What is meant when it is said that a molecule is excited?

[Ahmed Abdullah]

I know what an exited atom is : any of it's electron is elevated from the ground state. But I can't really figure out what is an excited molecule (e.g diene of polyene)...
I have read molecules like diene or polyene can be excited and they absorb light wave for this as atoms do to send some of their electrons to higher energy level.
Is it something like , hmm...the molecular orbital is enlarged so that the shared electrons can enjoy extra freedom (higher energy) or something else?
I think you have understood the problem.

 

[LtStorm]

Well, exciting a molecule is pretty much the same concept as exciting an atom. It just gets a little more complicated when you do it to electrons in bonds, or to non-bonded electrons on an atom bonded to other atoms.

The electrons in molecule bonds have excitation states they can be pushed into by the absorption of light just like non-bonded electrons. Of course, this leads to anti-bonding orbitals and such, as well as the concept of chromophores, organic molecules that absorb in the visible to ultraviolet spectrum.

So it's technically not the molecule, but the electrons within the molecule that are excited, just like with atoms I suppose.

Which, that absorption and the re-emission are taken advantage of for all kinds of molecular absorption spectrometry.

 

[pkleinod]

I know what an exited atom is : any of it's electron is elevated from the ground state. But I can't really figure out what is an excited molecule

A molecule can exist in many many states. One of these is the ground state. If the molecule is in any other state than the ground state, it is said to be in an excited state or, simply, excited.

Because a molecule consists of more than one atom, it has more degrees of freedom than a single atom: a molecule can rotate and vibrate. If you shine microwave radiation of the correct frequency on a molecule, it can be rotationally excited. Infra-red radiation excites the vibrational modes. Much higher frequencies (visible light, ultraviolet light) may excite the molecule into a higher electronic state.

 

[LtStorm]

Feh, shouldn't have typed my first post out just after waking up from a nap, as it came out ambiguous and confusing sounding, to me at least.

As pkleinod pointed out, there are different ways a atoms (and by extension molecules) can be excited. Visible light and higher frequencies have an effect on the electrons themselves, pushing them into higher states. Infrared causes the whole molecule to vibrate, and microwave and lower frequency radiation causes the molecule/atoms to spin (which can be prohibited by molecular bonds in some cases, presumably causing other effects).