Femtosecond Interatomic Coulombic Decay in Free Neon Clusters: Large Lifetime Differences between Surface and Bulk G. O ¨ hrwall, 1, * M. Tchaplyguine, 1,2 M. Lundwall, 1 R. Feifel, 1,† H. Bergersen, 1 T. Rander, 1 A. Lindblad, 1 J. Schulz, 2 S. Peredkov, 3 S. Barth, 4 S. Marburger, 4 U. Hergenhahn, 4 S. Svensson, 1 and O. Bjo ¨rneholm 1 1 Department of Physics, Uppsala University, Box 530, S-751 21 Uppsala, Sweden 2 MAX-lab, Lund University, Box 118, SE-221 00 Lund, Sweden 3 Institute of Physics, Lund University, Box 118, SE-221 00 Lund, Sweden 4 Max-Planck-Institute for Plasma Physics, EURATOM Association, Boltzmannstrasse 2, 85748 Garching, Germany (Received 28 May 2004; published 19 October 2004) A quantitative determination of 2s vacancy lifetimes in surface and bulk atoms of free Ne clusters has been made. While for free atoms the 2s inner-valence hole has a ps lifetime, it reduces to 6  1 fs for cluster bulk atoms. For surface atoms, the lifetime is on average longer than 30 fs. The lifetime estimate was obtained from fits of high-resolution photoelectron spectra of Ne clusters. The shortening of the lifetime is attributed to the coordination dependent interatomic Coulombic decay, which is extremely sensitive to internuclear distances. DOI: 10.1103/PhysRevLett.93.173401 PACS numbers: 36.40.Cg, 34.50.Gb It is well established that the binding energy of a core electron is dependent on its chemical surrounding. This is the basis of ESCA (electron spectroscopy for chemical analysis) [1], which today is a standard technique for surface analysis. Whether there is a dependence of the core-hole lifetime on the chemical surrounding is less well studied. Experimental attempts to find variations of the core-hole lifetime predicted in some calculations [2,3] did not have conclusive results [2,4,5], thus corrob- orating the picture of Auger decay as a local process. The only known exception to this seems to be molecules with strongly ionic (fluorine) ligands, in which some works found an influence of the number of ligands on other centers [6,7]. Recently, however, Cederbaum and co-workers have suggested a novel type of electronic decay mechanism, which should quite generally prevail in weakly bound complexes and which is only possible due to the presence of the chemical surroundings. In this process a vacancy in an inner-valence shell is filled by an outer-valence elec- tron, while a valence electron from a neighboring site in the complex is emitted into the continuum. It has been known that in a cluster or a solid, a delocalized two-hole state may well have a lower energy than an inner-valence hole, and a transition of this type is thus energetically possible, but it has not until now been realized that this mechanism will be the dominant relaxation pathway in many cases of inner-valence photoionization. According to investigations by Cederbaum and co-workers, vacancy lifetimes including this decay channel can be of the order tens to hundreds of femtoseconds, i.e., much shorter than the radiative decay, which has a lifetime in the order of hundreds of picoseconds. The first system they investi- gated was F 2s holes in the hydrogen bonded molecular cluster HF 3 [8], but also, in van der Waals bonded systems like Ne clusters, 2s vacancies are expected to show the same behavior [9,10]. A direct experimental proof for the existence of this type of decay was recently reported by Marburger et al. [11], where the low kinetic energy electrons emitted in the decay of 2s 1 states in Ne clusters were observed. There is a central difference between this decay mecha- nism and Auger decay: The Auger process is essentially an intra-atomic mechanism, whereas this new decay process is interatomic. Hence, it has been named inter- atomic or intermolecular Coulombic decay (ICD). The participation of neighboring sites in this process makes the lifetime of an inner-valence hole which decays by ICD much more sensitive to the environment than what has been observed for Auger decay. This again is backed by theoretical investigations of Cederbaum and co-workers, who predict that the Ne 2s lifetime strongly decreases with an increasing number of neighbors in Ne clusters. For the central atom in a Ne 13 cluster, the Ne 2s lifetime was estimated as 3 fs, compared to about 85 fs for the Ne dimer. The surface atoms in the Ne 13 cluster were pre- dicted to have an ICD lifetime on the order of 10 fs [10]. These findings are all related to a strong dependence of the interatomic decay rate on the number and distance of neighboring atoms [9,10]. A lifetime of 3 fs corresponds to a Lorentzian spectral width of more than 200 meV. This width would be easy to resolve in conventional photoelectron spectroscopy, where much higher resolutions can be achieved. In this Letter we present photoelectron spectroscopy results for Ne clusters with an experimental resolution of 30 meV , in which we observe a lifetime broadening consistent with predictions including the ICD process. Furthermore, we report a lifetime difference between surface and bulk features, as predicted by Cederbaum and co-workers. The experiments were performed at beam line I411 at the synchrotron facility MAX-lab in Lund, Sweden [12]. VOLUME 93, NUMBER 17 PHYSICAL REVIEW LETTERS week ending 22 OCTOBER 2004 173401-1 0031-9007= 04=93(17)=173401(4)$22.50  2004 The American Physical Society 173401-1