Question

by comparing the instrumental design of a spectrophotometer and the spectrofluorometer, why does the emission spectrum of the blank have well defined peaks while the absorption spectrum is nearly flat?

(hint how do you define absorbance)

Answer 1

First - many compounds do not emit light after absorption of radiation. so these molecules do not have an excitation spectrum. The reason is that the absorbed energy can dissipate in the molecule without emitting light/fotons.

Secondly, 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. In spectroscopy terms: the absorption spectrum shows the So-S1, the So-S2, So-S3 etc bands, but the excitation spectrum only shows the So-S1 band