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Physical Review

The Excitation of Sodium by Ionized Mercury Vapor

Harold W. Webb, S. C. Wang

Published Mar 1, 1929• 5 citations
Publisher

Abstract

Excitation of sodium by excited and by ionized mercury vapor.---The excitation of the sodium spectrum by ionized mercury distilling from an arc was studied in a tube in which conditions were under control. An auxiliary electrode in the stream of mercury vapor was used to increase the number of excited atoms at the point of observation until the excitation due to them predominated over that due to the ions. The spectrum produced by excited mercury atoms was thus compared with that due to mercury ions. The spectrum due to the excited atoms showed the marked "resonance" effect shown earlier by Beutler and Josephy. The spectrum due to mercury ions also showed excitation corresponding to the excitation energy of the metastable mercury atom. This may be accounted for as due to metastable atoms formed as the result of recombination of mercury ions. An alternative explanation is that the excitation of a sodium atom by a mercury ion is by a three body impact involving a mercury ion, a sodium atom and an electron. This was suggested by the fact that when sodium is introduced into the ionized mercury there is a strong enhancement of the mercury lines originating at the two $3S$ levels, which seems to be best explained as due to three body impacts. It seems probable that both this and the above process are of the same nature. It was noted further that the spectrum excited by ionized mercury resembles the chemiluminescence spectrum resulting from mixing sodium and mercuric chloride vapors. The spark line of mercury 3984, not previously found in the luminous mercury stream, was found when sodium was present.

DOI
10.1103/physrev.33.329

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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.

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Efficient laser ionization of sodium vapor—A possible explanation based on superelastic collisions and reduced ionization potential

Raymond M. Measures

The recently observed efficient ionization of sodium vapor by a laser tuned to the sodium resonance line can be explained in terms of the large population of resonance-state atoms created by laser-induced radiative equilibrium. This large population of excited atoms represents (a) a reservoir of energy that can rapidly be transferred to the free electrons via superelastic collisions and (b) a source of pseudo-ground- state atoms possessing a much reduced ionization energy. Three-photon ionization is proposed as the mechanism for creation of the primary pool of free electrons.

Journal of Applied Physics•Jul 1, 1977•78 citations
Excitation of sodium by excited and by ionized mercury vapor.---The excitation of the sodium spectrum by ionized mercury distilling from an arc was studied in a tube in which conditions were under control. An auxiliary electrode in the stream of mercury vapor was used to increase the number of excited atoms at the point of observation until the excitation due to them predominated over that due to the ions. The spectrum produced by excited mercury atoms was thus compared with that due to mercury ions. The spectrum due to the excited atoms showed the marked "resonance" effect shown earlier by Beutler and Josephy. The spectrum due to mercury ions also showed excitation corresponding to the excitation energy of the metastable mercury atom. This may be accounted for as due to metastable atoms formed as the result of recombination of mercury ions. An alternative explanation is that the excitation of a sodium atom by a mercury ion is by a three body impact involving a mercury ion, a sodium atom and an electron. This was suggested by the fact that when sodium is introduced into the ionized mercury there is a strong enhancement of the mercury lines originating at the two 3S3S3S levels, which seems to be best explained as due to three body impacts. It seems probable that both this and the above process are of the same nature. It was noted further that the spectrum excited by ionized mercury resembles the chemiluminescence spectrum resulting from mixing sodium and mercuric chloride vapors. The spark line of mercury 3984, not previously found in the luminous mercury stream, was found when sodium was present.
A mixture of potassium vapor and mercury vapor contained in a tube fashioned after the Cario-Lochte-Holtgreven sodium vapor lamp was radiated by light from a water-cooled mercury arc. The resulting fluorescence radiation contained potassium lines as well as two of the three known K2{\mathrm{K}}_{2}K2​ absorption bands and a new band with maximum at 5721A. On exposure of the mixture to light from a hot mercury arc or to light having all wave-lengths shorter than 2800A filtered out, the fluorescence was eliminated. This indicated that the excitation was caused by absorption of 2536A by the mercury. Absorption of the 2536A mercury line raises the mercury atom to the 63P16^{3}P_{1}63P1​ state, having an energy of 4.86 volts. The ionization potential of potassium is 4.32 volts; thus in a collision of the second kind, the excited mercury atom has enough energy to ionize the potassium atom. There was no evidence of a potassium emission from an energy state in the continuum equal in energy to the Hg 63P16^{3}P_{1}63P1​ or 63P06^{3}P_{0}63P0​ states. There were, however, two strong lines in fluorescence which could not be ascribed to the usual K energy levels nor to any probable impurity. To test whether or not ionization of potassium actually takes place, a filament was introduced into the tube to supply electrons. A large increase in intensity of the fluorescence of potassium lines was obtained with the hot filament. It was furthermore found that the intensity increased with filament temperature.
Collisional transfer of excitation from mercury to sodium was investigated using methods of sensitized fluorescence. A mixture of mercury and sodium vapors at low pressure was irradiated with Hg 2537 Å resonance radiation, producing a population of Hg(63P1) atoms whose inelastic collisions with ground-state sodium atoms resulted in a transfer of excitation energy to close-lying S, P, and D states in sodium. The subsequent decay of these states manifested itself in the emission of a sensitized fluorescent spectrum. Absolute intensity measurements on the components of the spectrum yielded 21 cross sections whose magnitudes range from 0.02 to 38.5 Å2 and which exhibit a pronounced resonance with ΔE, the energy defect between Hg (63P1) and the appropriate level in sodium.
The effect of foreign gases upon the fluorescence of sodium vapor excited by the second line of the principal series has been investigated. The gases used were helium, nitrogen, and hydrogen and the intensities of both the 3303A line and the DDD light were observed. From the variation in intensity of the 3303A line as gas is added it is concluded that the presence of an inert gas has no appreciable effect upon the relative transition probabilities in the 32P{3}^{2}P32P state of sodium. With nitrogen or hydrogen as the added gas the DDD light intensity variation followed that of the 3303A line very closely but with helium a marked increase in intensity was produced by the first gas added with a decrease at higher pressures. This increase was explained as due to a transfer of sodium atoms from the 32P{3}^{2}P32P to the 32D{3}^{2}D32D state by collisions, thus increasing the number of atoms which radiate the DDD light. In addition to this increase in number the collisional transfers produced an increased "Doppler width" in the DDD light which persisted up to pressures well over 100 mm of helium and caused a decreased absorption of the fluorescence light. The bearing of this work on the difference in the intensity ratio of the ${2}^{2}S\ensuremath{-}{3}^{2}P$ and ${1}^{2}S\ensuremath{-}{3}^{2}P$ transitions as reported by Weiss for an arc and by Christensen and Rollefson in fluorescence is discussed. It is concluded on the basis of experimental evidence that any explanation of the difference which depends upon multiplicity of the 32P{3}^{2}P32P state is incorrect. An explanation of this difference based upon the assumption that the life period of an excited atom varies inversely as the transition probability of the radiation emitted is presented. It is shown that recent work on the life periods of atoms in the 23S1{2}^{3}{S}_{1}23S1​ state of mercury at least partially supports this assumption. A new type of sodium arc with increased intensity and stability of operation is described.
The production of Na+{\mathrm{Na}}^{+}Na+ and Na2{\mathrm{Na}}_{2}Na2​+{\mathrm{}}^{+}+ in sodium vapor irradiated by pulsed laser light in the D-line region of the spectrum has been studied by varying the laser power density and the wavelength. The data, which were acquired with an order-of-magnitude higher resolution than those in our previous experiments, support our earlier conclusion that ions formed at laser wavelengths close to, but not coincident with, the D lines result from three-photon resonance-enhanced ionization of Na2{\mathrm{Na}}_{2}Na2​ molecules present in the vapor.
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.
The recently observed efficient ionization of sodium vapor by a laser tuned to the sodium resonance line can be explained in terms of the large population of resonance-state atoms created by laser-induced radiative equilibrium. This large population of excited atoms represents (a) a reservoir of energy that can rapidly be transferred to the free electrons via superelastic collisions and (b) a source of pseudo-ground- state atoms possessing a much reduced ionization energy. Three-photon ionization is proposed as the mechanism for creation of the primary pool of free electrons.