Including Ionizing and Radiating Potentials and Related Effects BY ARTHUR LLEWELYN HUGHES

Research Professor of Physics, Queen's University, Kingston, Canada

PREFACE

The writing of this report on the recent progress in photoelectricity was suggested by the Photo-Electric Committee of the Division of Physical Sciences of the National Research Council. The author wishes to express his appreciation of the valuable suggestions given by the other members of the Committee, viz., Professors R. A. Millikan, K. T. Compton, J. Kunz, and C. E. Mendenhall, to whom the report was sent for comment. Thanks are also due to Professors Compton and Mendenhall for supplying periodicals and summaries of articles, not accessible to the author, and to the authorities of Queen's University for giving ample opportunity for the preparation of the report.

II. Ionization of gases and vapors by light..

86

Fluctuations in the photo-electric threshold. Dependence on the

V. The photo-electric effect as a function of the frequency

and state of polarization of the light. Richardson's statistical theory.

110

110

112

115

119

VII.

Photo-electric effects of non-metallic elements and in

organic compounds..

122

VIII. Photo-electric effects of dyes, fluorescent and phos

phorescent substances.

124

IX. Positive rays produced by light.

X. Sources of light used in photo-electric experiments....

125

125

V. d. D. P. G. Verhandlungen der Deutsche Physikalische Gesellschaft.

127

135

139

150

153

158

165

INTRODUCTION

The principal monographs which have hitherto appeared on photo-electricity are the following:

R. Ladenburg: In the Jahrbuch für Radioaktivität, 1909.

H. S. Allen: "Photo-Electricity" (Longmans), 1913.

A. LI. Hughes: "Photo-Electricity" (Cambridge University Press), 1914.

R. Pohl and P. Pringsheim: "Die lichtelektrische Erscheinungen" (Vieweg), 1914. R. v. Schweidler: "Photo-Elecktrizität" (in Graetz' "Handbuch der Elektrizität

und des Magnetismuss" Barth), 1914.

W. Hallwachs: "Die Lichtelektrizität" (published privately).

The present report gives an account of the progress which has been made since 1913. It may, therefore, be regarded as supplementing, and bringing up to date, Allen's "Photo-Electricity" and Hughes's "Photo-Electricity." The division into chapters is that followed in the author's "Photo-Electricity." A chapter has been added on the new and most interesting group of investigations which may, for want of a shorter title, be designated as "Ionizing and Radiating Potentials and Related Effects." This subject, which in many aspects is so closely related to photoelectricity that it is natural to consider it a part of photo-electricity in an enlarged sense, has attracted an immense amount of attention from both theoretical and experimental physicists in recent years. Certain investigations, frequently designated as photo-electrical, because they deal with the change of resistance of materials like selenium under illumination, have not been touched upon. These investigations seem to have little in common with those dealt with in the report; it would be well if some such title as "photo-resistance effects" could be used for them to distinguish them from the effects commonly classed as photo-electric effects.

Perrin has recently published a remarkable paper1 in which he suggests the important and possible decisive rôle played by radiations in determining chemical reactions, fluorescence and phosphorescence, radioactivity, cosmical evolution and changes of state. According to this theory, the energetics of every change in the configuration of a structure from state A to state A' may be represented by equations of the type A + hv = A' + hv', where and are the radiation frequencies which change A to A' and A' into A, respectively, and h(v — v') is the heat of reaction. There is considerable direct support for this theory and it leads to a satisfactory explanation of certain facts of velocity of chemical reactions which were not explained on the older kinetic theory. Recent calculations by Langmuir prove that Perrin's theory cannot be applied to many cases of molecular dissociation in the simple form proposed, but suggest some additional source of energy for which the light, or the collision, may act as a releasing "trigger." Yet there is, in the theory, a suggestion that photoelectric action, in an enlarged sense, may be one of the most fundamental and important occurrences in nature.

1 A. d. P., 11, 5-108 (1919).

2 J. A. C. S., 42, 2190 (1920).

Throughout the report, it will be often necessary to make use of the quantum relation Ve = hv=hc/λ. It is generally convenient to express V in volts and λ in Ångstrom units, in which case the numerical relation between them is given by:

IONIZATION OF GASES AND VAPORS BY LIGHT

Comparatively little has been done on this subject since 1913, although it was obvious that the subject was very incompletely covered. The technical difficulties are very formidable. In the first place, most gases, if they can be ionized at all, require light of such short wave-length to effect ionization, that very special arrangements have to be made to secure the right kind of light. Fluorite, the most transparent of known substances, begins to absorb strongly just in the region of wave-lengths where ionization of air begins. As it is desirable to have a window between the source of light and the ionization chamber, it becomes very difficult to find a window transparent to the active light. Another obstacle to securing reliable results is that in the region where the ionization of gases begins, the photo-electric effect of metals is enormous, and it becomes a problem how to disentangle the small ionization in the gas from the very large photo-electric effect of the electrodes (even when apparently well shielded) which are generally necessary to separate the ions in a gas.

Ionization of Air. Hughes1 had obtained results which indicated that air could only be ionized by wave-lengths shorter than about X 1350. From time to time, various investigators (e. g., Bloch2) had obtained results which seemed to show that a weak ionization of air could be obtained by intense light from a mercury lamp (long wave-length limit probably λ 1800). In view of the spurious effects of slight traces of impurities, dust particles, nuclei, etc., so clearly shown in the extensive investigations of Lenard and Ramsauer, it may well be that the slight ionization observed with 1 Proc. Camb. Phil. Soc., 15, 483 (1910).

3

C. R., 1912, 903, 1076.

Ber. d. Heid. Akad., 1910-1911.

ultraviolet light from a mercury lamp is not due to a real ionization of the air molecules themselves, but to some obscure subsidiary effect.

Ionization of Alkali Vapors. Theoretical considerations indicate that the vapors of the alkali metals should be ionized by light of much longer wave-length than is necessary to ionize air. Against this advantage is to be set the difficulties of working with the alkali metals, the necessity for working at high temperatures to secure an appreciable vapor pressure, and the fact that, if the electrodes become covered with a film of the metal, their photoelectric effect becomes enormous and may well mask any ionization effect.

Gilbreath' obtained results which were interpreted as indicating a true ionization of potassium vapor at temperatures not over 65° C. The light used was that from an arc, or from a 500-watt lamp, filtered through glass (shortest wave-length transmitted, probably about λ 3300). Kunz and Williams2 recently found that caesium vapor was ionized by light of wave-length x 3190, and that wave-lengths longer than this were quite ineffective. Special care was taken to ensure absence of surface effects.

Discussion. In view of the remarkable utility of the quantum theory in linking up facts in other regions of photo-electricity one naturally looks to it for confirmation of some of the results obtained in ionization of gases by light. Other things being equal, one may perhaps be justified in accepting, tentatively at any rate, those results which link up best with the quantum theory, rather than those which have no obvious connection.

We know that gas molecules, struck by electrons, give out radiation when the energy of the electrons exceeds a certain critical value and become ionized when the energy exceeds another critical value. The potentials associated with these critical values are called radiating potentials and ionizing potentials, respectively. As will be shown in a later chapter, the radiating potential VR is related to the frequency R of the radiation emitted, as follows:

when h, e, m and VR are Planck's constant, the charge on and mass of an electron, and the velocity of the electron acquired from the fall of potential VR. The ionizing potential VI is related to a V1

1 P. R., 10, 166 (1917).

2 To be published shortly.