Collisions of Electrons with Atoms (AQA A Level Physics): Revision Note

Ionisation & Excitation

• Electrons in an atom occupy certain energy states called energy levels

• Electrons will occupy the lowest possible energy level as this is the most stable configuration for the atom

• When an electron absorbs or emits a photon, it can move between these energy levels, or be removed from the atom completely

Excitation

• Excitation is:

  When an electron absorbs enough energy to move up to a higher energy level

• When an electron moves to a higher energy level, the atom is said to be in an excited state

  • To excite an electron to a higher energy level, it must absorb a photon

• Electrons can also move back down to a lower energy level by de-excitation 

  • To de-excite an electron to a lower energy level, it must emit a photon

Ionisation

• Ionisation is:

  When an atom gains or loses an orbital electron and becomes charged

• When an electron is removed from an atom, the atom becomes ionised

• An electron can be removed from any energy level it occupies

• However, the ionisation energy of an atom is the minimum energy required to remove an electron from the ground state of an atom

Fluorescent Tube

• Fluorescence occurs when an electron in an atomic orbital absorbs energy from an interaction with a photon or a collision with another electron

• Fluorescent tubes are partially evacuated glass tubes filled with low-pressure mercury vapour with a phosphor coating on the glass

Fluorescent tubes operate on the basis of excitation and de-excitation of electrons leading to the emission of visible light

• When a high voltage is applied across the tube, electrons flow from the cathode to the anode producing an electron beam

• These beam electrons collide with the electrons in the mercury atoms transferring kinetic energy in the collision

• The atomic electrons in the mercury atoms gain enough energy for ionisation to occur

• A high energy state is unstable and so a high energy electron will de-excite i.e. move back to their original ground state

• As it de-excite, the electron releases energy by emitting photons in the UV range of wavelengths

• The UV photons then collide with electrons in the atoms of the phosphor coating and excite them into a higher energy level

• As these phosphor electrons de-excite, they do so in stages emitting photons in the visible light range of wavelengths