The minimum electron energies associated with the excitation of the spectra of helium

Abstract

In view of the recent developments in the theory of the possible stationary states of the helium atom, and the hearing on this theory of recent experimental work on the critical potential differences for the production of radiation and of ionisation in helium, it was thought that a spectroscopic investigation of the excitation of helium under different potential differences might he fruitful. An investigation of the minimum potential differences required for the stimulation of the different lines of the ordinary helium line spectrum, and also of the enhanced lines and the helium hand spectrum, was therefore made under various conditions of gas pressure and intensity of bombarding electron stream. In particular the possibility of exciting lines in the visible region of the spectrum, without the occurrence of ionisation, was examined. The conditions for the appearance of the band spectrum and for its maintenance, were investigated with special reference to the recent suggestion of Franck and Knipping that it originates from He2 molecules which are produced by the combination of pairs of abnormal helium atoms resulting from 20.4 volts electron impacts with normal helium. The experimental results which have been obtained are as follows: 1.(a) There are no genuine differences in the minimumvoltages required for the excitation of different individual lines in the ordinary series spectrum of helium. (b) The minimum voltages at which these lines were everexcited (as distinct from maintained) was 20.4 volts and this only with a high gas pressure and a dense bombarding electron stream. In a large number of observations the lines were notexcited until 25.2 volts (i.e. the ionisation voltage) was reached. (c) Under very special conditions the series lines weremaintained at potential differences as low as 13 - 14 volts. There seems, moreover, to be no reason for supposing that they cannot be maintained below this value. The minimum voltage for the excitation of 4686, a prominent line of the enhanced system of helium, has three differentwell defined values under different experimental conditions. Two of these values, 80 volts, and 54.2 volts correspond respectively to the voltages required theoretically (and confirmed experimentally) for the removal from the helium atom of both electrons simultaneously, and for their removal by two separate electron impacts. The third value, 50.8 volts, corresponds to the energy required by Bohr's theory for the removal of the remaining electron in an already ionised helium atom to orbit 4, the line 4686 being, on the same theory, the radiation which is emitted when the second electron in such an atom falls from orbit 4 to orbit 3.III. (a) The band spectrum of helium is not always present when the ordinary series spectrum is excited though the minimum voltage at which the band spectrum was ever excited was the same as the corresponding minimum for the ordinary series spectrum. (b) The band spectrum was never observed in the absenceof the ordinary series lines, but when these were maintained at voltages below 20.4 volts the band spectrum could be maintained below this value also. (c) The band spectrum became more intense relatively to the ordinary line spectrum as the pressure was increased, and at the same time the lowest voltage at which it could be seen was reduced.The results given under I lead to the conclusion that the occurrence of ionisation is essential for the production of the ordinary line series spectrum of helium. This conclusion is difficult to reconcile with some of the recent conclusions of Franck and Knipping, but it is in complete agreement with the results of the work of McLennan and Ireton, and of Foote and Meggers on certain metallic vapours.In contrast with this the results given under II lead to the conclusion that in the case of the helium positive ion, further ionisation is not essential for the production of the enhanced lines of helium. The results of the investigationof the conditions for the excitation of the enhanced line 4686 are in accord with Bohr's general theory of radiation. The results given under III lead to the conclusion that the hand spectrum originates from some system whose production depends upon the presence of abnormal helium atoms, and they therefore support the view that it originates from He2 molecules. The results of the present research alone afford no particular reason for concluding that the abnormal atoms concerned are those resulting from 20.4 volts electron impacts, but considered in conjunction with the results of a recent investigation by Horton and Davies, they lend strong support to this suggestion.