MCAT Chemistry Question — Photon | Next Step Test Prep MCAT Chemistry Question — Photon | Next Step Test Prep

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When a packet of light called a photon strikes the electron cloud, the electron may jump to a higher energy shell. Although the electron might stay in the excited state without returning to the ground state, we observe electrons returning to ground rather than staying in excited states. If an additional photon can strike the excited electron to stimulate a return to ground state, how many photons are given off by an electron returning to ground state from a shell at one higher energy level after excitation by a second photon?

A) 0

B) 1

C) 2

D) 3

Explanation

The photoelectric effect describes the phenomenon of electrons jumping to discrete energy levels after excitation by packets of light called photons. Photons can excite electrons to higher energy states in a process called absorption, and photons are given off as electrons return to ground states in a process called emission. We do not observe electrons staying at an excited state and spontaneously returning to ground. In a process called stimulated emission, described by Einstein, excitation from a second photon from incident light again excites the electron, which then returns to ground state.

As stated in the question stem, a second photon excites the electron before the electron returns to ground state. This phenomenon of one photon striking an excited electron in order to release two photons in a return to ground state is the theory behind lasers. One in yields two out, but the electron must be in an excited state in order to undergo stimulated emission.

A) 0, incorrect, When electrons return to ground state there is an emission of photons.

B) 1, incorrect, Although the initial excitation of the electron will yield one photon out in a return to ground state, the question stem states that an additional photon has struck the electron in order to stimulate the emission.

C) 2, correct, Both the initial excitatory photon and the second excitatory photon are released.

D) 3, incorrect, There is no third photon mentioned in the question.

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