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Physical Review

Polarization of Radiation Excited by Electron Impact

A. Ellett, Paul D. Foote, Fred L. Mohler

Published Jan 1, 1926• 19 citations
Publisher

Abstract

Polarized radiation following electron impact.---If a unidirectional beam of electrons, of velocity slightly in excess of the resonance potential, is projected through a vapor at a very low pressure so that effects due to atomic collision and resonance radiation are negligible, and in the absence of an applied field and resulting Larmor precession, there is reason to suspect that the radiation emitted perpendicular to the electron beam should be partially polarized, especially for lines corresponding to $\ensuremath{\Delta}j=1$.Polarization in mercury and sodium vapor.---Experiment showed a polarization of about 30 percent for $\ensuremath{\lambda}2537$ with the greatest intensity of the electrical vibration having a direction perpendicular to the electron beam. No polarization was detected for the unresolved sodium D-lines, confirming the recent observation of Kossel and Gerthsen.Polarization in mercury with field of 3 gauss.---A field parallel to the electron beam produced no effect. With the field perpendicular to the beam the radiation along the field was unpolarized while that perpendicular to the field was reduced in polarization to 13 percent. These results suggest that the atom radiates with the same distribution of polarization and intensity as in zero field but with the atom rotated through an angle determined by the time of excitation and the precessional velocity.

DOI
10.1103/physrev.27.31

Related papers

Effect of a Magnetic Field Upon the Polarization of Impact Radiation

W.H. McCorkle

Sodium $D$ line radiation excited by electrons moving parallel to a magnetic field of 300 gauss or more shows about 16 percent polarization, the major electric vibration being parallel to the direction of the exciting electron beam. In fields below 300 gauss the polarization is less, decreasing to zero in the absence of a field. That the polarization changes in a magnetic field must be due to the fact that the $3^{2}P_{\frac{3}{2}}$ level is a hyperfine multiplet which undergoes a Paschen-Back effect.

Physical Review•Sep 15, 1935•0 citations

Polarized Resonance Radiation in Weak Magnetic Fields

Robert Williams Wood, A. Ellett

Effect of a magnetic field on the polarization of resonance radiation of mercury and of sodium.---(1) Mercury $\ensuremath{\lambda}2536$. A beam of plane polarized monochromatic light was focussed on the bulb containing the vapor at low pressure (0\ifmmode^\circ\else\textdegree\fi{}C), and the polarization of the resonance radiation was determined photographically with the aid of a quartz wedge and a double image quartz prism. If the exciting beam is going east with the electric vector vertical, when the earth's field is carefully neutralized the polarization is 90 per cent in any horizontal direction and zero vertically; a field of only 2 gauss directed north reduces the polarization in its direction to nearly zero (the decrease with increasing field being exponential), changes the polarization east and also up to 60 percent, and increases the intensity directed upwards threefold. Similar effects are produced by fields in other directions. The polarization with zero field does not approach 100 per cent at low pressures, since 90 per cent was found also with the vapor at -50\ifmmode^\circ\else\textdegree\fi{}C, when the radiation first appeared. It was thought that the high value might be due to an orientation of the resonating atom by the field of the exciting light, but a beam of concentrated sunlight produced no effect. As found by Malinowski, a strong field of 10,000 gauss merely increased the intensity of the radiation about 10 per cent. This added light was found to be unpolarized. (2) Sodium $D$ line. In the case of sodium vapor the tube was heated to 185\ifmmode^\circ\else\textdegree\fi{}C, the observations were visual, and as a source of unreversed light, the glow on the surface of a sodium glass vacuum tube carrying a discharge, was used. The effects observed are similar to those for mercury, but differ for some directions; fields forty times stronger were required, and the polarization in zero field was only 6 per cent, probably due to traces of hydrogen. This was increased by the field in some directions to 30 per cent.Effect of magnetic field on fluorescent light of iodine vapor and on white scattered light from mercury and ether vapors.---The polarization was found not to be affected.

Physical Review•Sep 1, 1924•74 citations

Excitation of spin-polarized sodium and potassium atoms by electron impact

Muhammad Abdul-Hakim Bukhari, H -J Beyert, M. A. Chaudhry +3 more

A beam of unpolarized electrons is crossed at right angles with a beam of partially polarized sodium or potassium atoms (degree of polarization P A approximately=21%) and resonance lines from the excited 2 P 3/2, 1/2 states are observed in a direction perpendicular to both the atom and electron beams for electron energies from threshold to 200 eV. The polarization of the emitted photons is measured in the form of the Stokes parameters P 1 , P 2 and P 3 . The present measurements for sodium atoms are in better agreement with the theoretical values than the previously published experimental results. No other experimental or theoretical results are available for potassium atoms.

Journal of Physics B Atomic Molecular and Optical Physics•May 14, 1995•4 citations

On the excitation of polarised light by electron impact. II.—Mercury

H. W. B. Skinner, E. T. S. Appleyard

Abstract In a number of recent papers descriptions have been given of how the light from atoms excited by a directed stream of electrons is polarised, in the absence of any external field of force. In particular we may refer to the paper by one of the present writers (Paper I), where some of the characteristics of the effect in mercury were discussed. It was found that the different lines of the spectrum were differently polarised, both in magnitude and direction. The behaviour of the polarisation when a magnetic field is applied to the mercury atoms was found to be identical with the effect produced by a similar field on the polarisa­tion when the light is excited by the absorption of plane polarised light, the electric vector of which is parallel to the direction of the stream of electrons. In this way, the polarisation of the light excited by electron impact was shown to be closely related to the polarisation of resonance radiation, which had been investigated by Wood and Ellett and others. In the paper referred to, the measurement of the polarisation of the spectral lines was, except in an individual instance, only qualitative. Since its appearance two papers have appeared in which polarisation measurements of the mercury lines excited by electron impact have appeared. Eldridge and Olsen gave qualitative results, and these confirmed those which we had given. Quarder gives quantitative measure­ments which are also for the most part in general accord. The aim of the present paper is to describe quantitative results, and, as will be seen, these are not in all cases in detailed agreement with those given by Quarder. The second aim is to determine the variation of the polarisation with the velocity of the electron stream, a point which has not been previously attempted. In Paper I it was shown that the magnitude of the polarisation is intimately connected with the dynamics of the collision process. By the electron impact, the atom is raised from its normal state into an upper quantum state, and by its return into the normal state, or into another state, light is emitted. The magnitude of the polarisation depends on both of these processes, but the second process, the actual emission of the light, is well understood. Polarisation measurements therefore give a powerful means of investigating the collision of an electron with an atom in the case when excitation of the atom takes place, and it is in this that their main interest lies.

Proceedings of the Royal Society of London Series A Containing Papers of a Mathematical and Physical Character•Dec 1, 1927•72 citations

The polarization of atomic line radiation excited by electron impact

Ian Colin Percival, M. J. Seaton

Abstract The dipole radiation emitted by an atom excited by a unidirectional electron beam has a nonuniform angular distribution which is simply related to the percentage polarization P of the radiation emitted perpendicular to the beam. P was first calculated using the OppenheimerPenney (O.-P.) theory. In this theory the probability of excitation of an upper quantum state and the probability of subsequent emission of a polarized photon from such a state are considered independently. P is finally expressed in terms of the cross-sections QM for excitation of states of definite component of angular momentum along the direction of the electron beam. In general, P is dependent on detailed numerical calculations of QM j, but the selection rule A = 0 removes this dependence at threshold. In the O.-P. theory allowance may be made for fine structure and hyperfine structure, but the theory is ambiguous when the f.s. or h.f.s. separations are comparable with the line width. A theory is therefore developed which is based on the calculation of the probability of a polarized photon being emitted by the complete system of atom + electron. The ambiguity of the O.-P. theory is removed by integration over line profiles, but the expressions reduce to O.-P. expressions when the f.s. or h.f.s. separations are much smaller or much larger than the line width. The Lyot line of hydrogen is an intermediate case for which the line widths and the h.f.s. separations are comparable. Assuming the validity of the Born approximation, a simple expression is obtained which allows the QM to be calculated from the angular distribution of the scattered electrons. Theoretical predictions are compared with experimental results. For the Na£) lines the predicted polarization is small enough to escape experimental detection. Polarizations observed by - Skinner & Appleyard in 1927 for various Hg lines rise to maxima with decreasing electron energy, and then tend to values close to zero at threshold. These experimental results at low energies appear to be inexplicable in terms of the reactions considered, but if the polarization curve above the maximum is extrapolated to threshold, the theory and experiment are found to be in reasonable agreement. Further experimental work is thought to be desirable

Philosophical Transactions of the Royal Society of London Series A Mathematical and Physical Sciences•Nov 6, 1958•573 citations

Polarization of Resonance Radiation in Mercury

Harry F. Olson

$1^{1}S_{0}\ensuremath{-}2^{3}P_{1}$ of mercury excited by plane polarized light shows incomplete polarization both in the absence and in the presence of a weak field parallel to the electric vector. The amount of initial polarization depends upon the relative intensities of the hyperfine structure lines in the exciting light. The polarization with various relative orientations of field and light vector and the variation of polarization with field intensity in weak fields may be interpreted successfully by means of a semi-classical model, with proper relative intensities parallel and prependicular to the light vector, rotating after excitation with an angular velocity $\frac{\mathrm{geH}}{2mc}$ and emitting a damped wave. From curves connecting depolarization, rotation of maximum of polarization, etc., with field intensity $\ensuremath{\alpha}$, the damping constant, has been found to be 1.02 (\ifmmode\pm\else\textpm\fi{}0.02) ${10}^{7}$ ${\mathrm{sec}}^{\ensuremath{-}1}$.

Physical Review•Sep 1, 1928•14 citations
Polarized radiation following electron impact.---If a unidirectional beam of electrons, of velocity slightly in excess of the resonance potential, is projected through a vapor at a very low pressure so that effects due to atomic collision and resonance radiation are negligible, and in the absence of an applied field and resulting Larmor precession, there is reason to suspect that the radiation emitted perpendicular to the electron beam should be partially polarized, especially for lines corresponding to \ensuremathΔj=1\ensuremath{\Delta}j=1\ensuremathΔj=1.Polarization in mercury and sodium vapor.---Experiment showed a polarization of about 30 percent for \ensuremathλ2537\ensuremath{\lambda}2537\ensuremathλ2537 with the greatest intensity of the electrical vibration having a direction perpendicular to the electron beam. No polarization was detected for the unresolved sodium D-lines, confirming the recent observation of Kossel and Gerthsen.Polarization in mercury with field of 3 gauss.---A field parallel to the electron beam produced no effect. With the field perpendicular to the beam the radiation along the field was unpolarized while that perpendicular to the field was reduced in polarization to 13 percent. These results suggest that the atom radiates with the same distribution of polarization and intensity as in zero field but with the atom rotated through an angle determined by the time of excitation and the precessional velocity.
Sodium DDD line radiation excited by electrons moving parallel to a magnetic field of 300 gauss or more shows about 16 percent polarization, the major electric vibration being parallel to the direction of the exciting electron beam. In fields below 300 gauss the polarization is less, decreasing to zero in the absence of a field. That the polarization changes in a magnetic field must be due to the fact that the 32P323^{2}P_{\frac{3}{2}}32P23​​ level is a hyperfine multiplet which undergoes a Paschen-Back effect.
Effect of a magnetic field on the polarization of resonance radiation of mercury and of sodium.---(1) Mercury $\ensuremath{\lambda}2536$. A beam of plane polarized monochromatic light was focussed on the bulb containing the vapor at low pressure (0\ifmmode^\circ\else\textdegree\fi{}C), and the polarization of the resonance radiation was determined photographically with the aid of a quartz wedge and a double image quartz prism. If the exciting beam is going east with the electric vector vertical, when the earth's field is carefully neutralized the polarization is 90 per cent in any horizontal direction and zero vertically; a field of only 2 gauss directed north reduces the polarization in its direction to nearly zero (the decrease with increasing field being exponential), changes the polarization east and also up to 60 percent, and increases the intensity directed upwards threefold. Similar effects are produced by fields in other directions. The polarization with zero field does not approach 100 per cent at low pressures, since 90 per cent was found also with the vapor at -50\ifmmode^\circ\else\textdegree\fi{}C, when the radiation first appeared. It was thought that the high value might be due to an orientation of the resonating atom by the field of the exciting light, but a beam of concentrated sunlight produced no effect. As found by Malinowski, a strong field of 10,000 gauss merely increased the intensity of the radiation about 10 per cent. This added light was found to be unpolarized. (2) Sodium DDD line. In the case of sodium vapor the tube was heated to 185\ifmmode^\circ\else\textdegree\fi{}C, the observations were visual, and as a source of unreversed light, the glow on the surface of a sodium glass vacuum tube carrying a discharge, was used. The effects observed are similar to those for mercury, but differ for some directions; fields forty times stronger were required, and the polarization in zero field was only 6 per cent, probably due to traces of hydrogen. This was increased by the field in some directions to 30 per cent.Effect of magnetic field on fluorescent light of iodine vapor and on white scattered light from mercury and ether vapors.---The polarization was found not to be affected.
A beam of unpolarized electrons is crossed at right angles with a beam of partially polarized sodium or potassium atoms (degree of polarization P A approximately=21%) and resonance lines from the excited 2 P 3/2, 1/2 states are observed in a direction perpendicular to both the atom and electron beams for electron energies from threshold to 200 eV. The polarization of the emitted photons is measured in the form of the Stokes parameters P 1 , P 2 and P 3 . The present measurements for sodium atoms are in better agreement with the theoretical values than the previously published experimental results. No other experimental or theoretical results are available for potassium atoms.
Abstract In a number of recent papers descriptions have been given of how the light from atoms excited by a directed stream of electrons is polarised, in the absence of any external field of force. In particular we may refer to the paper by one of the present writers (Paper I), where some of the characteristics of the effect in mercury were discussed. It was found that the different lines of the spectrum were differently polarised, both in magnitude and direction. The behaviour of the polarisation when a magnetic field is applied to the mercury atoms was found to be identical with the effect produced by a similar field on the polarisa­tion when the light is excited by the absorption of plane polarised light, the electric vector of which is parallel to the direction of the stream of electrons. In this way, the polarisation of the light excited by electron impact was shown to be closely related to the polarisation of resonance radiation, which had been investigated by Wood and Ellett and others. In the paper referred to, the measurement of the polarisation of the spectral lines was, except in an individual instance, only qualitative. Since its appearance two papers have appeared in which polarisation measurements of the mercury lines excited by electron impact have appeared. Eldridge and Olsen gave qualitative results, and these confirmed those which we had given. Quarder gives quantitative measure­ments which are also for the most part in general accord. The aim of the present paper is to describe quantitative results, and, as will be seen, these are not in all cases in detailed agreement with those given by Quarder. The second aim is to determine the variation of the polarisation with the velocity of the electron stream, a point which has not been previously attempted. In Paper I it was shown that the magnitude of the polarisation is intimately connected with the dynamics of the collision process. By the electron impact, the atom is raised from its normal state into an upper quantum state, and by its return into the normal state, or into another state, light is emitted. The magnitude of the polarisation depends on both of these processes, but the second process, the actual emission of the light, is well understood. Polarisation measurements therefore give a powerful means of investigating the collision of an electron with an atom in the case when excitation of the atom takes place, and it is in this that their main interest lies.
Abstract The dipole radiation emitted by an atom excited by a unidirectional electron beam has a nonuniform angular distribution which is simply related to the percentage polarization P of the radiation emitted perpendicular to the beam. P was first calculated using the OppenheimerPenney (O.-P.) theory. In this theory the probability of excitation of an upper quantum state and the probability of subsequent emission of a polarized photon from such a state are considered independently. P is finally expressed in terms of the cross-sections QM for excitation of states of definite component of angular momentum along the direction of the electron beam. In general, P is dependent on detailed numerical calculations of QM j, but the selection rule A = 0 removes this dependence at threshold. In the O.-P. theory allowance may be made for fine structure and hyperfine structure, but the theory is ambiguous when the f.s. or h.f.s. separations are comparable with the line width. A theory is therefore developed which is based on the calculation of the probability of a polarized photon being emitted by the complete system of atom + electron. The ambiguity of the O.-P. theory is removed by integration over line profiles, but the expressions reduce to O.-P. expressions when the f.s. or h.f.s. separations are much smaller or much larger than the line width. The Lyot line of hydrogen is an intermediate case for which the line widths and the h.f.s. separations are comparable. Assuming the validity of the Born approximation, a simple expression is obtained which allows the QM to be calculated from the angular distribution of the scattered electrons. Theoretical predictions are compared with experimental results. For the Na£) lines the predicted polarization is small enough to escape experimental detection. Polarizations observed by - Skinner & Appleyard in 1927 for various Hg lines rise to maxima with decreasing electron energy, and then tend to values close to zero at threshold. These experimental results at low energies appear to be inexplicable in terms of the reactions considered, but if the polarization curve above the maximum is extrapolated to threshold, the theory and experiment are found to be in reasonable agreement. Further experimental work is thought to be desirable
$1^{1}S_{0}\ensuremath{-}2^{3}P_{1}$ of mercury excited by plane polarized light shows incomplete polarization both in the absence and in the presence of a weak field parallel to the electric vector. The amount of initial polarization depends upon the relative intensities of the hyperfine structure lines in the exciting light. The polarization with various relative orientations of field and light vector and the variation of polarization with field intensity in weak fields may be interpreted successfully by means of a semi-classical model, with proper relative intensities parallel and prependicular to the light vector, rotating after excitation with an angular velocity geH2mc\frac{\mathrm{geH}}{2mc}2mcgeH​ and emitting a damped wave. From curves connecting depolarization, rotation of maximum of polarization, etc., with field intensity $\ensuremath{\alpha}$, the damping constant, has been found to be 1.02 (\ifmmode\pm\else\textpm\fi{}0.02) 107{10}^{7}107 ${\mathrm{sec}}^{\ensuremath{-}1}$.