Explain photon absorption and emission as energy-level transitions

Follow the direction of the transition

Absorption and emission are opposite energy changes. Draw an energy axis upward, mark the initial and final levels, and let the arrow direction decide which process occurs.

For a one-photon transition, the photon energy equals the gap between the two allowed levels: |ΔE| = hf.

Absorption and emission

Absorption occurs when a molecule takes in a photon and moves from a lower-energy state to a higher-energy state. The upward transition requires hf = Ehigh − Elow.

Emission occurs when a molecule moves from a higher-energy state to a lower-energy state and releases a photon. The emitted photon has energy hf = Ehigh − Elow.

If the photon energy does not match an allowed transition, it is not absorbed in this simple spectroscopic model.

Energy is conserved in one event

The energy gap is a difference, not a sum of the printed energy values. Whether the levels are positive or negative, calculate final minus initial for absorption and take the positive magnitude for the photon.

An excited state can sometimes lose energy through several steps, so an emitted photon need not have the same energy as the photon originally absorbed. Only a direct return across the same gap gives the same photon energy.

A spectrum records the energies or wavelengths at which these allowed processes occur. It does not show electrons resting halfway between levels.

Worked example: a 2.8 eV gap

A molecule has levels at 1.8 eV and 4.6 eV. The energy needed for an upward transition is 4.6 − 1.8 = 2.8 eV.

The molecule absorbs a 2.8 eV photon to move upward. A direct downward transition between the same levels emits a 2.8 eV photon.

Practise using arrow direction

An upward arrow always means the system gains energy, so label it absorption. A downward arrow means the system loses energy, so label it emission.

Write the molecular change and photon change together to avoid saying that a molecule absorbs while dropping to a lower level.

Test a non-matching photon

Suppose the only permitted gap is 4.2 × 10⁻¹⁹ J. A photon with a different energy does not place the molecule at an intermediate stationary energy.

Unless another allowed transition or multiphoton process is specified, treat the photon as not absorbed.

Common mistake: reversing the energy flow

The molecule absorbs energy when it moves upward and emits energy when it moves downward. Do not choose the word from whether an arrow enters or leaves a sketch without checking the level direction.

Also avoid adding the two level energies. Spectroscopy depends on their separation.

Check your understanding

A complete explanation states the initial and final levels, calculates the positive gap, gives the arrow direction and names absorption or emission correctly.

Next, apply this transition model to chromophores and the four common UV/visible electronic transitions.