Collisions of Electrons with Atoms

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Ionisation and Excitation

Both electrons being removed from an atom and the movement of electrons within energy levels in the atom explain commonly observed phenomena.

Ionisation

To ionise an atom means to remove or add electrons to an atom.
Ionisation in the context of quantum phenomena almost always means the removal of electrons completely from the atom.
The energy needed for an electron to go from the ground state to being completely removed is called the ionisation energy.

Excitation

Electrons can also be made to move from one energy level to the other.
To do this, the electron needs exactly the energy difference between energy levels.
One way to do this is for a photon to interact with the electron.
The electron will absorb all the energy of the photon.

Excitation 2

If the energy of the photon is exactly the energy difference between the energy levels, the electron will move up the energy levels.
When an electron has moved up energy levels we called the electron "excited".
The process of making the electron move up energy levels is called "excitation".

Fluorescent Tubes

It is important to understand how fluorescent tubes work.

Constituents of the tube

A fluorescent tube has mercury vapour inside it.
A fluorescent tube is coated on the inside with phosphor.
There are free electrons in the fluorescent tube.
- A high voltage will accelerate these free electrons.

Excitation of electrons

The high-energy free electrons then ionise the mercury vapour.
This means that there are more free electrons than previously.
The free electrons then excite electrons in the mercury atoms.

Release of photons

The excited electrons then move back to their ground state.
When they do so, they release the excess energy in the form of photons.
- These photons are high-frequency, high-energy, ultraviolet photons.

Absorption of high-energy photons

The ultra-violet photons then collide with the phosphor coating on the inside of the tube.
The electrons in the phosphor are now excited.

Release of visible light photons

The excited electrons then eventually move back to their ground state. When they do so, they release the excess energy in the form of photons.
These photons have a frequency in the visible range.

The Electron Volt

When dealing with energy levels of individual atoms, the SI unit of energy (the Joule, J) is far too big. So we need to define a more appropriate unit of energy, the electron volt, eV.

Definition of electron volt

The electron volt is defined as the energy given to a fundamental charge, e, accelerated through a potential difference of 1 Volt.

Conversion

To convert between J and eV, simply multiply or divide by the charge of the electron, e = 1.6×10^-19.
- 1 eV = 1.6 ×10^-19 J

Example - hydrogen

Let's say the energy of an electron in a hydrogen atom is 13.6 eV.
To find this in Joules, use the equation shown previously:
- 13.6 eV = 13.6 × 1.6 ×10^-19 J
- 13.6 eV = 2.18 ×10^-18 J (3 s.f.)