Optical factors in the photoemission of thin films.
Expressions are derived for the dependence of the photoemission from thin films deposited on transparent substrates on the direction of illumination, as determined by optical factors alone. For normal incidence, the photoemission of very thin-film transparent photocathodes may be expected to be about 1.4 times as great for illumination from the glass side as for illumination from the vacuum side. Furthermore, if the index of refraction of the film is large compared with that of the substrate, the ratio, for natural light, is also almost independent of the angle of incidence for a wide angular range. For thicker films, both the optical constants of the film and the mean depth of origin of the photoelectrons play an important role. However, for strongly absorbing as well as nearly transparent photoemissive films, substantially higher photocurrent should be obtainable from a photoemissive film of optimum thickness illuminated through the substrate than for a thick film with similar optical constants illuminated from the vacuum side. An assumed simple proportionality between optical stimulus, here given by the square of the field intensity within the photosensitive film, and photoexcitation implies that the photoeffect studied is a volume effect and not dependent on the direction of the electric vector within the film.
Experimental evidence of the angular dependence of electron excitation in photoemission
The dependence of the photoemission from thin layers of trialkali antimonide on the direction of polarisation of obliquely incident light (vector effect) was measured at temperatures between 80 and 300 °K. At low temperatures a decrease of the ratio of polarisation yield is found at increasing light intensities up to I = 1015 photons/cm2 s of 1.9 eV photon energy. The decrease is nearly parallel to the measured decrease of the quantum yield Q of the relation Q ∼ I 1/2. This change is related to an increase of the surface potential of the cathode and to filling of surface traps by photoelectrons. The change of K is describable by decreasing the exit cone of the photoelectrons. The stronger decrease of K than it could be explained by that of Q with I indicates an angular dependence of the excitation process of the electrons in the cathode. A qualitative conclusion of a Compton-like preferential direction somehow perpendicular to the E-vector of the light is given. 
 
 
 
Es wurde die Abhangigkeit der Photoemission aus dunnen Schichten von Tri-Alkali-antimonid in Abhangigkeit von der Polarisation des schrag einfallenden Lichtes (Vektor-effekt) bei Temperaturen zwischen 80 und 300 °K gemessen. Bei niedrigen Temperaturen wurde ein Abfall des Polarisationsverhaltnisses mit steigender Lichtintensitat bis zu I = 1015 Photonen/cm2 s mit 1,9 eV Photonenenergie gefunden. Der Abfall verlauft nahezu parallel mit dem gemessenen Abfall der Quantenausbeute Q mit der Beziehung Q ∼ I−1/2. Die Anderung ist mit einem Ansteigen des Oberflachenpotentials der Katode und dem Fullen von Oberflachenhaftstellen durch Photoelektronen verbunden. Die Anderung von K ist mit der Verkleinerung des Austrittskegels der Photoelektronen erklarbar. Der Abfall von K, der starker ist, als der durch die Abhangigkeit von Q von I erklarbare, weist auf eine Winkelabhangigkeit des Anregungsprozesses der Elektronen in der Katode hin. Eine qualitative Schlusfolgerung auf eine Compton-ahnliche Vorzugsrichtung, etwa senkrecht zum E-Vektor des Lichts, wird angegeben.
Line Arrester Energy Discharge Duties
Surge arresters could practically eliminate lightning flashovers on distribution lines, if they are properly deployed in large enough numbers, and if they could survive the current and energy discharge duties imposed by high-magnitude lightning strokes. The discharge duty depends on complex interactions between the arrester locations, grounding, shielding from nearby objects, and the local lightning environment. But many factors of traditional concern with distribution arrester application, such as the lead length, surge front time, and protective margin, play little or no role in the discharge duty. Probabilistic methods determine the expected arrester failure rate. This should be much lower than the original flashover rate on the line insulation, otherwise the design is not acceptable. A transient program such as EMTP/ATP provides the best tool for calculating the discharge duty, but simplified methods are also available