Thesis (Ph. D.)--University of Pennsylvania.

Thesis (Ph. D.)--University of Pennsylvania, 1927.

Thesis (Ph. D.)--University of Pennsylvania.

Thesis (Ph. D.)--Ohio State University, 2005. / Title from first page of PDF file. Includes bibliographical references (p. 182-189).

Thesis (Ph. D.)--Washington State University, May 2007. / Includes bibliographical references (p. 234-257).

Thesis (Ph.D.)--Kent State University, 2008. / Title from PDF t.p. (viewed Oct. 5, 2009). Advisor: D. Mark Manley. Keywords: partial-wave analysis; multichannel; KN scattering; crystal ball. Includes bibliographical references (p. 125-127).

A method is described for investigating the ionisation produced in helium by collisions between quasi-stationary atoms of helium - that is, atoms possessing only thermal velocities - and a beam of neutral helium atoms with kinetic energy less than one hundred electron volts. Ionisation is shown to begin when the kinetic energy of the impinging particle is twice as great as the minimum energy which an electron must possess in order to ionise helium. This value is lower than any which have been reported previously, and agrees with that predicted by a simple theory of collision, in which the impacting atoms are regarded as perfectly elastic spheres. Electrons resulting from the bombardment of metal surfaces by helium atoms were detected, and methods are described by means of which they were distinguished from those produced as a result of ionisation of the gas by atomic impact. Approximate values are obtained for the coefficient of liberation of electrons from nickel, by normally incident helium ions,and for the variation of the secondary electron emission from the same target, when the kinetic energy of an impacting beam of neutral helium atoms is increased from 42 to 70 electron volts. The theoretical implications of the results are discussed. The sensitivity of the apparatus used in this research iscompared with that of the arrangements employed by Varney* and by Rostagni** in similar investigations. Reasons are suggested for the complete failure of the former author to detect ionisation in helium,with impacting helium atoms of kinetic energy up to 400 electron volts, and for the high value reported by Rostagni for the critical energy for ionisation.

General formulations of the quantal theory of collisions between two atomic systems are available and the relevant sections are summarized in Chapter I. The results of Born can then be applied in Chapter II to the problem of electron loss from fast H passing through its parent gas at impact energies lying between 2.5 kev and 10 Mev. The sensitivity of the cross sections for which either of the resultant H atoms is excited to the 2p state, to the choice of bound and free wave functions is examined and the total loss cross section is compared with data on electron loss in H2 and H. The same theory is used in Chapter III to study the excitation and ionization of Lithium by fast electron and proton impact at incident energies up to 1 kev and 1 Mev respectively. The ionization cross sections obtained using Seaton's method (1959) yield comparison material, and the excitation calculation so far tested for sensitivity to choice of orbital parameters. In Chapter IV the rather different methods used for treating slow collisions between electrons and positive ions are employed to gain information on the diffusion and viscosity cross sections and hence on the transport coefficients, in particular the electrical and thermal conductivities. Various numerical and analytical approximations are obtained for these quantities, in the ranges 10 °K - 10 K and 10 -101 per cm3 of temperature and electron number density respectively It is shown that the electrical conductivity passes through a maximum at a temperature which is possibly in the laboratory range for high number densities. The numerical results are summarized in a series of tables.

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.

This thesis is in two parts, the first portion of which describes experiments which were carried out to detect a positive photo-electric emission from potassium. The result of these was that if such an emission occurs it is of the order of 10 times the negative emission at most, i.e. a thousand times as small as the value found by Dember, who claimed to have definite evidence Of a photo-electric effect. The major portion concerns the two or wore threshold frequencies of potassium. A brief historical account is given ending with the work of Richardson and Young in which they predict for the sensitised potassium surface a threshold at 10,000 AU and another at 30,000 AU. Precautions were taken to obtain a clean surface in a high vacuum, the apparatus being of a form which made the saturation of the photo-electric currents easy and the potassium was subjected to approximately black body radiation. Using Richardson's formula for the saturated currents, the work functions, b, and the corresponding threshold frequencies were determined. The values of the frequencies found were 7,100 A.U., 10,000 A.U. 21,000 A.U. A tentative explanation is given of certain Hysteresis phenomena which were observed.