Past exam of the quantum, atomic and molecular physics course of the University of Oxford / 2016 / 1

Potassium has a closed 18-electron core and one valence electron. In $3d{}^2{D}_{5/2}$ the orbital label specifies principal quantum number $n=3$ and single-electron orbital angular momentum $l=2$. The term letter D specifies total orbital angular momentum $L=2$, the superscript $2S+1=2$ specifies $S=1/2$, and the subscript is total electronic angular momentum $J=5/2$. The $5p{}^2{P}_{3/2}$ state instead has $n=5$, $l=L=1$, the same spin, and $J=3/2$. The closed electron shell that forms the core contributes no net $L$ or $S$ in this alkali-like approximation. The dash identifies a pair of levels rather than by itself specifying the direction of emission.

The leading electric dipole transition rules for one active electron areThe electronic parity must change, since the electric dipole operator is odd under spatial reflection. The electric-dipole selection rule imposes no general restriction on changing $n$ by more than one. The rule for magnetic sublevels depends on the radiation's polarization relative to the quantization axis: the $q=0$ component couples equal $m_J$, whereas $q=\pm 1$ supplies the other changes. In a many-electron description using LS coupling the total orbital rule is $\Delta L=0,\pm 1$, excluding $L=0\leftrightarrow 0$; for a single active electron the stronger $\Delta l=\pm 1$ applies. Thus the displayed D–P pair satisfies the leading rules. Relativistic mixing and higher-multipole transitions can permit weaker lines outside the approximation of electric dipole transitions.

Use the common ionization continuum consisting of a divalent calcium ion and a free electron as the upper reference, with the singly ionized calcium ground state $4s{}^2{S}_{1/2}$ at zero energy. The resonance doublet isThe higher-$J$ P component is higher in energy because its emission to the common lower level has the shorter wavelength. The full series is $np{}^2{P}_{1/2,3/2}\to 4s{}^2{S}_{1/2}$. As $n\to \infty$, its two upper levels approach the same continuum, so the ground-state ionization energy is approximately $hc/(104.4\mathrm{nm})=11.88$ eV.

The second doublet has a common upper S level and two lower P levels:Both combinations put $5s{}^2{S}_{1/2}$ near $6.47$ eV above the ground state. The series is $ns{}^2{S}_{1/2}\to 4p{}^2{P}_{1/2,3/2}$ with $n\ge 5$. Its two limits measure the ionization energy from each lower P level, giving slightly different limit wavelengths near 142 nm, not two unrelated continua. From the rounded wavelengths, $E(4p_{1/2})\simeq 3.124$ eV and $E(4p_{3/2})\simeq 3.152$ eV, leaving roughly $8.74$ eV to the same continuum. The tabulated level ordering and labels independently agree with these assignments.^[ref]

The quoted wavelengths and limits are rounded; air and vacuum conventions also matter for precision spectroscopy. They justify a labelled schematic and the approximate energy differences, not an artificial claim of exact closure at every printed decimal.

The limit near 121.8 nm lies about $hc/(121.8\mathrm{nm})\simeq 10.18$ eV above its lower term. Subtracting from the approximately $11.88$ eV ground-state ionization energy puts that lower term near $1.70$ eV: the $3d{}^2{D}_{3/2,5/2}$ pair. The first upper levels are the already identified $4p{}^2{P}_{1/2,3/2}$ pair. Their allowed decays areThe fourth possible pairing, $P_{1/2}\to D_{5/2}$, has $|\Delta J|=2$ and is forbidden as an electric dipole transition. The three lines therefore arise from doublet terms, not a spin-triplet term: “triplet series” counts three spectral lines.

For example, the first two wavelengths together with the $4p_{3/2}$ energy give $E(3d_{3/2})\simeq 1.693$ eV and $E(3d_{5/2})\simeq 1.700$ eV. The longer 854.2 nm wavelength from the same upper level places its lower D level higher. The 866.2 nm transition independently returns the lower D energy within the rounding precision. Increasing the upper principal quantum number gives $np{}^{2}P\to 3d{}^{2}D$ triplets with two nearby lower-term ionization limits near 121.8 nm. The arrows are included in b; their emission direction is P to D, not the reverse. The independent tabulation confirms the D-term position and ordering.^[ref]

In the regime of weak-field Zeeman splitting, $J,m_J$ remain good labels and $\Delta E={\mu }_B{g}_J{m}_{J}B$. For the 393.4 nm line, $J_u=3/2$ in $4p{}^2{P}_{3/2}$ and $J_l=1/2$ in $4s{}^2{S}_{1/2}$. The Landé g-factors are $g_u=4/3$ and $g_l\simeq 2$. Apply $\Delta m_J=0,\pm 1$ to the four upper and two lower sublevels. The allowed pairs $(m_u,m_l)$ areTheir frequency displacements in units of ${\mu }_{B}B/h$ are respectively $1,-1/3,-5/3,5/3,1/3,-1$, so none coincide. Looking perpendicular to the field permits both the $\pi$ radiation from $q=0$ transitions and the $\sigma$ radiation from $q=\pm 1$ transitions. ConsequentlyA linear polarizer parallel to the field isolates the two $\pi$ components; a transverse polarizer selects the four $\sigma$ components. Without polarization selection both sets are present. This count assumes a field small compared with fine structure coupling and neglects isotope-dependent hyperfine structure; the strong-field Paschen-Back effect or a resolved nonzero-spin isotope would require a different model. The spectral spacings are unnecessary for the count, but the distinct shifts verify that counting allowed transitions does not inadvertently count degenerate lines twice.