Fourth Case Study Photochemistry at Nanoparticles

It is a very interesting question which additional modifications are introduced if one uses nanoparticles instead of surfaces of bulk materials. As our earlier review [13] has summarized, for MNPs new electronic excitations - notably the Mie or particle Plasmon [20] - can be expected to feed photochemical reactions on the MNPs. But also the lifetimes and thus the reactivity of excited states may be changed by going to nanoparticles the main expected effect here is the confinement of excitations in the small particles, which would quickly disperse in the volume in bulk materials [13]. 

We found that the cross section, o, for NO desorption from (NO)2 monolayers on Ag NPs is indeed strongly enhanced by excitation of the plasmon, which is known to lie at approximately 3.5 eV (with a weak dependence on NP size [89, 90]). Compared to Ag(l 11), an enhancement of o by up to a factor 40 (depending on NP size see item 3 in this list) has been found [14]. 

There is also an enhancement of o off the plasmon resonance, albeit weaker, which we interpret as due to confinement of excitations, here of hot electrons in the Ag NP. The photochemical mechanism of all the processes seen is believed to involve TNI states [87, 94], which is consistent with detailed characterizations 

On the other hand, the dynamics of the bond dissociations (i.e. the motion of the representative wave packet on the potential energy surfaces of ground and excited states for the various molecular entities) turns out to be always the same. This is concluded from the fact that the final state of the desorbing NO stays constant all energy distributions, translational, rotational, and vibrational, as well as their correlations, are identical despite the strong variations of cross sections [14, 96], These characteristics are compatible with the proposed TNI mechanism (see item 2 in 

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