Considering the photoionization of Ar@C 60 , we predict resonant femtosecond decays of both Ar and C 60 vacancies through the continua of atom-fullerene hybrid final states. The resulting resonances emerge from the interference between simultaneous autoionizing and intercoulombic decay (ICD) processes. For Ar 3s → np excitations, these resonances are far stronger than the Ar-to-C 60 resonant ICDs, while for C 60 excitations they are strikingly larger than the corresponding Auger features. The results indicate the power of hybridization to enhance decay rates, and modify lifetimes and line profiles. [4] research has gone into studying ICD processes in weakly bound atomic systems. These involve the observation of ICD in rare gas dimers [5], rare gas clusters [6], surfaces [7], and small water droplets [8,9]. ICD followed by resonant Auger decay has been identified in Ar dimers using momentum resolved electron-ion-ion coincidence spectroscopy [10,11]. Ultrafast ICDs of a dicationic monomer in a cluster to produce a cluster tricataion [12] Of particular interest is the resonant ICD (RICD) where the precursor excitation to form an inner-shell vacancy is accomplished by promoting an inner electron to an excited state by an external stimulant, generally electromagnetic radiation [17,18] or, more recently, chargeparticle impact [21]. A theoretical study of RICD followed by Ne 2s → np excitations in MgNe clusters suggested the leading contribution of RICD among other interatomic decay modes [22]. Photoelectron spectroscopy with Ne clusters for 2s → np excitations measured the signature of RICD processes [17]. Similar excitations in the double photoionization of Ne dimers were utilized to observe RICD by tracking the formation of energetic Ne + fragments [18]. Most recently, strong enhancement of the HeNe + yield, as He resonantly couples with the radiation, is detected [19], confirming an earlier prediction [20].Atoms confined in fullerene shells forming endofullerene compounds are particularly attractive natural laboratories to study RICD processes. There are two compelling reasons for this: (i) such materials are highly stable, have low-cost sustenance at the room temperature and are enjoying a rapid improvement in synthesis techniques [23]; and (ii) the effect of correlation of the central atom with the cage electrons have been predicted to spectacularly affect the atomic photoionization [24]. A first attempt to predict ICD in endofullerenes was made by calculating ICD rates for Ne@C 60 [25]. While some speculation on the role of Coulomb interaction mediated energy transfer from atom to fullerene to broaden Auger lines has been made [26,27], no studies, theoretical or experimental, of RICD resonances in the ionization cross section of endofullerenes have been performed. Furthermore, ICD of endofullerene molecules can uncover effects not yet known. This is because: (i) endofullerenes being spherical analogues of asymmetric dimers consisting of an atom and a cluster can also induce reverse RICD processes, the decay of...
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