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University Quantum Mechanics II · unit UQMII-08

The Fine Structure of Hydrogen and the Lamb Shift

Connect the ideas, make your working visible, and test the result. Use the hints when you need a starting point; open the full solution after your attempt.

Extra challenging questions

A selection from this pathway. Open a topic below for its complete practice and challenge set.

Extra challenging · 01

The hierarchy of corrections set by α: infer an input with uncertainty

The fine-structure hierarchy is the ordering of hydrogen's energy corrections by their powers of the dimensionless coupling α = e²/(4πε₀ħc) ≈ 1/137 and of the mass ratio mₑ/mₚ, each rung being mc² times a higher power and therefore a smaller effect. Use the supplied topic model Y_out = (n*137.036/z)^2, where Y_out is mc² ÷ BOHR BAR (dimensionless). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. z is Nuclear charge Z, dimensionless. Keep n (Principal quantum number n) = 2 fixed. For this exercise use only 1 ≤ z ≤ 81, at the specified test settings separated by 1. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 30.046184, with a bounded reading uncertainty ±6008.3. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. Determine every allowed value of z consistent with that bound. Compare with a straight-line interpolation between the two endpoint predictions, and explain why a fractional setting may be inadmissible. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

mc² ÷ BOHR BAR against z

Predictions from the supplied The hierarchy of corrections set by α model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): mc² ÷ BOHR BAR

mc² ÷ BOHR BAR against zPredictions from the supplied The hierarchy of corrections set by α model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: z: Nuclear charge Z. Vertical axis: mc² ÷ BOHR BAR. Exact values are available in the data table below.-29931.73e43.76e45.78e47.81e4-2.219.44162.684.2xy

Horizontal axis (x): z: Nuclear charge Z

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1z: Nuclear charge Z: 1mc² ÷ BOHR BAR: 7.51e4
Read the exact data as a table
mc² ÷ BOHR BAR against z — supplied values
z: Nuclear charge Zmc² ÷ BOHR BAR
175115.461184
21170.32984395464854
4144.68498583224271
6120.186901688793338
8111.448782378295991

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

Enumerate the permitted settings in the stated range, evaluate Y_out at each, and retain every setting satisfying |Y_out − Y_measured| ≤ u. The plotted predictions are supplied calibration evidence.

Reveal the worked solution
  1. Endpoint predictions are Y_out(1) = 75115.5 and Y_out(81) = 11.4488. The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives z_line = 1 + (30.0462 − 75115.5)(81 − 1)/(11.4488 − 75115.5) = 80.9802. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Evaluate the model at each permitted setting. The central reading corresponds to z = 50; only the allowed settings 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81 lie inside the stated reading bound.
  4. The interpolation error in the input is |z_line − z| = 30.9802.
  5. The compatible allowed values span 4 to 81. Values between specified test settings are outside this exercise's input domain even if an interpolation passes through them.
  6. The quoted output bound restricts which specified settings remain compatible. It does not change physical restrictions on quantum numbers or counts, and the chosen grid alone does not establish physical quantisation.

Answer Compatible z: 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81. Endpoint interpolation predicts 80.9802; check admissibility against the discrete settings.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

Extra challenging · 02

The relativistic kinetic-energy correction: infer an input with uncertainty

The relativistic kinetic-energy correction is the first-order shift produced by H′ᵣ = −p⁴/(8m³c²), the leading amount by which the true kinetic energy √(p²c² + m²c⁴) − mc² falls below p²/2m. Use the supplied topic model Y_out = 4*n/(min(l,n-1)+0.5)-3, where Y_out is ⟨p⁴⟩/⟨p²⟩² (dimensionless). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; min(a,b) selects the smaller value. l is Orbital quantum number l, dimensionless. Keep n (Principal quantum number n) = 3 fixed. For this exercise use only 0 ≤ l ≤ 2, at the specified test settings separated by 1. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 5, with a bounded reading uncertainty ±1.536. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. Determine every allowed value of l consistent with that bound. Compare with a straight-line interpolation between the two endpoint predictions, and explain why a fractional setting may be inadmissible. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

⟨p⁴⟩/⟨p²⟩² against l

Predictions from the supplied The relativistic kinetic-energy correction model, with all other inputs fixed as stated in the question. 3 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): ⟨p⁴⟩/⟨p²⟩²

⟨p⁴⟩/⟨p²⟩² against lPredictions from the supplied The relativistic kinetic-energy correction model, with all other inputs fixed as stated in the question. 3 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: l: Orbital quantum number l. Vertical axis: ⟨p⁴⟩/⟨p²⟩². Exact values are available in the data table below.1.0326.21611.416.5821.77-0.080.4611.542.08xy

Horizontal axis (x): l: Orbital quantum number l

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1l: Orbital quantum number l: 0⟨p⁴⟩/⟨p²⟩²: 21
Read the exact data as a table
⟨p⁴⟩/⟨p²⟩² against l — supplied values
l: Orbital quantum number l⟨p⁴⟩/⟨p²⟩²
021
15
21.7999999999999998

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

Enumerate the permitted settings in the stated range, evaluate Y_out at each, and retain every setting satisfying |Y_out − Y_measured| ≤ u. The plotted predictions are supplied calibration evidence.

Reveal the worked solution
  1. Endpoint predictions are Y_out(0) = 21 and Y_out(2) = 1.8. The target lies between them. On the selected branch the response is strictly decreasing.
  2. Endpoint interpolation gives l_line = 0 + (5 − 21)(2 − 0)/(1.8 − 21) = 1.66667. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Evaluate the model at each permitted setting. The central reading corresponds to l = 1; only the allowed settings 1 lie inside the stated reading bound.
  4. The interpolation error in the input is |l_line − l| = 0.666667.
  5. The compatible allowed values span 1 to 1. Values between specified test settings are outside this exercise's input domain even if an interpolation passes through them.
  6. The quoted output bound restricts which specified settings remain compatible. It does not change physical restrictions on quantum numbers or counts, and the chosen grid alone does not establish physical quantisation.

Answer Compatible l: 1. Endpoint interpolation predicts 1.66667; check admissibility against the discrete settings.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

Extra challenging · 03

Spin-orbit coupling from the electron's rest frame: infer an input with uncertainty

Spin-orbit coupling is the energy of the electron's spin magnetic moment in the magnetic field the moving nucleus produces in the electron's rest frame, reduced by Thomas precession to H′_so = (e²/(8πε₀))(1/(m²c²r³)) S⋅L. Use the supplied topic model Y_out = 10.949*(g-tau), where Y_out is PREDICTED 2P DOUBLET (GHz). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793. g is Spin g-factor g, dimensionless. Keep tau (Thomas precession (0 off, 1 on)) = 0 fixed. For this exercise use only 0.31875 ≤ g ≤ 2.68125 on this continuous branch. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = 19.268871 GHz, with a bounded reading uncertainty ±1.5132 GHz. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. First estimate the unknown g by straight-line interpolation between the two endpoint predictions. Then solve the actual model on the stated branch to an absolute input tolerance of 0.00023625, using a numerical bracket if required. Find the full input interval consistent with the bounded reading uncertainty and compare it with a symmetric input-error assumption. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

PREDICTED 2P DOUBLET against g

Predictions from the supplied Spin-orbit coupling from the electron's rest frame model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): PREDICTED 2P DOUBLET / GHz

PREDICTED 2P DOUBLET against gPredictions from the supplied Spin-orbit coupling from the electron's rest frame model, with all other inputs fixed as stated in the question. 5 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: g: Spin g-factor g. Vertical axis: PREDICTED 2P DOUBLET / GHz. Exact values are available in the data table below.2.4559.43916.4223.4130.390.22420.86211.52.1382.776xy

Horizontal axis (x): g: Spin g-factor g

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1g: Spin g-factor g: 0.3187PREDICTED 2P DOUBLET / GHz: 3.49
Read the exact data as a table
PREDICTED 2P DOUBLET against g — supplied values
g: Spin g-factor gPREDICTED 2P DOUBLET / GHz
0.318753.4899937499999996
0.90937499999999999.956746874999999
1.516.4235
2.090624999999999722.890253124999997
2.6812529.357006249999998

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

The model's endpoint predictions bracket the reading. Solve Y_out(g) = Y_measured and then solve again at Y_measured − u and Y_measured + u; a nonlinear inverse can give unequal input deviations.

Reveal the worked solution
  1. Endpoint predictions are Y_out(0.31875) = 3.48999 GHz and Y_out(2.68125) = 29.357 GHz. The target lies between them. On the selected branch the response is strictly increasing.
  2. Endpoint interpolation gives g_line = 0.31875 + (19.2689 − 3.48999)(2.68125 − 0.31875)/(29.357 − 3.48999) = 1.75987. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Use the actual equation, not the chord, and bracket its root. Evaluating the midpoint and retaining the sign-changing half-interval gives g = 1.75987; substituting back gives Y_out = 19.268871 GHz.
  4. The interpolation error in the input is |g_line − g| = 4.44089e-16. This is within the requested numerical tolerance.
  5. Invert both limits Y_out = 17.755671 GHz and 20.782071 GHz. Sort the roots even if the response decreases: 1.62167 ≤ g ≤ 1.89808.
  6. The lower and upper input deviations are 0.138204 and 0.138204. They agree on this branch; this does not establish linearity for changes in other inputs or outside the interval. This interval includes reading uncertainty only, not model error or uncertainty in fixed inputs.

Answer g ≈ 1.75987; compatible interval [1.62167, 1.89808]. Endpoint interpolation gives 1.75987.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

Extra challenging · 04

Evaluating spin-orbit coupling in the coupled basis: infer an input with uncertainty

The first-order spin-orbit shift of a hydrogen level is the expectation value of Hₛₒ in a coupled state |n, l, ½, j, mⱼ⟩: the eigenvalue of S⋅L, fixed by j and l, times the radial average of e²/(8πε₀m²c²r³). Use the supplied topic model Y_out = -350.377*min(l,1)/(4*(nr+l+1)^3*max(l,1)*(l+0.5)), where Y_out is SHIFT OF j = l − ½ (GHz). The output symbol Y_out is distinct from all input symbols. ^ denotes a power; π = 3.141592653589793; min(a,b) selects the smaller value; max(a,b) selects the larger value. l is Orbital quantum number l, dimensionless. Keep nr (Radial nodes nᵣ = n − l − 1) = 0 fixed. For this exercise use only 1 ≤ l ≤ 4, at the specified test settings separated by 1. A grid of test settings is an input restriction for this exercise, not by itself a claim of physical quantisation. This calculation retains the topic model's stated assumptions; it does not extend them to a different physical setup. A reading gives Y_out = -0.13034859 GHz, with a bounded reading uncertainty ±0.58085 GHz. Treat the model and all fixed inputs as exact for this exercise; the bound is not a standard deviation. Determine every allowed value of l consistent with that bound. Compare with a straight-line interpolation between the two endpoint predictions, and explain why a fractional setting may be inadmissible. Do not assume that the supplied calibration points prove monotonicity outside this branch.

Explore the question data

SHIFT OF j = l − ½ against l

Predictions from the supplied Evaluating spin-orbit coupling in the coupled basis model, with all other inputs fixed as stated in the question. 4 calibration points support a trend comparison; they are not measured data or a straight-line law.

Vertical axis (y): SHIFT OF j = l − ½ / GHz

SHIFT OF j = l − ½ against lPredictions from the supplied Evaluating spin-orbit coupling in the coupled basis model, with all other inputs fixed as stated in the question. 4 calibration points support a trend comparison; they are not measured data or a straight-line law. Horizontal axis: l: Orbital quantum number l. Vertical axis: SHIFT OF j = l − ½ / GHz. Exact values are available in the data table below.-7.59-5.63-3.669-1.7090.25150.881.692.53.314.12xy

Horizontal axis (x): l: Orbital quantum number l

Axes show the supplied calibration interval; the origin may lie outside the plot.

Reading 1l: Orbital quantum number l: 1SHIFT OF j = l − ½ / GHz: -7.3
Read the exact data as a table
SHIFT OF j = l − ½ against l — supplied values
l: Orbital quantum number lSHIFT OF j = l − ½ / GHz
1-7.299520833333333
2-0.6488462962962963
3-0.1303485863095238
4-0.03893077777777778

These are the supplied values, not a worked answer. No fitted trend is drawn.

Show a hint

Enumerate the permitted settings in the stated range, evaluate Y_out at each, and retain every setting satisfying |Y_out − Y_measured| ≤ u. The plotted predictions are supplied calibration evidence.

Reveal the worked solution
  1. Endpoint predictions are Y_out(1) = -7.29952 GHz and Y_out(4) = -0.0389308 GHz. The target lies between them. On the selected branch the response is strictly increasing.
  2. Endpoint interpolation gives l_line = 1 + (-0.130349 − -7.29952)(4 − 1)/(-0.0389308 − -7.29952) = 3.96223. This estimates the inverse of the endpoint chord, rather than assuming that chord is the physical model.
  3. Evaluate the model at each permitted setting. The central reading corresponds to l = 3; only the allowed settings 2, 3, 4 lie inside the stated reading bound.
  4. The interpolation error in the input is |l_line − l| = 0.962227.
  5. The compatible allowed values span 2 to 4. Values between specified test settings are outside this exercise's input domain even if an interpolation passes through them.
  6. The quoted output bound restricts which specified settings remain compatible. It does not change physical restrictions on quantum numbers or counts, and the chosen grid alone does not establish physical quantisation.

Answer Compatible l: 2, 3, 4. Endpoint interpolation predicts 3.96223; check admissibility against the discrete settings.

Compare the assumptions, method and units with your attempt. Explain any correction before trying another question.

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