2.2 electromagnetic radiation and quantum phenomena

• The Photoelectric Effect

  If you shine UV light onto a metal surface, it can emit electrons

  • First observed in 1839 by Alexandre Edmond Becquerel
  • The intensity of the light doesn't affect the quantity of electrons emitted
  • They transfer the same energy
  • Light’s energy is determined by its frequency 
  • E = hf
• • UV light gives the electrons enough energy to leave the surface of the metal
  • Any photon with a frequency less than UV won’t result in the emission of electrons as they won’t have enough energy
  • Electrons closer to the surface of the metal are emitted at a higher speed as they require less energy to be emitted

  
• Work function - minimum energy required to remove one electron from the surface of a metal

  • E = hf = hc 
  • hf0 = + KE

  Threshold frequency - the minimum frequency of electromagnetic radiation required to emit photoelectrons from a metal’s surface
• Conduction electrons

  • Move around randomly with energies ~ 610-21 J
  • Work functions of metals are about 10-19J so electrons don't have enough energy to leave the metal’s surface
  • Dissipated energy isn’t transferred to other electrons

  Stopping voltage - voltage required to just stop the electrons Vs

  • Millikan’s experiment, proves Einstein’s theory
• ElectronVolt - the kinetic energy gained by an electron when accelerated through a potential difference (voltage) of 1 volt

  • Energy = charge x potential difference
  • 1eV = 1.6x10-19J
• Ionisation - when atoms gain or lose electrons to have full outer shells

  
  

  Excitation

  • Gas atoms can absorb energy from colliding electrons without being ionised
  • Happens specific energies called excitation energies
  • Colliding electrons make electrons in atoms move to outer shells
  • If electrons have sufficient kinetic energy in a collision to excite an electron, no energy will be transferred

  Hydrogen excitation

  • Orbital 1 - ground state 
  • 13.6 eV
  • Orbital 2 - first excited level 
  • 3.4 eV
• De-excitation

  • Vacancy in shells means outer electrons drop to lower energy shells
  • The electron emits a photon
  • The photon has an energy equal to the difference between energy levels
• Ionisation and Excitation in a fluorescent tube

  • Electrons in atomic orbital shells absorb energy 
  • interaction with photon
  • collision with another atom
  • Fluorescent tubes are partially evacuated glass tubes filled with low-pressure mercury vapour and phosphor coating on glass
  • When electric current passes through vapour the electrons in mercury are excited and move to higher energy level
  • This is unstable so the electron de-excites and moves to its original state
• • As it de-excites, the electron releases some energy in the form of a UV photon
  • The UV excites electrons in phosphor coating
  • Visible light photons are released when electrons then de-excite
  • This provides the flourescerent glow

  
• Absorption and emission spectra   hf=E1-E2

  • Electrons absorb photons
  • Photons must have exactly the same energy as the energy between the two energy levels to jump up shells
  • Spectrum 400-700nm
  • Continuous - all colours are shown
  • Absorption - Coloured spectrum with black lines where the photons are absorbed

  Emission - black spectrum with coloured lines where photons are emitted

  
• Series/spectrum

  • Lyman series - UV spectrum
  • Balmer series - visible spectrum
  • Paschen series - infrared spectrum

  
• wavelength = hp  wavelength =h / mv 

  De Broglie wavelength =planck's constant / momentum

  • Prince louis de broglie (1924)
  • Matter-wave equation

  Thermionic emission - heating metal causing electrons to be released

  
  

  Wave theory prediction

  • Emission should take place at any frequency
  • Emission would happen at any intensity of light but would take longer at lower intensities
  • Quantity and energy of photoelectrons should be proportional to the light intensity
• Main conclusions

  1. Number of photoelectrons emitted per second is proportional to the intensity of the light
  2. For a given metal, there is a minimum frequency called the threshold frequency (f0), below which there is no emission
  3. The energy of emitted photoelectrons is proportional to the frequency of the incident light
  4. Photoelectric emission occurs without delay as soon as the incident radiation is directed at the surface - no matter how weak the radiation

  • Only conclusion 1 can be explained by light being a wave
  • Conclusions 2-4 can be explained by photons of energy
• Experiment 

  • Beams of electrons directed at a thin metal foil
  • Rows of atoms cause electrons to be diffracted by the same amount from grains of different directions
  • Wave particle duality - exhibits both wave-like and particle-like behaviour