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The energy \(E\) and degeneracy \(d\) of the second excited state of a three-dimensional, isotropic quantum harmonic oscillator with angular frequency \(\omega\) are

  • A.\(E = \frac{7}{2}\hbar\omega, \quad d = 6\)
  • B.\(E = \frac{7}{2}\hbar\omega, \quad d = 3\)
  • C.\(E = \frac{5}{2}\hbar\omega, \quad d = 3\)
  • D.\(E = \frac{5}{2}\hbar\omega, \quad d = 6\)

Answer: A

The second excited state has \(E = \frac{7}{2}\hbar\omega\) and \(d = 6\).

For the 3D isotropic oscillator \(E = (n_{x} + n_{y} + n_{z} + \frac{3}{2})\hbar\omega\). The ground state has n = 0, the first excited state n = 1, and the second excited state n = 2, so \(E = \frac{7}{2}\hbar\omega\).

Its degeneracy is the number of ways to write 2 as \(n_{x} + n_{y} + n_{z}\): (2,0,0), (0,2,0), (0,0,2), (1,1,0), (1,0,1), (0,1,1), six in all, matching \((n + 1)(n + 2)/2 = 6\). d = 3 counts only one of the two kinds of combination. \(\frac{5}{2}\hbar\omega\) is the first excited state, whose degeneracy is 3, not the second.

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