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Physical Review

The Sensitized Fluorescence of Potassium

Ernst H. Krause

Published Jan 15, 1939• 1 citation
Publisher

Abstract

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 ${\mathrm{K}}_{2}$ 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 $6^{3}P_{1}$ 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 $6^{3}P_{1}$ or $6^{3}P_{0}$ 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.

DOI
10.1103/physrev.55.164

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Physical Review•Jul 15, 1960•9 citations
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.
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.
Sensitized fluorescence in potassium vapor at pressures between 10−5 mm Hg and 10−1 mm Hg was investigated to determine the cross sections for collisions of the second kind between unexcited and excited potassium atoms. An idealized fluorescence tube was used in the experiment and the fluorescent light was analyzed with a photoelectric recording spectrometer. The collision cross sections at low vapor pressures, where imprisonment of radiation is absent, are identical for both the 42P1/2 → 42P3/2 and 42P1/2 ← 42P3/2 transitions and equal 6.6 × 10−12 cm2 at 2 × 10−4 mm Hg. The experimental results are interpreted on the basis of new selection rules for sensitized fluorescence.
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.
Abstract It is known that the fluorescence of mercury vapour can be excited by absorption in the region λ 1850, situated near the resonance line 1 1S0- 1 1P1, and in the region of band absorption λ 2345. It can also be excited by absorption in the region of the resonance line 1 1S0— 2 3P1 2536.52. In this latter case it has, I believe, been generally supposed that fluorescence is not obtained unless excitation is fairly close to the resonance line. Thus Prings-heim and Terenin found that a hot mercury lamp (punktlampe of Heraeus) or an overloaded ordinary 220-volt mercury lamp does not give band fluorescence of any kind. In the hot lamp the core of the resonance line is removed by self reversal. It is, however, well known that unexcited mercury vapour has an absorption band beginning near the resonance line 2537, and extending, if the vapour is dense, to a distance of some hundreds of Ångstroms towards longer waves. The question arises whether this absorption is accompanied by fluorescent emission or not.
Abstract As I have shown in numerous publications, the vapour of mercury in vacuo, even at temperatures below the solidifying point of the metal, when illuminated by the light of a water-cooled quartz mercury lamp, emits the monochromatic radiation of wave-length 2536. According to Bohr’s theory, this resonance radiation results from the fall of an electron from the 2p2 orbit, to which it has been raised from the IS orbit by the absorption of one quantum of 2536 radiation, back to the IS orbit, the orbit occupied by the electron when the atom is in its normal state. An atom in which the electron has been brought to the 2p2 orbit is in a condition to absorb the radiation of any other line produced by the fall of electrons from outer orbits to orbit 2p2, and Fuchtbauer has shown, in an extremely interesting and important paper (‘Physik. Zeit.,’ 1920), that a large number of the arc lines of mercury are radiated by cold mercury vapour illuminated by the mercury arc. He showed that, if the 2536 radiation was screened off by glass, none of the other lines appeared, and explained their presence as the result of the absorption by excited atoms with electrons at the 2p2 level, of the radiations terminating at this level, by which the electrons are carried out to other-orbits, from which they can fall back in various ways, giving rise to emission lines in the process. Though Fuchtbauer‘s experiment did not prove that this process actually took place, his explanation was the only one possible on modern theory, and the experiments which are to be considered in the present paper show that it was correct. Twenty years ago the most that could have been said would have been that the absorption of the 2536 radiation “shook” the atom in some way, causing it to emit its other frequencies as a sort of fluorescence, since Fuchtbauer’s experiment did not show that excitation by lines other than the 2536 line was necessary for the emission of the arc spectrum. To demonstrate experimentally that the step-by-step transfer of electrons from orbit to orbit actually takes place, it is necessary to illuminate the mercury vapour with various combinations of monochromatic light, and study the resultant emission spectrum.
An investigation of the sensitized fluorescence of a mercury thallium mixture without and with the addition of argon and helium gases is discussed. Data taken without the addition of foreign gases are used to extend the theory of Frish and Karaulinya to a mercury thallium mixture. Relative collision cross sections for the excitation of the thallium energy states, 92S129^{2}S_{\frac{1}{2}}92S21​​, 72D52,327^{2}D_{\frac{5}{2},\frac{3}{2}}72D25​,23​​, 82S128^{2}S_{\frac{1}{2}}82S21​​, 62D526^{2}D_{\frac{5}{2}}62D25​​, and 62D326^{2}D_{\frac{3}{2}}62D23​​ were calculated for mercury thallium collisions. Data taken with the addition of argon and helium gases are given to indicate the variation of the intensity of the thallium lines as a function of argon and helium gas pressures at one constant thallium and three mercury temperatures. The explanation of the results depends on the role of mercury 63P16^{3}P_{1}63P1​ excited atoms, metastable atoms, and mercury molecules in collision with thallium atoms for energy exchange, and also requires the use of Winans' partial selection rule and other generally accepted ideas concerning energy transfer, emission, and absorption.