Question

In a laser, the population inversion is achieved between which two energy levels?

• A. Ground state and the first excited state
• B. Two vibrational levels of the ground state
• C. Two closely spaced excited states
• D. The first and second excited states

Hint:

Lasers prefer working between excited states — it’s easier to get more electrons up there and trigger photon emission!

Answer 1

The Correct Option is C

In a laser, population inversion is a condition where more electrons are in an excited state than in a lower energy state, enabling stimulated emission of photons.
Why Between Two Closely Spaced Excited States?

• In most practical lasers (like solid-state or gas lasers), population inversion is created between two excited states rather than between an excited state and the ground state.
• Typically, electrons are pumped to a higher excited level and then quickly decay (non-radiatively) to a slightly lower metastable state.
• From this metastable state, they transition to another excited state (not the ground state), emitting coherent photons.
• This technique makes it easier to achieve and maintain population inversion.

Why Other Options Are Incorrect:

• (A) Ground state is usually the most populated; achieving inversion with it is difficult.
• (B) Vibrational levels apply to molecular transitions, not standard laser operation.
• (D) While this might seem correct, the key phrase is “closely spaced” — which enables easier and faster transitions.

Hence, for effective laser operation, population inversion is typically achieved between two closely spaced excited states.