TRB PG Assistant - Linear harmonic oscillator, angular momentum and addition of angular momenta, perturbation theory, transition probabilities for constant and harmonic perturbations Question Bank with Options

1. If a system has a Hamiltonian H = H0 + λH' and λ is a small parameter, the energy of the system in the first order of perturbation theory is approximately:

A) E_n ≈ E_n^(0) + λ <ψ_n^(0)|H'|ψ_n^(0)>

B) E_n ≈ E_n^(0) + λ^2 <ψ_n^(0)|H'|ψ_n^(0)>

C) E_n ≈ E_n^(0)

D) E_n ≈ λ <ψ_n^(0)|H'|ψ_n^(0)>

2. In the addition of angular momenta, the state vector |j1 j2; J M> is a linear combination of |j1 m1> |j2 m2> states. The coefficients are known as:

A) Fourier coefficients

B) Clebsch-Gordan coefficients

C) Eigenvectors

D) Lagrange multipliers

3. What is the normalization constant for the ground state wave function of a 1D quantum harmonic oscillator?

A) (mω / (πħ))^(1/4)

B) (mω / (2πħ))^(1/4)

C) (mω / ħ)^(1/4)

D) (mω / (πħ))^(1/2)

4. The selection rules for transitions in a harmonic oscillator are typically determined by:

A) The parity of the initial and final states.

B) The change in the principal quantum number being ±1.

C) The change in the angular momentum quantum number being ±1.

D) The perturbation Hamiltonian.

5. A quantum harmonic oscillator in its ground state has:

A) Zero kinetic energy and zero potential energy.

B) Non-zero kinetic energy and zero potential energy.

C) Zero kinetic energy and non-zero potential energy.

D) Non-zero kinetic energy and non-zero potential energy.

6. Which operator commutes with L^2 and Lz?

A) Lx

B) Ly

C) L+

D) None of the above, as they do not commute with L^2 and Lz simultaneously except for trivial cases.

7. What is the condition for resonance in stimulated absorption or emission of radiation by an atom?

A) The frequency of the radiation matches the energy difference between atomic levels.

B) The intensity of the radiation is very high.

C) The atom is in its ground state.

D) The atom is in an excited state.

8. The probability of transition from a state |i> to a state |f> under a time-dependent perturbation is proportional to:

A) |<f|H'|i>|^2

B) |<f|H'|i>|^2 * t

C) |<f|H'|i>|^2 / t

D) |<f|H'|i>|

9. What is the primary outcome of the addition of angular momenta in quantum mechanics?

A) A single, unique total angular momentum value.

B) A set of possible total angular momentum values.

C) A decrease in the total angular momentum.

D) An increase in the total angular momentum.

10. In perturbation theory, if E_n^(0) = E_m^(0) for n ≠ m (degenerate case), what is required to find the correct energy shifts?

A) Second-order perturbation theory only.

B) The first-order energy correction is sufficient.

C) Diagonalization of the perturbation Hamiltonian within the degenerate subspace.

D) Ignoring the perturbation entirely.

11. Which of the following is NOT a fundamental commutation relation for angular momentum operators?

A) [Lx, Ly] = iħ Lz

B) [L^2, Lx] = 0

C) [Lz, L+] = ħ L+

D) [L+, L-] = 2ħ Lz

12. What is the physical interpretation of the operator L^2?

A) The z-component of angular momentum.

B) The square of the angular momentum magnitude.

C) The orbital angular momentum in the xy-plane.

D) The total angular momentum including spin.

13. In the context of angular momentum addition, what is the degeneracy of a total angular momentum state J for a given pair of j1 and j2?

A) 2J + 1

B) (2j1 + 1)(2j2 + 1)

C) 2j1 + 1

D) 2j2 + 1

14. The transition probability for a constant perturbation grows quadratically with time for small times. What happens at very long times?

A) It continues to grow quadratically.

B) It oscillates indefinitely.

C) It saturates.

D) It becomes zero.

15. For a harmonic perturbation H'(t) = V e^(-iωt) + V† e^(iωt), what condition leads to transitions from state |i> to state |f> with E_f < E_i?

A) E_f - E_i ≈ ħω

B) E_f - E_i ≈ -ħω

C) E_f - E_i ≈ 0

D) E_f - E_i ≈ ħ

16. What is the role of the delta function δ(E_f - E_i) in Fermi's Golden Rule?

A) It ensures that the transition occurs only between states of different energy.

B) It represents the density of states at the final energy.

C) It enforces energy conservation for the transition.

D) It accounts for the time duration of the perturbation.

17. What is the primary advantage of using perturbation theory?

A) It provides exact analytical solutions for complex problems.

B) It allows approximate solutions for problems that cannot be solved exactly.

C) It simplifies the Hamiltonian to be always diagonal.

D) It eliminates the need for quantum numbers.

18. What is the condition for applying time-independent perturbation theory?

A) The perturbation must be very strong.

B) The perturbation must be time-dependent.

C) The perturbation must be small compared to the unperturbed Hamiltonian, and the system is in a stationary state.

D) The unperturbed system must be exactly solvable.

19. In perturbation theory, if the perturbation H' commutes with the unperturbed Hamiltonian H0, then:

A) The energy levels shift significantly.

B) The first-order energy correction is always zero.

C) The first-order wave function correction is zero.

D) The degeneracy is lifted.

20. Degeneracy in a quantum system refers to:

A) States with different energies but the same wave function.

B) States with the same energy but different wave functions.

C) States with the same quantum numbers.

D) States with zero energy.

21. The addition of angular momenta is crucial for describing:

A) The motion of a single particle in a potential well.

B) The interaction of particles with external electromagnetic fields.

C) The structure of atoms and nuclei, where different angular momenta combine.

D) The behavior of non-relativistic particles.

22. When adding two angular momenta j1 and j2, the total angular momentum quantum number J can take values from |j1 - j2| to j1 + j2 in integer steps. What is the maximum possible value of J?

A) j1

B) j2

C) j1 + j2

D) |j1 - j2|

23. What is the physical significance of the magnetic quantum number 'm'?

A) It determines the magnitude of the angular momentum.

B) It determines the orientation of the angular momentum vector in space.

C) It determines the energy of the state.

D) It determines the spin of the particle.

24. The commutation relation for angular momentum is [Li, Lj] = iħ εijk Lk. What does this imply?

A) Lx, Ly, and Lz can be simultaneously diagonalized.

B) Only one component of angular momentum can be precisely known at a time.

C) Angular momentum is conserved.

D) The magnitude of angular momentum is quantized.

25. The angular momentum operator Lz commutes with which other angular momentum operator?

A) Lx

B) Ly

C) L^2

D) L+

26. What does the operator a do to a state |n> of the harmonic oscillator?

A) It annihilates the state, lowering the energy.

B) It creates a state, raising the energy.

C) It leaves the state unchanged.

D) It measures the energy of the state.

27. What does the operator a† do to a state |n> of the harmonic oscillator?

A) It annihilates the state, lowering the energy.

B) It creates a state, raising the energy.

C) It leaves the state unchanged.

D) It measures the energy of the state.

28. The ladder operators for a harmonic oscillator are defined as a and a†. What is the relationship between the Hamiltonian and these operators?

A) H = ħω (a†a + 1)

B) H = ħω (a†a - 1)

C) H = ħω a†a

D) H = ħ (a† + a)

29. What is the effect of the zero-point energy in a quantum harmonic oscillator?

A) It causes the oscillator to have maximum energy at absolute zero.

B) It prevents the oscillator from being at rest at the bottom of the potential well.

C) It allows the oscillator to have zero energy at absolute zero.

D) It is a purely classical effect.

30. What is the transition probability per unit time for a harmonic perturbation when E_f - E_i ≈ ħω?

A) W_{i→f} = 2π/ħ |<f|V†|i>|^2 δ(E_f - E_i - ħω)

B) W_{i→f} = 2π/ħ |<f|V|i>|^2 δ(E_f - E_i + ħω)

C) W_{i→f} = 2π/ħ |<f|V|i>|^2 δ(E_f - E_i - ħω)

D) W_{i→f} = 2π/ħ |<f|V†|i>|^2 δ(E_f - E_i + ħω)

31. For a harmonic perturbation of the form H'(t) = V e^(-iωt) + V† e^(iωt), what condition leads to transitions from state |i> to state |f> with E_f > E_i?

A) E_f - E_i ≈ ħω

B) E_f - E_i ≈ -ħω

C) E_f - E_i ≈ 0

D) E_f - E_i ≈ ħ

32. Fermi's Golden Rule describes the transition rate for which type of perturbation?

A) Time-independent perturbation

B) Harmonic perturbation

C) Constant perturbation

D) Time-dependent perturbation that is not harmonic

33. What is the transition probability per unit time for a constant perturbation H' when E_f ≠ E_i?

A) W_{i→f} = 2π/ħ |<f|H'|i>|^2 δ(E_f - E_i)

B) W_{i→f} = π/ħ |<f|H'|i>|^2 δ(E_f - E_i)

C) W_{i→f} = 4π/ħ |<f|H'|i>|^2 δ(E_f - E_i)

D) W_{i→f} = |<f|H'|i>|^2 δ(E_f - E_i)

34. For a constant perturbation H' applied for a time t, what is the transition probability from state |i> to state |f> (where E_i ≠ E_f)?

A) P_{i→f} = |<f|H'|i>|^2 * t^2 / ħ^2

B) P_{i→f} = 4 |<f|H'|i>|^2 * sin^2((E_f - E_i)t / 2ħ) / (E_f - E_i)^2

C) P_{i→f} = |<f|H'|i>|^2 * t / ħ

D) P_{i→f} = 4 |<f|H'|i>|^2 * t^2 / ħ^2

35. What does the term 'transition probability' refer to in quantum mechanics?

A) The probability of a system changing from one energy state to another

B) The probability of finding a particle in a certain position

C) The probability of measuring a specific angular momentum

D) The probability of the system remaining in the same state

36. What is the first-order correction to the wave function in time-independent perturbation theory?

A) ψ_n^(1) = Σ_{m≠n} <ψ_m^(0)|H'|ψ_n^(0)> ψ_m^(0) / (E_n^(0) - E_m^(0))

B) ψ_n^(1) = Σ_{m≠n} <ψ_m^(0)|H'|ψ_n^(0)> ψ_m^(0) / (E_m^(0) - E_n^(0))

C) ψ_n^(1) = <ψ_n^(0)|H'|ψ_n^(0)> ψ_n^(0)

D) ψ_n^(1) = Σ_{m≠n} <ψ_m^(0)|H'|ψ_n^(0)> ψ_m^(0)

37. In first-order time-independent perturbation theory, what is the correction to the energy eigenvalue E_n^(0) of the unperturbed system?

A) E_n^(1) = <ψ_n^(0)|H'|ψ_n^(0)>

B) E_n^(1) = Σ_{m≠n} |<ψ_m^(0)|H'|ψ_n^(0)>|^2 / (E_n^(0) - E_m^(0))

C) E_n^(1) = <ψ_n^(0)|H'|ψ_m^(0)>

D) E_n^(1) = Σ_{m≠n} <ψ_m^(0)|H'|ψ_n^(0)>

38. Perturbation theory is used when the Hamiltonian of a system can be written as H = H0 + H', where H' is small compared to H0. What does H0 represent?

A) The perturbed Hamiltonian

B) The exact Hamiltonian

C) The unperturbed Hamiltonian, whose solutions are known

D) The interaction Hamiltonian

39. What is the Clebsch-Gordan coefficient related to?

A) The energy levels of a system

B) The probabilities of different total angular momentum states

C) The angular distribution of particles

D) The magnetic moment of a particle

40. If we add two spin-1/2 angular momenta, what are the possible total spin quantum numbers?

A) S = 1/2

B) S = 1

C) S = 0, 1

D) S = 0, 1/2

41. When two angular momenta, J1 and J2, are added, what is the resulting total angular momentum quantum number, J?

A) J = |j1 - j2|, ..., j1 + j2

B) J = j1 + j2

C) J = |j1 - j2|

D) J = 0, 1, 2, ..., j1 + j2

42. For a given orbital angular momentum quantum number 'l', what are the possible values of the magnetic quantum number 'm'?

A) m = 0, 1, 2, ..., l

B) m = -l, -l+1, ..., 0, ..., l-1, l

C) m = -l, l

D) m = 0, 1, 2, ..., ∞

43. What are the eigenvalues of the z-component of the angular momentum operator, Lz?

A) l ħ

B) m ħ

C) m^2 ħ^2

D) l(l+1) ħ

44. What are the eigenvalues of the square of the angular momentum operator, L^2?

A) m^2 ħ^2

B) l(l+1) ħ^2

C) l ħ^2

D) m ħ^2

45. Which operator corresponds to the angular momentum in quantum mechanics?

A) L = r x p

B) L = iħ (r x ∇)

C) L = ħ (r x ∇)

D) L = p x r

46. What is the ground state energy of a quantum harmonic oscillator?

A) 0

B) ħω

C) 1/2 ħω

D) kT

47. What are the allowed energy levels for a one-dimensional quantum harmonic oscillator?

A) En = nhf, where n is an integer

B) En = (n + 1/2)ħω, where n = 0, 1, 2,...

C) En = nhf + 1/2 ħω, where n is an integer

D) En = nħω, where n is an integer

48. In the context of quantum mechanics, what does the Hamiltonian operator represent for a simple harmonic oscillator?

A) The momentum operator

B) The potential energy operator

C) The total energy operator (kinetic + potential)

D) The position operator

49. What is the potential energy function for a one-dimensional simple harmonic oscillator?

A) V(x) = kx

B) V(x) = 1/2 kx^2

C) V(x) = kx^3

D) V(x) = k/x

Linear harmonic oscillator, angular momentum and addition of angular momenta, perturbation theory, transition probabilities for constant and harmonic perturbations - Question Bank