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In a photochemical reaction, everything starts with absorption of a photon. I.e. a ground state reactant is excited to the first excited state. Sometimes the reaction barrier on the ground state surface is too high; perhaps lower barriers exist in other dimensions that lead to undesirable products or dissociation. Perhaps the relevant barrier is much lower on the excited state surface, which makes the reaction possible in the excited state.

Is the difference, then, that thermal reactions take place exclusively on the ground state surface, while a photochemical reaction takes place, at least in part, on the excited state surface?

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I agree with the other answer; thermal reactions take place on the ground state, but I would add that once an excited state is produced thermal reactions can occur there also. A nice example is the photochemical isomerisation of trans stilbene but other molecules such as octatetraene behave similarly. Photo-induced electron transfer is another example of a thermal reaction in the excited state.

A sketch of the potential energy profile along the reaction coordinate is shown below.

trans-cis stilbene

Once in the trans excited state there is a barrier (the transition state) that prevents the molecule from isomerising immediately. The barrier height is approx 15 kJ/mol and there is fluorescence from the excited state (lifetime 70 ps) which is in competition with isomerisation. The long fluorescence lifetime means that thermalisation occurs in the excited state. Once over the barrier there is a conical intersection which is crossed ballistically, that is without thermalisation, and either the trans ground state is produced or the cis ground state.

The small well in the excited trans state is at a greater bond extension than the ground state, meaning that before isomerisation the bond is stretched then it is twisted on the pathway to the cis isomer.

If the cis ground state is excited, this moves ballistically to the conical interaction as there appears to be no barrier to isomerisation.

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Yes, you are right thermal reactions take place exclusively on the ground state surface, while a photochemical reaction takes place, at least in part on the excited state surface.

But there is another difference between them as photochemical processes offer an advantage over thermal methods by forming thermodynamically disfavored products. Therefore, they overcome large activation barriers in a short period of time, and allow reactions that were inaccessible by thermal processes.

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