Question

What is the difference between UV-vis fluorescence and UV absorption (please be detailed with your response and explain the correlation between the two as well as their distinct properties)?

Answer 1

UV-visible refers to absorption spectroscopy in the UV and visible portion of the electromagnetic spectrum. When a molecule (typically starting out in a singlet ground state) absorbs a photon it will become an excited state molecule. Following quantum mechanical selection rules, the molecule will be in a singlet excited state.

Fluorescence is one of many relaxation processes by which a molecule in the excited state can relax back down to the ground state. In fluorescence, a molecule in the lowest vibrational level of of a singlet excited state emits a photon to return to the ground singlet state.

Most commonly molecules return to the ground state by a mechanism referred to as internal conversion and energy is conserved by heat being absorbed by the surroundings. Other relaxation mechanisms include phosphorescence, quenching by either an electron transfer or fluorescence energy transfer. An excited state molecule can also undergo photo-decomposition or photochemistry.

The bottom line is that a molecule in the excited state is there for a limited amount of time. Fluorescence is but one mechanism to depopulate the excited state.