Atomic excitation by electron collisions

An atom is excited when one of its electrons gains energy and moves to a higher permitted energy level while remaining bound to the atom. Atomic electrons can occupy only particular energy levels, not the energies between them.

Excitation by a collision

An electron travelling towards an atom is called an incident electron. When it collides with the atom, it can transfer some of its kinetic energy to an atomic electron. If the atomic electron gains exactly the energy difference between its current level and a higher permitted level, it can move to that higher level.

The incident electron must have at least enough kinetic energy to supply that difference. If it has more than the required amount, it can retain the excess as kinetic energy after the collision. Having enough energy makes excitation possible, but it does not mean that every collision will excite the atom.

Photon emission after excitation

An excited atom can return to a lower energy level when its excited electron moves down. The atom emits the lost energy as a photon. The photon’s energy equals the difference between the electron’s initial and final energy levels. The electron may return directly to its original level or move first to another lower permitted level.

Atomic ionisation by electron collisions

An atom can become ionised when one of its electrons gains enough energy to escape. The atom is left with more protons than electrons, so it becomes a positive ion. The removed electron is no longer bound to the atom.

An electron travelling towards an atom is called an incident electron. In a collision, it can transfer some of its energy to an electron bound in the atom. If the energy transferred is at least the energy needed to remove that electron, ionisation can occur. Any energy transferred beyond what is needed for removal can become kinetic energy of the freed electron.

The energy needed to remove an electron is its ionisation energy. It depends on the atom and on the initial energy state of the electron being removed. An incident electron must transfer enough energy in the collision; having enough energy itself does not mean that it will necessarily transfer enough.

In excitation, an atomic electron gains energy but remains bound to the atom. In ionisation, it gains enough energy to become unbound.

Fluorescent tube operation

A fluorescent tube produces visible light by passing an electric current through a low-pressure gas and using a coating to convert ultraviolet radiation into visible light. The gas inside the tube includes mercury vapour.

An electric field accelerates electrons through the gas. When these electrons collide with gas atoms, they can transfer energy to the atoms in two ways:

• Excitation: an electron in an atom gains energy and moves to a higher energy level.
• Ionisation: an electron is removed from an atom, producing a free electron and a positive ion.

The free electrons and positive ions produced by ionisation can move through the gas, helping it conduct an electric current.

Excited mercury atoms return to lower energy levels and emit ultraviolet photons. The fluorescent coating on the inside of the tube absorbs this ultraviolet radiation and emits visible light.

Electron volt

An electron gains energy when it is accelerated through a potential difference. The energy gained by an electron accelerated through a potential difference of one volt is called one electron volt.

The electron volt, written $$\mathrm{eV}$$, is a unit of energy, not a unit of potential difference. In joules, its approximate value is:

Converting electron volts and joules

To convert an energy from electron volts to joules, multiply its numerical value by $$1.60 \times 10^{-19}$$. To convert from joules to electron volts, divide its numerical value by the same factor.

For example, $$2.0,\mathrm{eV}$$ is approximately $$3.20 \times 10^{-19},\mathrm{J}$$. Dividing $$3.20 \times 10^{-19}$$ by $$1.60 \times 10^{-19}$$ converts it back to $$2.0,\mathrm{eV}$$.