Discussion
The energy separation between two consecutive Stokes lines in Raman spectrum depends on :
Answer: A
The separation depends on the energy separation between vibrational levels in the excited state, option A.
In a Stokes line the molecule gains energy from the photon and is left in a higher vibrational level (v = 1, 2, ...) of the electronic ground state. The first Stokes line is the shift \(E_{1} - E_{0}\) and the next is \(E_{2} - E_{0}\), so their separation is \(E_{2} - E_{1}\), the spacing between the vibrational levels the molecule is left in. For an anharmonic oscillator this spacing shrinks, which is why overtone lines are not equally spaced.
Options B and D are wrong because the lines are shifts from the incident line: the position of the incident light changes the absolute wavelengths but not their separation, and the intensity affects only how strong the lines are. Option C is the weaker reading: the upper vibrational levels reached are themselves levels of the ground electronic state, so the wording is loose, but a single level v = 0 has no spacing, and the key reads the stem as referring to the excited levels.
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