P3 quantum phenomena

• the photoelectric effect is when photoelectrons are emitted from the surface of a metal after light above a certain frequency is shone onto it
• the threshold frequency is the minimum frequency required for the photoelectric effect to take place, it is different depending on the metal
• the photoelectric effect shows that light cannot be a wave because:

  • photoelectrons are not emitted if the light is under the threshold frequency, so wavelength must be less than a maximum value as lambda = c / f
  • number of photoelectrons emitted per second is proportional to the light intensity
  • the emission of photoelectrons occurs immediately as light is shone on the surface
• if the wave theory was correct, any frequency of light should be able to cause photoelectric emission because the energy absorbed by each electron will gradually increase with each wave
• the photon model of light is that:

  • light travels in discrete packets called photons, their energy is directly proportional to their frequency
  • each electron can only absorb a single photon, so a photoelectron is only emitted if the frequency is above the threshold frequency
  • if light intensity is increased, number of photoelectrons emitted per second increases
• the work function of a metal is the minimum energy required for electrons to be emitted from the surface of a metal
• work function is represented by $\phi$
• the stopping potential is the potential difference you would need to apply across the metal to stop the photoelectrons which have maximum kinetic energy
• stopping potential is represented by $V_s$
• wavelength is represented by $\lambda$
• measuring stopping potential allows you to find the maximum kinetic energy of photoelectrons, by using the equation Ek(max) = eVs
• the photoelectric equation is E = hf - phi

  emission can only take place if Ek(max) > 0 or if hf > phi
• the electrons in a metal move about at random, like the molecules of a gas
• the average kinetic energy of an electron in a metal depends on the temperature of the metal
• the work function of the metal is the minimum energy needed by an electron to escape from the metal surface when the metal is at zero potential, it should be to the order of 10^-19 Joules
• when an electron absorbs a photon, its kinetic energy increases by an amount equal to the energy of the photon, if this exceeds the work function of the metal then the electron can leave, if it does not exceed the work function then the electron cannot leave, and collides with other electrons and ions so quickly loses the energy
• an ion is a charged atom, it has gained or lost electrons so the number of electrons is not equal to the number of protons
• the process of creating ions is ionisation, examples include alpha, beta and gamma radiation passing through a substance, and electrons passing through a fluorescent tube
• ionisation energy is the energy required to remove an electron from an atom to form an ion
• the electron volt is a unit of energy equal to the work done when an electron is moved through a potential difference of one volt, represented by eV
• to convert from J to eV: divide by 1.6*10^-19
• to convert from eV to J: multiply by 1.6*10^-19
• excitation is the process of electrons gaining energy from collisions, so moving up energy levels
• excitation energy is the energy required for the electron to move from an inner shell to an outer shell, it is always less than ionisation energy as the electron is not completely removed
• the electrons in an atom stay there because of the electrostatic forces of attraction of the nucleus, they move about the nucleus in orbits or shells
• the energy of an electron in a shell is constant
• an electron in a shell near the nucleus has less energy than an electron in a shell further away from the nucleus, as more energy is required to prevent it from leaving
• the lowest energy state of an atom is called the ground state
• when an atom in the ground state absorbs energy, one of its electrons moves to a shell with a higher energy, so the atom is now in an excited state
• an energy level diagram shows the possible energy values of an atom
• an energy level diagram which uses excitation energy would have positive values and the ground state as zero
• an energy level diagram which uses ionisation energy would have negative values and the ground state as the most negative value
• de-excitation is the process of the gap left by an excited electron being filled by another electron, which emits a photon so that it has the required energy to move to a lower level

  the energy of the photon is equal to the energy lost by the electron
• when an electron moves from energy level E1 to energy level E2, the energy of the emitted photon is shown by hf = E1 - E2
• an electron in an atom can absorb a photon and move to an outer shell with a vacancy, but only if the energy of the photon is equal to the difference between the energy levels, if it is smaller or larger then the photon will not be absorbed by an electron
• fluorescent tubes are glass tubes filled with mercury vapour which produce light when a high voltage is applied across them
• fluorescent tube process:

  • the high voltage accelerates free electrons through the tube which collide with the mercury atoms
  • mercury atoms become ionised and release more free electrons
  • free electrons collide with the mercury atoms, causing them to become excited
  • mercury atoms deexcite, releasing photons, most of which are in the UV range
  • the fluorescent coating on the inside of the tube absorbs the UV photons
  • electrons in the atoms of the coating become excited, and deexcite releasing photons of visible light
• if you pass the light from a fluorescent tube through a diffraction grating or prism, you get a line spectrum where each line represents a different wavelength of light emmitted by the tube
• a line spectrum is discrete not continuous so each line represents the corresponding photon energy which is required for this wavelength to be emitted
• line absorption spectra can be produced by passing white light through a cooled gas, this produces a continuous spectrum of light with black lines at certain wavelengths