Excitation and Emission

Introduction

An atom can gain energy without losing its electron. It may enter an excited state and later return toward a lower state, sometimes emitting light. These transitions link energy diagrams to observable spectra. The familiar language of electrons “jumping” describes changes of state rather than tiny balls following visible paths between rings.

Core explanation

Excitation raises an atom from an occupied lower energy state to a higher allowed bound state. Energy can be supplied by absorbing light or through collisions. For a photon-driven transition, the photon energy must match an allowed energy difference within the relevant physical linewidth and transition rules.

Excitation is not the same as ionisation . In excitation the electron remains part of a bound atomic state. Ionisation removes it from the atom. A photon above the ionisation threshold can supply removal energy and leave additional energy as kinetic energy of the outgoing electron, rather than needing to match one discrete bound-state gap.

When an excited atom moves to a lower state, energy must be transferred elsewhere. In radiative emission, it leaves as a photon. The photon energy is the positive difference between the initial higher level and the final lower level. Other interactions can sometimes transfer energy without producing the particular photon imagined in a simple diagram.

An atom can relax directly or through a sequence of intermediate levels where allowed. If it drops through two steps, it emits two photons whose combined energy equals the overall energy decrease, assuming the steps are radiative. The individual photon energies differ from that of a single direct-transition photon.

For an ensemble, observed light intensity depends on how many atoms populate excited states and how likely particular transitions are. A possible energy gap does not guarantee a prominent line under every condition. Ground-state atoms cannot keep emitting the same transition indefinitely without further energy input.

Energy accounting therefore connects the state diagram, excitation source and observed light. It also prevents the misconception that an atom manufactures energy when it emits a photon: the emitted energy comes from an earlier higher-energy state.

Step-by-step reasoning

1. Write the initial and final state energies. 2. Decide whether the atomic energy increases or decreases. 3. Calculate the positive magnitude of the difference. 4. Identify energy input for excitation or energy output for emission, distinguishing any multi-step path from a direct transition.

Visual explanation

Draw levels at 0, 3 and 5 arbitrary energy units, using zero as a diagram reference rather than an ionisation limit. Show a direct arrow from 5 to 0 and a two-arrow cascade from 5 to 3 to 0. Label their photon energies 5, or 2 followed by 3.

Real-world analogy

Descending from the fifth floor to ground level can happen directly or with a stop on the third floor. The total change in height is the same, but the segments differ. Atomic cascades likewise partition a total energy decrease, while actual transitions obey quantum rules absent from the elevator analogy.

Real-world example

Gas-discharge lamps supply energy to atoms through electrical processes. Excited atoms can then emit their characteristic radiation. Continuous light requires a continuing energy source because the atoms do not remain able to emit indefinitely after reaching their lower states.

Why?

Why must photon energy equal a state difference for a bound-state radiative transition? Energy is conserved. The atom's energy loss must appear in the emitted radiation and any small recoil contributions; introductory calculations normally neglect the recoil correction.

Common misconception

“An electron emits light when it jumps to a higher energy level.” Reaching a higher level requires energy input. Emission normally accompanies a downward transition to lower atomic energy, with the released energy carried away by the photon.

Worked example

A hypothetical atom has levels at 1, 4 and 9 eV on a common arbitrary reference. Moving from 1 to 9 eV requires 8 eV. Returning through the 4 eV level can emit photons of 5 eV and 3 eV. Their energies add to 8 eV, matching the total decrease. A direct return would instead emit one 8 eV photon if that transition is allowed.

Quick check

1. Does an excited atom necessarily have fewer electrons than its ground-state form? Answer: No. Excitation changes electronic energy state; ionisation is the process that removes an electron.

Exam focus

State which direction an arrow represents. Use a difference of levels for photon energy, and sum photon energies for a cascade rather than averaging them. Do not confuse an arbitrary zero in a sketch with a stated ionisation threshold.

Advanced insight

Some excited states have relatively long lifetimes because direct radiative decay is weak or restricted. Such metastable states illustrate why knowing an energy gap is not enough to predict how quickly light will be emitted. Transition probabilities are an additional part of the quantum description.

Summary

Excitation requires energy and retains a bound electron, whereas ionisation removes it. Radiative relaxation transfers a positive energy difference into a photon. Cascades divide the same total decrease among several photons, and continuing emission requires populated excited states and an energy supply.

Practice questions

1. Find photon energy for a downward transition from 7 eV to 2 eV on one reference. Answer: 5 eV, the positive difference between the two state energies. 2. A cascade emits photons of 2 eV and 4 eV. What is the total atomic energy decrease? Answer: 6 eV, because energy transfers add. 3. Why does a ground-state atom not radiate a further downward electronic transition indefinitely? Answer: No lower electronic state is available; further repeated emission requires energy input to populate excited states again.