This A-level lab connects discrete atomic energies with the photons
absorbed and emitted. The diagram uses zero at the first ionisation
limit: less negative levels have more energy. For a transition, the
photon energy is the magnitude of the energy difference.
Three ways to picture an atom
The classical model illustrates the failure of a radiating planetary
orbit: it loses energy and collapses. Its collapse time is
illustrative. Bohr orbits are a historical representation with
schematic radii. The cloud displays |ψ|² for normalised hydrogen
eigenfunctions; it is a stationary probability distribution, not an
electron trajectory. The s, p and d examples use real m = 0 orbitals.
Repeated position measurements are independent samples of identically
prepared atoms.
Clouds are automatically rescaled with n² to remain legible; the scale
note states the distance represented by one scene unit in Bohr radii
a₀. Multi-electron atoms use a schematic atomic picture and evaluated
atomic-state energies; they do not use a hydrogen cloud as a
many-electron solution.
Transitions and spectra
Hydrogen uses Eₙ = −13.6/n² eV, with s, p and d states through n = 6.
Dipole transitions require Δl = ±1 here; fine structure, spin,
polarisation and higher multipoles are omitted. In particular, 2s → 1s
is not a supported one-photon dipole decay. Metastable and unsupported
channels are not invented. Selected transitions of neutral helium,
sodium, neon and mercury are taken from NIST. An unlisted pair may
have other physical channels outside this subset.
Single-atom trials prepare the selected initial state. Photons retain
their energy in flight. Absorption requires a supported upward
transition within the declared 0.002 eV source bandwidth. Above the
ionisation threshold, excess energy becomes electron kinetic energy;
atomic recoil is neglected. Re-emission is shown sideways after an
illustrative delay, so it need not return to the forward detector.
Cascade delays are also illustrative, not measured atomic lifetimes.
Model emission spectra assume equal populations of the included upper
states and weight photon rates by the tabulated A coefficients.
Hydrogen and custom atoms use equal transition weights. Absorption
spectra use equal peak optical depths for the occupied lower states.
“Prepared lower states” is an artificial ensemble, not a
thermal-equilibrium gas. No plasma temperature, collisional rates or
calibrated lamp brightness are predicted. NIST line sets are
incomplete; omitted lines may exist.
Gaussian instrument broadening is controlled by its full width at half
maximum. Model curves are predictions, while the detector view counts
the actual illustrated photons. Spectra-workspace photons are
representative samples chosen to make events visible; their arrival
counts are not a calibrated scan of the continuum. UV and IR markers
are neutral display colours. Every wavelength is a vacuum wavelength
derived from energy differences, so it differs slightly from familiar
wavelengths tabulated in air.
A-level connections
AQA 3.2.2.2–3.2.2.3: excitation, ionisation, the electronvolt, photon
emission and line spectra. OCR A 4.5.1 and 5.5.2: photons, atomic
levels and stellar spectra. Orbital mathematics and selection-rule
detail are enrichment. Balmer absorption needs atoms in n = 2; cold
ground-state hydrogen does not acquire visible Balmer absorption
simply because its emission spectrum contains those lines.
Sources
AQA: stellar absorption and the n = 2 population
·
NIST: transitions and line strengths
Controls & saved work
Use the initial/final selectors or select two lines on the diagram.
Drag the 3D view to rotate; use the labelled zoom and reset buttons.
Space plays/pauses when you are outside a form; Step advances 0.1
illustration seconds. Replay does not add detections. Changing the
atom prepares a new trial; the notebook is retained. Local recovery
and experiment files save settings and captured readings, but restart
transient photons and timers. Escape closes a panel and restores
focus.
