[Ahmed Abdullah]

We know that when an atom is excited electron is elevated from the ground state. But I can't really figure what is an excited molecule..
Is it something like , hmm....the molecular orbital is enlarged so that the shared electrons can enjoy extra freedom or something else?
I think you have understood the problem.

[Borek]

We better start from the very beginning - what does it mean that atom is excited? What happens to the excited electron?

[Ahmed Abdullah]

We better start from the very beginning - what does it mean that atom is excited? What happens to the excited electron?

1. At least one of the electron absorbs energy and rises from the ground state.

2. Excited electron remains in some higher energy level (as long as it is an excited atom).

I like your approach BOREK. Can you be more help?

[Borek]

2. Excited electron remains in some higher energy level (as long as it is an excited atom).

And in terms of orbitals?

[Ahmed Abdullah]

2. Excited electron remains in some higher energy level (as long as it is an excited atom).

And in terms of orbitals?

electrons jump from one orbital to other; where the latter is somehow more energetic than the other. Right?

[Borek]

Exactly. Now, molecules are not different - just instead of atomic orbitals they have molecular orbitals.

[Ahmed Abdullah]

we know atoms can be ionized by absorbing photons of relevant energy.
So molecules like Cl2 should be able to undergo similar process to give rise to: Cl2 + photon-------> 2Cl+
Is it reality?

[Borek]

You have to decide what you are talking about - 2Cl⁺ are not an excited Cl₂.

[Ahmed Abdullah]

absorbing sufficient energy- atoms can eject out electron from the orbital so that the electron is no longer under the influence of the nucleus; that is a positive ion.
From the analogy I think similar thing can happen to electron pair in a molecular orbital.

[Borek]

From the analogy I think similar thing can happen to electron pair in a molecular orbital.

Not necesarilly an electron pair - it may happen to a single electron leading to ion like H₂⁺. But this is completely new entity, it is NOT an excited molecule. So you have to decide if you are asking about excited molecule, or ionized molecule.

[Yggdrasil]

Often light can break chemical bonds in a molecule without ionizing the substance (i.e. without ejecting any electrons).  For example, Cl₂ can break down into two chlorine radicals (Cl^.).