Question about excited atoms etc.

[Denver Dang]

A quick question...

When you excite electrons or so in molecules, atoms etc., do the molecule/atom change, or what does happen ?

I mean, if you have water, and excite an electron, do it turn from "liquid" to "gas", or the other way around, or doesn't it change its structure or what you can call it ?

I know that you can ionize an atom if you excite an electron to "infinity". But if you just go from the ground state to the 1st excited state, does anything happen there, if you take into account that it doesn't decay to the ground state almost instant.

Hope you know what I mean.

Regards :)

 

[nismaratwork]

A quick question...

When you excite electrons or so in molecules, atoms etc., do the molecule/atom change, or what does happen ?

I mean, if you have water, and excite an electron, do it turn from "liquid" to "gas", or the other way around, or doesn't it change its structure or what you can call it ?

I know that you can ionize an atom if you excite an electron to "infinity". But if you just go from the ground state to the 1st excited state, does anything happen there, if you take into account that it doesn't decay to the ground state almost instant.

Hope you know what I mean.

Regards :)

I'm not sure what you mean... are you asking if ELECTRONS undergo some kind of phase change (they don't)? If you're asking what amount of energy causes a phase transition in given material... that's the question, and it's simple and a question of chemistry. Excitation is achieved by adding energy... so take a simple example: you have an ice-cube, and you put it over a burner... you're adding energy, causing molecular motion to increase, and the ice phase-changes to water (it melts). Keep going and you get steam... and so forth.

The amount of energy needed depends on the volume of material, i.e. how many molecules of the substance. You can't just take some water and pick a single electron and excite it, you just dump energy into it. If I've missed your question, I apologize, but you're mixing the macroscopic with the microscopic.

 

[alxm]

When you excite electrons or so in molecules, atoms etc., do the molecule/atom change, or what does happen ?

They will change, yes. Exciting a single electron will change the charge density around the atom/molecule, its bond strength, its dipole moment, its geometry, and quite a few things.

If you look at the simplest possible molecule, H₂⁺, then any electronic excitation will lead to it dissociating (because there's only the one electron holding it together, and only one 'bonding' electronic state). But with most molecules a single electron excitation isn't usually enough to cause a (covalent) bond to break. Ionization might, though.

It doesn't cause a phase change though. Phase (gas/liquid etc) is a macroscopic property. It just doesn't make sense to say whether a single molecule is gas or liquid. E.g. a water molecule might hydrogen bond to 3-4 other water molecules in the liquid. But a cluster of 5 water molecules in vacuum dosen't constitute a water droplet. In simple terms, phase is whether or not the average thermal energy exceeds the average intermolecular bonding energy. But the electronic energy is largely decoupled from the thermal energy.

In most cases, an excited electron simply returns to its ground state before much can happen chemically (with some exceptions like photochemical reactions, which often involve ionization rather than a bound excitation). In the cases where an excited electronic state is long-lived enough to persist for chemically-relevant timescales, it's effectively a different molecule. When chemists talk about O₂, they mean oxygen in its triplet spin state, the ground state. If they mean the excited singlet spin state, which is relatively stable, then they say "singlet oxygen".

Singlet oxygen is much more reactive than triplet oxygen for the same reason that it's relatively stable as an excited state. Electrons can't spontaneously flip their spin and change from singlet to triplet, due to selection rules/conservation laws. If it hadn't been for that, we'd all spontaneously combust immediately, because the same rules stop triplet oxygen from immediately reacting. (because both the molecules it'd react with, and the product molecules would be singlets.)

 

[nismaratwork]

They will change, yes. Exciting a single electron will change the charge density around the atom/molecule, its bond strength, its dipole moment, its geometry, and quite a few things.

If you look at the simplest possible molecule, H₂⁺, then any electronic excitation will lead to it dissociating (because there's only the one electron holding it together, and only one 'bonding' electronic state). But with most molecules a single electron excitation isn't usually enough to cause a (covalent) bond to break. Ionization might, though.

It doesn't cause a phase change though. Phase (gas/liquid etc) is a macroscopic property. It just doesn't make sense to say whether a single molecule is gas or liquid. E.g. a water molecule might hydrogen bond to 3-4 other water molecules in the liquid. But a cluster of 5 water molecules in vacuum dosen't constitute a water droplet. In simple terms, phase is whether or not the average thermal energy exceeds the average intermolecular bonding energy. But the electronic energy is largely decoupled from the thermal energy.

In most cases, an excited electron simply returns to its ground state before much can happen chemically (with some exceptions like photochemical reactions, which often involve ionization rather than a bound excitation). In the cases where an excited electronic state is long-lived enough to persist for chemically-relevant timescales, it's effectively a different molecule. When chemists talk about O₂, they mean oxygen in its triplet spin state, the ground state. If they mean the excited singlet spin state, which is relatively stable, then they say "singlet oxygen".

Singlet oxygen is much more reactive than triplet oxygen for the same reason that it's relatively stable as an excited state. Electrons can't spontaneously flip their spin and change from singlet to triplet, due to selection rules/conservation laws. If it hadn't been for that, we'd all spontaneously combust immediately, because the same rules stop triplet oxygen from immediately reacting. (because both the molecules it'd react with, and the product molecules would be singlets.)

@ Bolded portion: That's the kind of line you just don't hear outside of PF too often.