838 BELL SYSTEM TECHNICAL JOURNAL 



The validity of Einstein's equation precludes the possibility of ex- 

 plaining the new maximum in the spectral response curve for a treated 

 surface by a "Raman shift" of the incident light frequencies, even 

 though the separation of these maxima is equal to certain well-known vi- 

 bration-rotation frequencies of the dielectric molecules. It may be that 

 the natural frequency of the alkali metal atom is diminished by the 

 vibration frequency of the complex atom in which it is incorporated. 



The Lindemann formula for the frequency of the selective photo- 

 electric maximum [Iw = (ne'^/mr^)^, primitive though it seems in the 

 light of modern theory, has always given values for the pure metals in 

 close agreement with experimental determinations. The n term is 

 determined by the valence of the substance, a choice of unity being 

 used for the monovalent alkali metals corresponding to an electron 

 revolving around a singly charged ion. A choice of 2, 3, — for divalent, 

 trivalent, — substances corresponds to electrons revolving around 

 doubly, triply-charged ions. Under certain conditions the alkali 

 metals manifest different valencies, such for instance, as those exhibited 

 in the oxide series Na202, Na^O, NasO, Na40. These compounds can 

 be prepared in vacuum and are light-sensitive. Spectral response 

 curves for such cells exhibit all the selective maxima always separated 

 from it by the frequency of a well-known line in the vibration-rotation 

 spectrum of the dielectric molecules, usually the 1.5ju line so charac- 

 teristic of oxygen-hydrogen, carbon-hydrogen or nitrogen-hydrogen 

 linkages. The long wave limit shifts an amount agreeing with the 

 separation of the maxima. 



With a cell so designed that the cathode could be sensitized in a side 

 chamber and then slipped into its proper place (thus keeping the anode 

 free from light-sensitive materials), stopping potentials were obtained 

 for electrons, liberated by monochromatic light, from a sodium cathode 

 before and after treating it with sulphur vapor and air. For light of 

 wave-lengths ranging from X35C0A to X8000A falling on the treated 

 cathode, the electron retarding potentials are found to vary linearly 

 with the frequency of the exciting light, thus establishing the validity 

 of Einstein's photoelectric equation for composite surfaces. From 

 the slope of the straight line depicting this relationship, the value of 

 Planck's constant h is found to be 6.541 X 10^", significant to three 

 figures. An almost identical value is obtained for untreated sodium. 

 The apparent stopping potentials, or voltages at which the photo- 

 electric currents become zero are the same before and after the sulphur 

 and air treatment. The voltage at which the current just saturates 

 is always greater after treatment than before. This is a measure of 

 the change in contact potential of the cathode called for by the Linde- 



