rightarrow}6^{3}P_{1}$ in mercury, induced in collisions with

{\mathrm{N}}_{2}

molecules, has been determined by the method of delayed coincidences. The Hg

^{3}P_{1}

state was excited by pulses of 2537-\AA{} Hg resonance radiation and the decay of the long-lived 2537-\AA{} afterglow was studied in relation to

{\mathrm{N}}_{2}

pressure. The resulting cross section ${Q}_{12}(^{3}P_{0}\ensuremath{\rightarrow}^{3}P_{1})=4.18\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$

{\mathrm{\AA{}}}^{2}

was found to be good agreement with the prediction of the principle of detailed balancing when compared with the cross section ${Q}_{21}(^{3}P_{1}\ensuremath{\rightarrow}^{3}P_{0})$. It was also found that, within the sensitivity of the method, collisions with

{\mathrm{N}}_{2}

molecules do not quench

^{3}P_{1}

^{3}P_{0}

Hg atoms to the ground state.

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}

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.

The

2^{3}S_{1}

(metastable) and

2^{3}P_{1,2}

excited-state atoms of

{\mathrm{He}}^{4}

are aligned by an unpolarized beam of resonance radiation. The alignment is detected by the depolarization of the resonance fluorescence at 1\ensuremath{\mu}, which occurs when either the

2^{3}S_{1}

level or the

2^{2}P_{1,2}

levels are saturated by a rf magnetic field at the Zeeman frequency. The observed signals are several orders of magnitude larger than the corresponding signals observed by monitoring intensity changes in the transmitted beam. Considerably smaller signals are observed even in the absence of the pumping beam. Expressions for the intensity of the $\ensuremath{\pi}$ and $\ensuremath{\sigma}$ components of the scattered radiation are derived in terms of the alignment and density of the metastable atoms. The alignment of the

2^{3}S_{1} (F=\frac{3}{2})

and the

2^{3}P_{2} (F=\frac{5}{2})

levels of

{\mathrm{He}}^{3}

is also observed. The excited-state linewidth as a function of pressure leads to a radiative decay time of $\ensuremath{\tau}\ensuremath{\approx}0.96\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$ sec and a mixing cross section of 53\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}16}$

{\mathrm{cm}}^{2}

. The alignment of the

3^{3}P

level of

{\mathrm{He}}^{4}

is also observed by monitoring polarization changes of the 3889 \AA{} line.

Collisional transitions between Zeeman substates of Cs

\\rm 6\\sp2P\\sb{1/2}

atoms in collisions with He atoms have been investigated by methods of atomic fluorescence spectroscopy. Cs vapour contained in a quartz cell together with helium at low pressure, was selectively excited by pulsed dye-laser radiation to the

\\rm 6\\sp2P\\sb{1/2,-1/2}

Zeeman substate in a magnetic field of strength ranging from 1.5 T to 7 T. The fluorescence spectrum consisting of components emitted from the directly excited state and from the collisionally populated state, was resolved with a Fabry-Perot interferometer. Measurements of the relative intensities of the fluorescence components yielded the Zeeman mixing cross section for Cs-He collisions. It was found that the enhancement in the cross section varied as B

\\sp2

in the range 0 $$ 5 T. A theoretical calculation of the magnetic-field enhancement of the cross section was also carried out, using time-dependent perturbation theory. The results of the calculation are in very good agreement with the experimental results at fields in the range 0

<

<

5 T, though not beyond 5 T where further theoretical work remains to be done.Dept. of Physics. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis1993 .G365. Source: Masters Abstracts International, Volume: 32-02, page: 0636. Adviser: L. Krause. Thesis (M.Sc.)--University of Windsor (Canada), 1993.

2s

levels of neon where the changes are completely due to collisions. By analyzing the intensity changes in spontaneous emission originating from a gas-discharge cell placed within the maser resonator, we have observed the approach to a partial thermalization of two closely spaced levels. In this way we have been able to obtain the atomic-collision cross section for excitation transfer between the

2{s}_{2}

and

2{s}_{3}

levels. In pure neon this has been found to be ${\ensuremath{\sigma}}_{23}=(2.3\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$

{\mathrm{cm}}^{2}

. For a helium-neon mixture the measured cross section is ${\ensuremath{\sigma}}_{23}=(1.8\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}$

{\mathrm{cm}}^{2}

. The experimental method used to determine these cross sections eliminates the uncertainties caused by the effects of electron-atom collisions and radiative cascade. These techniques also yield the ratio of the Einstein coefficients for the transitions $2{s}_{2}\ensuremath{\rightarrow}2{p}_{10}$ and $2{s}_{3}\ensuremath{\rightarrow}2{p}_{10}$. The measured ratio is $\frac{{A}_{2}}{{A}_{3}}=3.0\ifmmode\pm\else\textpm\fi{}0.4$.