Photoelectric effect

Transcription

1 Experimental Physics EP3 Atoms and Molecules Photoelectric effect energy quantization, photons Experimental Physics III - Photoelectric effect 1

2 Light-matter interaction Light absorption and radiation is not a continuous process, but is quantized, i.e. occurs in some finite portions or quanta. E h Planck: Light propagation is described by the classical Maxwell equations, the quantization occurs only during the interaction events with matter. Einstein: Light may considered as a collection of particles With the energies given by the Planck s equation. Relativistic mechanics: 2 2 E / c p m c 2 m photon =0 0 E pc m m 0 v / 2 1 c Number of photons is not conserved Energy and momentum conservation Free electron can scatter photon, but cannot absorb or emit photon. Planck's constant = m 2 kg / s Experimental Physics III - Photoelectric effect 2

3 Photoelectric effect Hertz helped establish the photoelectric effect (which was later explained by Albert Einstein) when he noticed that a charged object loses its charge more readily when illuminated by ultraviolet light. In 1887, he made observations of the photoelectric effect and of the production and reception of electromagnetic (EM) waves, published in the journal Annalen der Physik. Upon irradiating negatively charged metallic objects by an ultraviolet light the object looses its negative electric charge. Neutral objects can be charged to a potential of a few volts. From 1888 to 1891: Stoletov invented a new experimental setup which was more suitable for a quantitative analysis of photoeffect. Using this setup, he discovered the direct proportionality between the intensity of light and the induced photo electric current (the first law of photoeffect or Stoletov's law). Based on this effect, he built the first solar cell. Soon after the discovery of electron in 1897, it has been shown that the ratio e/m for the particles forming photoelectric current is the same as for those forming cathode rays, i.e. they are electrons.. Experimental Physics III - Photoelectric effect 3

4 Photoelectric effect I Element Work Function (ev) Calcium 2.9 Iron 4.5 Platinum 6.35 Zinc 4.3 V A e The work function required to remove an electron to a close vicinity above the surface. Experimental Physics III - Photoelectric effect 4

5 Response time P = 100 W L = 1 m E max 4 P L 2 t Maximal energy obtained by an atom. E e E max To estimate assume that E e E max To remove an electron from the metal d 2 M N A 2 / 3 E max 610 A 20 m 2 But the photoelectric effect is non-inertial! t A 4L P s Zinc: M = 65 g/mol; = 7 g/cm 3 ; Experimental Physics III - Photoelectric effect 5

6 2 2 E / c p m c 2 0 Compton effect X-Ray pc + m 0 c 2 = p s c + E e p = p s + p e λ s = λ + h m 0 c 1 cos θ λ K h m 0 c m Experimental Physics III - Photoelectric effect 6

7 To remember! Light has properties of both particles and electromagnetic waves the wave-particle duality principle. The emw particles are called photons and their energy is given by E = h. Photons are massless, but they have a momentum which is related to their energy E = pc. In photoelectric effect, photons are absorbed by electrons and thus increase their kinetic energy. If the kinetic energy is higher than the work function, photoelectric current is observed. The Compton effect is the change of the wave length due to scattering by a particle (electron). Experimental Physics III - Photoelectric effect 7