The absorption spectrum of a semiconductor features a sharp region in the middle and flat regions on the sides due to light absorption behaviors of solution molecules that transition to various excited states. The emission peak is typically sharp because it usually originates from the lowest excited state, known as the s1 state. In cases where excitation occurs from higher states, the molecule can transition directly to the ground state without emitting radiation, affecting the excitation spectrum.

1. Why does the absorption spectrum of as semiconductor have a sharp/steep region in the middle
and two flat regions on either side? For emission spectrum, why is the emission peak typically
sharp (narrow)?
Solution
molecules absorb light to different excited states. Emission comes from the lowest of these
states, called the S1 state. When a molecule is excited to a higher state it often ends up in this
lowest excited state S1 and then emits radiation. In this case the excitation spectrum is the same
as the absorption spectrum. However from a higher excited state a molecule does not have to go
to the lowest excited state. It can directly go to the ground state for instance. Then it does not
emit radiation. In this case the excitation spectrum only has the lowest energy absoption features.