5
$\begingroup$

We learn in school that the color we perceive is determined by the light that is reflected back at us. Not all wavelengths of light reflect back at our eyes, some wavelengths are absorbed.

I'm confused about this absorption. From my understanding, molecules can absorb light typically rotationally, vibrationally, and electronically. rotationally and vibrationally are associated with the microwave and infrared region. Electronic excitations can be in the visible, but I always had the impression that the excited electrons would collapse down to the ground state and emit light. Thus, I don't think I know any mechanism which visible light can be absorbed without emitting light, albeit, longer wavelength light typically.

$\endgroup$
  • $\begingroup$ Related: chemistry.stackexchange.com/q/8099/16683 [As far as I know, they are usually due to electronic transitions. The question of what happens after excitation has been covered several times here, I believe, but the links are tough to find.] $\endgroup$ – orthocresol Jan 12 '18 at 22:50
  • 1
    $\begingroup$ I changed the title to more specific one, feel free to improve further. $\endgroup$ – Mithoron Jan 13 '18 at 0:35
7
$\begingroup$

You are correct in most of what you write. Absorption in the visible (and uv) parts of the spectrum are of far larger energy that IR and microwave and the lowest electronic transition involves promoting an electron from the highest populated orbital into the lowest unoccupied one. Once the excited state is formed it can emit via fluorescence and phosphorescence, which are both at a slightly lower energy that the light absorbed and this is because higher energy vibrational levels can be populated in the ground state. (Look for picture of a Jablonski diagram).

However, fluorescence is emitted into all directions ($4\pi$ radians due to molecular motion between absorption and emission) and only a small part of this could enter the eyes but also and most importantly there are non-radiative processes that competitively remove the energy from the excited state ultimately populating high vibrational levels in the ground state from which energy is rapidly lost into the surroundings. These non-radiative events mean that for most types of molecules the fluorescence yield is small, which means that only a few % of the excited state's energy ends up as photons. The non radiative processes are called intersystem crossing between singlet and triplet excited state and internal conversion between states of the same spin state. The processes causing these (besides and chemical reaction) are anharmonicity in molecule vibrations and particularly heavy atoms or unpaired electrons as part of the molecule itself or in nearby molecules such as in a solvent. Some molecules may also have symmetry forbidden optical transitions in emission but not in absorption: carotenes are of this type.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.