Site-Specific Excitation in Free Krypton Clusters

Abstract

Site-specific excitation in Kr clusters is investigated by high resolution inner-shell excitation in combination with model calculations, which are based on a core exciton model. Partial cation and total electron yield spectra of variable size Kr clusters are reported for the Kr 3d excitation regime (90 -96 eV) using synchrotron radiation. A cluster size-dependent spectral evolution is observed, corresponding to the transition from low lying cluster Rydberg states into surface and bulk exciton states. The results indicate that individual sites in perfectly and imperfectly shaped clusters are clearly distinguished.[S0031-9007 (98)05596-3] PACS numbers: 36.40.Mr, 61.10.HtClusters play a central role in linking properties of isolated atoms to those of the condensed phase of matter [1,2]. The size-dependent evolution of the electronic and geometric structure is therefore of primary interest. Van der Waals clusters, such as rare gas clusters, are ideal systems to study size effects in a systematic way. Rare gas clusters are known to consist of geometrical structures which are not well defined since perfect icosahedral structures [3] and imperfect structures [4] may be generated in jet expansions, a typical cluster production technique. Resonant photon excitation of clusters is a sensitive method to probe changes in electronic structure as a function of cluster size. Recent instrumental developments in the field high resolution x-ray spectroscopy using synchrotron radiation make high spectral resolution experiments on free clusters in the gas phase possible [4,5]. The inherent advantage in exciting highly localized core levels is due to element specific, and often site specific, processes which cannot be obtained by valence shell excitations.In this Letter we present experimental results supported by model calculations. High spectral resolution in combination with results from model calculations gives, to our knowledge for the first time, site-specific information on absorption processes of homogeneous clusters occurring in the inner-shell excitation regime. We have chosen the Kr 3d regime (90-100 eV), since it is well investigated for the atom [6] and the solid [7]. The preedge regime is dominated by two series of Rydberg states (Kr3d 5͞2 ! np and Kr3d 3͞2 ! np) in the atom, whereas in the solid the broad features of low lying core excitons are observed.The experimental setup consists of a jet expansion [8]. The skimmed jet is crossed with the beam of monochromatic synchrotron radiation (BESSY-I, beam line: U2-FSGM [9]). Cations are accelerated into a time-of-flight mass spectrometer. Total electron yields (TEY) are extracted into the opposite side using a channeltron adjacent to the cluster beam. Yields of mass-selected cluster cations are used for experiments on electronic properties of small clusters (typical size range: Kr n , with n , 70). Fragmentation is initiated in the Kr 3d regime by resonant MNNAuger decays of the core hole leading predominantly to doubly charged clusters [10], which are known to decay via ...

2s-Excitation and Photoionization of Neon Clusters

“…Several contributions may be considered in order to rationalize this finding, to which the excited-state lifetime may contribute most efficiently. Furthermore, the band width of the monochromator as well as different surfaces and bulk sites may contribute to the experimentally observed broadened line shapes, similar to previous work, where site-selective excitations in krypton clusters were discussed [8]. This aspect is discussed further below along with threshold photoelectron spectra.…”

“…This is due to characteristic energy shifts of the core exciton states relative to the atomic Rydberg states and shifts in core ionization energies [5][6][7][8][9]. The lowest energy Rydberg-/excitontransitions of clusters are significantly shifted to higher energy compared to the atomic transition, corresponding to a spectral blue-shift, whereas the higher ones are barely shifted or even show a spectral red-shift.…”

“…This allows us to measure with high accuracy the energy shifts between resonant excitations into Rydberg states in atoms and exciton states in clusters, similarly to previous high-resolution work on atomic and molecular clusters [8][9][10]. Threshold photoelectron spectra are taken by using a continuous extraction field strength of 7.5 V/cm in the ionization region.…”

“…The energy resolution of this beamline is adjusted by varying the slit width, ranging for the present work at E/E  3000 (cf. [8]). Cations and electrons are detected alternatively by either a time-of-flight mass spectrometer (cf.…”

“…These are also clearly distinguished from bulk sites [8,9]. Inner valence shell excitations of rare gas clusters, such as the Ar 3s-excitation, have also been investigated before [12][13][14][15].…”