In 1997, it was predicted (1) that an electronically excited atom or molecule placed in a loosely bound chemical system (such as a hydrogen-bonded or van-der-Waals-bonded cluster) could efficiently decay by transferring its excess energy to a neighbouring species that would then emit a low-energy electron. This intermolecular Coulombic decay (ICD) process has since been shown to be a common phenomenon (2-12), raising questions about its role in DNA damage induced by ionizing radiation, in which low-energy electrons are known to play an important part (13,14). It was recently suggested (15) that ICD can be triggered efficiently and site-selectively by resonantly core-exciting a target atom, which then transforms through Auger decay into an ionic species with sufficiently high excitation energy to permitICD to occur. Here we show experimentally that resonant Auger decay can indeed trigger ICD in dimers of both molecular nitrogen and carbon monoxide. By using ion and electron momentum spectroscopy to measure simultaneously the charged species created in the resonant-Auger-driven ICD cascade, we find that ICD occurs in less time than the 20 femtoseconds it would take for individual molecules to undergo dissociation. Our experimental confirmation of this process and its efficiency may trigger renewed efforts to develop resonant X-ray excitation schemes (16,17) for more localized and targeted cancer radiation therapy.
The experiment presented here shows that resonant excitation of a K-shell electron to a bound state is followed by Auger decay to an ionic species that can then undergo ICD, as sketched in Fig. 1 and proposed in ref. 15. The initial resonant excitation of the electron occurs as in the experiments that probed resonant interatomic Coulombic decay (5,6), but the state undergoing ICD is created after partial de-excitation of the system through a local Auger decay. The Auger decay can lead to the ground state of the molecular ion through 'participator Auger decay', although in many cases the excited electron will act as just a 'spectator' to an Auger decay in which an electron from the valence or inner valence shell fills the core hole and a second electron from the valence shell is emitted. This spectator pathway produces ionic states which are high enough in excitation energy to allow ICD to occur, and in the case of carbon monoxide accounts for the decay of approximately 75% of core-excited molecules (18). Our experiment explores the overall scenario for two simple model systems--clusters of just two carbon monoxide or two nitrogen molecules--that can be investigated in great detail. This allows us to follow the Auger decay occurring after resonant excitation of an inner-shell electron into the lowest unoccupied molecular orbital (in a [PI]* excitation) and the subsequent ICD:
hv 1 [N.sub.2]/[N.sub.2][right arrow] [N.sub.2]*([1.sub.s.sup.-1] [PI]*)/[N.sub.2] (local, resonant excitation) (1)
[N.sub.2]*([1.sub.s.sup.-1] [PI]*)/[N.sub.2] [right arrow] [N.sub.2.sup.+*]/[N.sub.2] + [e.sub.Auger] (spectator Auger decay) (2)
[N.sub.2.sup.+*]/[N.sub.2] [right arrow] [N.sub.2.sup.+] + [N.sub.2.sup.+] [e.sub.ICD] (ICD 1 two-site Coulomb explosion) (3)
where hv is the incident radiation, [PI]* is the excited molecular orbital, and [e.sub.Auger] and [e.sub.ICD] are the Auger- and ICD-emitted electrons ('[1s.sup.-1]' refers to...