There are two modes of surface plasma waves: 1) Non-radiative modes with phase velocities Cü/k smaller than the velocity of light c. They cannot decay into photons in general.2) Radiative modes with (o/k > c which couple directly with photons 1 .The following paper is concerned with the excitation of these modes by light and their decay into photons. It has been shown that the radiative mode on thin silver-and potassium-films can be excited by light and that the mode reradiates light almost into all directions with an intensity maximum at the plasma frequency cOp 2 . It had been further observed that the non-radiative modes radiate under certain conditions if they are excited by electrons (grazing incidence of electrons on . a rough surface 3 or at normal incidence on a grating 4 ).The mechanism of this emission is in these cases always the same: The "wave vector" of the roughness of the surface or its irregularity changes the plasmon wave vector k so that a) in the case of the radiative mode light emission is found in directions in addition to that of reflexion and transmission, b) in the case of the non-radiative mode its wave vector is reduced so that the condition /c0, the wave vector of the inhomogeneous wave is (co/c) • Vsq' sin 0O (fq = 2.16 for quartz) and thus can excite a non radiative mode on the boundary of the prism for j/fq sin 0O > 1 or 90° > @o > 43°. If one vaporises a silver film directly on the quartz surface the inhomogeneous light wave penetrates into the silver film and excites a non-radiative mode on the boundary silver/air. The excitation will be highest for those frequencies which fulfill the dispersion relation of these surface plasmons.The dispersion relations at the boundary of (bulk) silver/air and silver/quartz are 7 /c 2 = £ + 1 and k 2 = _E Eq £ + £q with e, £q the real parts of the dielectric constants of silver and quartz. These relations are little modified by the fact that the silver has a finite thickness. At a thickness of about 1000 Ä Ag the difference between the exact formula and the formulas given above is not important. Fig. 1 demonstrates the dispersion relations and shows that those surface oscillations on the silver-airboundary can be excited which lie between co/k = c and

The Determination of the Optical Constants of Metals by Excitation of Surface Plasmons A method is given to determine accurately the optical constants and the thickness of thin films when the real and the imaginary part of the dielectric constants obey the condition er < --1, gi <[er I-The method makes use of the possibility to excite surface plasma waves with the help of the inhomogeneous light wave obtained by total reflexion. The accuracy of the method is pointed out. As an example the optical constants of silver foils in the wavelength interval 4000 to 6000 A are determined. A. EinleitungUm den Realteil und den Imagin~irteil der dielektrischen Konstanten er, e~ zusammen mit der Schichtdicke yon dfinnen Schichten genau zu bestimmen, mul3 man eine Mel3methode verwenden, bei der die MeBwerte empfindlich und in verschiedener Weise von drei genannten Parametern abh~ingen.In ~ ist gezeigt, dab in der Umgebung der Plasmafrequenz yon Metallen (e~0) die Transmission von parallel zur Einfallsebene polarisiertem Licht bei verschiedenen Winkeln gtinstige Funktionen in diesem Sinne sind. Man nutzt dabei das Phgnomen der ,,Plasmaresonanzabsorption" der ,,strahlenden" Oberfl~ichenschwingungen aus. Wie man auch die Resonanzabsorption der zweiten Art der Obeffl~ichenplasmaschwingungen, der sog. ,,nicht strahlenden", benutzen kann, um sehr genaue optische Konstanten zu erhalten, wird in dieser Arbeit angegeben. In ghnlicher Weise kann man auch durch Streulichtmessungen die Plasmaresonanzemission dazu benutzen, genaue optische Konstanten zu bestimmen 2.Die nicht strahlenden Oberfl/ichenschwingungen k6nnen an der Grenzfl~iche zwischen MetaU und Vakuum auftreten, wenn die komplexe Dielektrizit/itskonstante des Metalls e=er+iel der Bedingung gentigt ~,<-1, ~e< I~1. Sie sind dadurch charakterisiert, dab die Komponente

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