Physics · Particles & Radiation · AS Level

Ionisation, Excitation & the Electron Volt

Zooming back out from quarks to whole atoms, this lesson looks at what happens when an electron in an atom absorbs energy — and introduces the electron volt, the unit of energy that particle and atomic physics actually work in, because a joule is an absurdly large amount of energy to be describing a single electron with.

9 min read

Ionisation and excitation: two different outcomes

An atom's electrons sit in a set of fixed, allowed energy levels. When an atom absorbs energy — from a collision or from absorbing a photon — one of two things can happen to an electron, depending on how much energy is available.

Excitation happens when an electron absorbs just enough energy to jump to a higher, still-bound energy level, without leaving the atom. The atom is left in an "excited state" — still neutral, but with more internal energy. Excited states aren't stable: the electron typically falls back down shortly afterwards, releasing the extra energy again, usually as a photon.

Ionisation happens when an electron absorbs enough energy to escape the atom's pull completely, leaving the atom behind as a positive ion. The ionisation energy is the minimum energy required to remove a specific electron from an atom entirely.

The physical distinction to hold onto

Excitation moves an electron between energy levels inside the atom — the electron stays bound, the atom stays neutral. Ionisation removes an electron from the atom altogether — the electron becomes free, the atom becomes a positive ion. Enough energy to reach a higher bound level is excitation; enough to escape entirely is ionisation.

Ionisation and excitation in a fluorescent tube

A fluorescent tube shows both processes at work. The tube contains mercury vapour at low pressure, with a high voltage across electrodes at either end, accelerating free electrons along the tube.

These accelerated electrons collide with mercury atoms. Some collisions transfer enough energy to excite electrons to higher energy levels. Those excited atoms don't stay excited for long — the electrons drop back down almost immediately, and each mercury atom emits a photon whose energy matches the gap between the two levels. For mercury, these photon energies correspond mostly to ultraviolet light, invisible to the human eye.

That's where the phosphor coating on the inside of the tube comes in: it absorbs the UV photons and re-emits the energy as a cascade of lower-energy visible photons — the tube's white light. Without the coating, the same atomic physics would happen inside, but almost none of the light would be visible.

Some of the accelerated electrons carry enough energy to ionise mercury atoms outright, knocking an electron out completely — part of what sustains the current through the low-pressure gas.

The electron volt as a unit of energy

Working in joules at the level of a single electron produces uncomfortably small numbers — ionisation energies and photon energies come out around $10^{-18}$ to $10^{-19}$ J. Particle and atomic physics instead uses a unit built around this scale: the electron volt, symbol eV.

One electron volt is the energy gained by a single electron accelerated through a potential difference of exactly 1 volt.

Derivation1 eV = 1.6 × 10⁻¹⁹ J

The energy transferred when a charge $Q$ is accelerated through a potential difference $V$ is $E = QV$.

This follows directly from the definition of potential difference as energy transferred per unit charge.

For one electron volt, take the charge to be exactly one electron's charge, $Q = e = 1.6\times10^{-19}\ \text{C}$, and the potential difference to be exactly $V = 1\ \text{V}$.

Substitute: $E = (1.6\times10^{-19}\ \text{C})\times(1\ \text{V}) = 1.6\times10^{-19}\ \text{J}$.

So $1\ \text{eV} = 1.6\times10^{-19}\ \text{J}$ — a fixed, exact conversion between the two units, since both $e$ and the definition of the volt are fixed.

Because eV is defined this way, converting eV to J is always just multiplying by $1.6\times10^{-19}$, and converting J to eV is always dividing by the same number.

▪ derived

Ionisation, excitation and photon energies in atomic and particle physics are conventionally quoted in eV (or its larger multiples — keV, MeV, GeV), since the numbers come out as ordinary, easy-to-work-with values. A typical atomic ionisation energy might be 10.4 eV, versus a much less legible $1.66\times10^{-18}$ J.

Converting fluently, both directions

To convert eV into J, multiply by $1.6\times10^{-19}$. To convert J into eV, divide by $1.6\times10^{-19}$. Exam questions routinely give a quantity in one unit and expect an answer in the other.

Key takeaways

Excitation: an electron absorbs energy and moves to a higher bound energy level, without leaving the atom — the atom remains neutral.
Ionisation: an electron absorbs enough energy to escape the atom entirely, leaving behind a positive ion.
In a fluorescent tube, accelerated electrons excite mercury atoms, which emit UV photons on de-excitation; a phosphor coating converts this UV into visible light. Ionisation of mercury atoms also occurs and contributes to sustaining the current.
One electron volt (eV) is the energy gained by an electron accelerated through a potential difference of 1 V.
From $E=QV$ with $Q=e$ and $V=1\ \text{V}$: $1\ \text{eV} = 1.6\times10^{-19}\ \text{J}$.
The eV is used throughout atomic and particle physics because it gives conveniently sized numbers at the scale of single particles, unlike the joule.
Conversion is fluent in both directions: multiply by $1.6\times10^{-19}$ to go eV→J, divide by $1.6\times10^{-19}$ to go J→eV.

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