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Chem 155 <strong>Final</strong> review topics and questions: c) Describe how you might prepare a 0.500 ppm calibration standard from a 1000.0 ppm standard solution. a. Pipet 1.00 mL and dilute to 100.0 mL 1000 ppm 10 ppm b. Pipet 1.00 mL and dilute to 20.0 mL 10 ppm * 1/20 0.500 ppm List 3 desired properties of a light detector. a. High sensitivity b. Low noise c. High quantum efficiency d. High speed 2. Sketch a photomultiplier tube and label the important parts a. See notes 3. How many dynodes are needed for a PMT to produce at least 1,000,000 electrons per photoelectron Assume that the secondary electron yield is 4. n 10 6 4 n ln ln 10 6 2.303log 10 6 n ( 2.3036) ln( 4) n 9.968 10 dynodes 4 10 1.049 10 6 4. Is it possible to detect near infrared radiation (wavelength = 1500 nm) with a photomultiplier tube using a Na photocathode The work function of Na is 2.75 eV. h 6.62610 34 J s c 3.0010 8 m s h c 1eV E 150010 9 m 1.60210 19 J 150010 9 m eV 1.60210 19 J E 0.827eV Since E the photoelectric effect is not promising for detection!
Chem 155 <strong>Final</strong> review topics and questions: 5. By what basic mechanism does the photodiode produce an electric current when light strikes it The photodiode is reverse-biased. This means that without light, no current flows. The PN junction of the photodiode is exposed to light. When light strikes the PN junction it may create electron-hole pairs. The bias will then sweep apart the electron-hole pairs and convert the light into an electrical current. 6. What happens at a p-n junction when a reverse-bias is applied Draw a diagram to support your answer. Why are photodiodes operated in reverse bias + + + + + - - - - The photon (red arrow) strikes the photodiode in the depletion region and an electron-hole pair is generated. The electron and hole (red circles) are swept apart by an electric field. In reverse bias, the diode produces zero current in the dark. Low or zero dark current is a good quality in a photo-detector. 7. What is the maximum gain of a photodiode transducer Is this better than a phototube Photodiode max gain = 1 = equal to the phototube max gain.
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