energy levels +photon emission

• 1 eV = 1.60 × 10–19 J
• Energy Levels and Photon Emission

  1. Fluorescent tubes contain mercury vapour, across which an initial high voltage is applied

  2. Free electrons collide with mercury atoms, exciting electrons

  3. Excited electrons emit photons in the UV range

  4. Phosphor coating absorbs photons, emitting visible light
• Fluorescent Tubes


  • Use excited electrons to produce light
  • Produce line emission spectra
• Fluorescent Tubes Produce Line Emission Spectra


  1. Split light from a fluorescent tube with a prism

  2. A line spectrum is seen as bright lines against a black background

  3. Each line corresponds to a particular wavelength of light emitted

  4. Only certain photon energies are allowed
• Quantum theory doesn’t really make much sense — to anyone. It works though, so it’s hard to argue with.
• Electrons in Atoms

  • Exist in discrete energy levels
  • Can only exist in certain well-defined energy levels
• Electron Transitions


  1. Electrons can move down energy levels by emitting a photon

  2. Energy of each photon emitted can only take a certain allowed value

  3. Electrons can move up energy levels if they absorb a photon with exact energy difference

  4. Movement to a higher energy level is called excitation

  5. If an electron is removed from an atom, the atom is ionised
• The ground state is the lowest energy state of the atom.
• The energy carried by each photon is equal to the difference in energies between the two levels.
• The ionisation energy of an atom is the amount of energy needed to completely remove an electron from the atom from the ground state (n = 1).
• The energies are only negative because of how “zero energy” is defined.
• Absorption Spectrum

  1. Shining white light through a cool gas gives an absorption spectrum

  2. Spectrum of white light is continuous

  3. Hot things emit a continuous spectrum

  4. Electrons in gas atoms absorb photons to excite them to higher energy levels

  5. Wavelengths are missing from the continuous spectrum
• You see a continuous spectrum with black lines in it corresponding to the absorbed wavelengths.
• If you compare the absorption and emission spectra of a particular gas

  The black lines in the absorption spectrum match up to the bright lines in the emission spectrum
• Cool gases remove certain wavelengths from the continuous spectrum.
• The kinetic energy gained by an electron (eV) = accelerating voltage (V).
• electron can exist on an energy level without losing energy it has the energy of the energy level
• bohr model

  electrons exist in energy levels but not inbetween

  A) valence state

  B) ground state

  
• for a electron to move up levels it needs to gain energy this is done by collisions from delocalised electrons
• for electron to move down levels it must give energy to surroundings this is done by emitting photons

  
• for electron to jump up a level it needs the exact amount of energy from a photon

  but from colliding electrons it can have more than exact amount to cause excitation
• excitaion is process of moving an electron up an energy level
• excitation is proof of particle nature of light
• ionisation

  electron is accelerated by potential difference and collides with an atomic electron transferring sufficient energy to release the electron and ionise the atom
• threshold voltage is the minimum voltage required to initiate a current flow of photoelectrons
• excitation by collision with another electron only happens when energy received is exact energy difference
• excitation by photon absorption must have exact energy because the electron switch frequencies
• what is this?

  no breaks

  visible light through it

  A)

  B)

  
• cold gas creates absorption spectrum

  and emissions gas or hat gas creates emission spectrum

  A)

  B)

  C)

  
• You get a line absorption spectrum when light with a continuous spectrum of energy (white light) passes through a cool gas. 2) At low temperatures, most of the electrons in the gas atoms will be in their ground states. 3) The electrons can only absorb photons with energies equal to the difference between two energy levels. 4) Photons of the corresponding wavelengths are absorbed by the electrons to excite them to higher energy levels
• 5)These wavelengths are then missing from the continuous spectrum when it comes out the other side of the gas.

  6) You see a continuous spectrum with black lines in it corresponding to the absorbed wavelengths.

  7) If you compare the absorption and emission spectra of a particular gas, the black lines in the absorption spectrum match up to the bright lines in the emission spectrum.
• absorption spectra caused by excitation (absorption of photons)
• emmission spectra caused by de-excitation emmitting photons
• both absorbtion and emmision spectra is proof od particle nature of light