E2 52 film thickness for tance. In Fig. 6 we show the reflectance at the plasmon was also observed. When the reflected light was detected angle as a function of film thickness in order to estimate with a RCA 1P28 photomultiplier whose output was mea- the optimum film thickness in the following section sured with a voltmeter, the results shown in Fig. 8 were obtained. The minimum in reflected intensity observed in EXPERIMENT AND RESULTS this experiment was comparable to the extinction obtained In this section we describe a simple apparatus for with crossed polarizers evaporating thin silver films, discuss the experimental re- The only experimental technique requiring a little ex- sults, and make suggestions for further experiments. Our perience is coating the correct film thickness. Referring to apparatus which produced a vacuum of 10-3 Torr con- Fig. 6, we see that films either less than 300 A thick or sisted of an Edwards 3-in. diffusion pump connected to a greater than 800 A thick will produce shallow reflectivity Vector 75 rotary roughing pump and a 6-in. bell jar with a No. 8 rubber stopper at its top through which the two electrodes entered the vacuum chamber. A coiled filament basket was made from tungsten or molybdenum wire. A 1.5-cm length of 99.999% silver wire(1-mm diameter) was placed at the bottom of the coil which was attache to the electrodes. Before the prism was placed at the bot- tom of the bell jar, the hypotenuse face was cleaned with reagent grade methanol and rinsed with distilled water. A metal shutter, magnetically coupled to the outside of the ell jar, was placed between the silver and block any impurities that might boil off while the silver was initially heated. A sketch of this apparatus is shown A current of 4-8 A supplied by a variac caused the filament to gradually glow red hot and the silver to slowly melt into a small ball at the bottom of the filament. The shutter was then opened and the coating process begun. After a few minutes of evaporation, the current was turmed off and the pressure inside the bell jar yas returned to atmospheric SHUT TER The prism was then removed from the evaporator and placed on a suitable rotary platform or a student grade spectrometer. A He-Ne laser beam polarized in the plane of incidence by a Polaroid sheet was incident on the prism, and the reflected beam could initially be observed on a white piece of paper. At the critical angle for total internal reflection, the reflected intensity approximately equaled the incident intensity, but as the external angle ASEPLATE of incidence was increased by 2-3, the reflectivity was observed first to decrease sharply and then to increase again. A simultaneous increase in the scattered-light in- Fig. 7. Sketch of simple evaporator apparatus. Prism with hypotenuse tensity from the laser spot on the back silver-air surface face up rests on the base plate 634/Am,J.Phys,.43,No.7,Jwy1975 Simon, Mitchell, and Watson