K-shell photodetachment from C Ϫ has been investigated in the photon energy range between 280 and 285 eV using the merged ion-beam-photon-beam technique. C Ϫ ions were produced using a Cs sputtering negative-ion source, while the photons were produced by the undulator beam line 10.0.1 of the Advanced Light Source. C ϩ ions formed by double detachment were detected as a function of incident photon energy. Using this collinear arrangement, the relative cross sections were measured and compared with theoretical predictions. The measured spectrum shows the first experimental evidence of the 1s2s 2 2p 4 ( 4 P) shape resonance near 281.7 eV, which is in excellent agreement with two independent R-matrix calculations for the 1s photodetachment cross section of C Ϫ producing C ϩ .
The photoionization of free Xe clusters is investigated by angle-resolved time-of-flight photoelectron spectroscopy. The measurements probe the evolution of the photoelectron angular distribution parameter as a function of photon energy and cluster size. While the overall photon-energy-dependent behavior of the photoelectrons from the clusters is very similar to that of the free atoms, distinct differences in the angular distribution point at cluster-size-dependent effects. Multiple scattering calculations trace their origin to elastic photoelectron scattering. Several decades after its beginnings, cluster research continues to be an exciting area of science as most cluster properties remain much less known than those of their constituent atoms and molecules. Moreover, the scalability of clusters allows interpolation between the individual atom, the surface, and the bulk, bridging the gap between single atoms or molecules and condensed matter systems. This makes clusters unique targets to advance both the fundamental understanding of the many-body problem as well as nanotechnological applications ͓1-3͔.Of particular interest are phenomena exhibiting clustersize dependence that underline the transition from individual atoms and molecules to large cluster systems with typical solid-state behavior, such as changes in cluster geometry and electronic structure. As each atom in the cluster is surrounded by an increasing number of neighbors, its electronic structure is altered by the resulting changes in the cluster potential and the atomic orbitals evolve into the band structure of the solid state. Cluster-size-dependent electronic properties can be probed directly using angle-resolved photoelectron spectroscopy, a well established technique for the study of the electronic structure of atoms, molecules, as well as condensed matter ͓4,5͔. However, while photoelectron spectroscopy of clusters, and in particular rare-gas clusters, has been a growing field since the late 1980s ͓6-16͔, there is a paucity of angle-resolved measurements, mainly due to low target densities in the cluster beam and the resulting low signal intensities in angle-resolved measurements. To date, measurements of the photoelectron angular distribution parameter are only available for small metal clusters ͑e.g., ͓17͔͒. For rare-gas clusters, recent qualitative studies by Öhr-wall et al. ͓10͔ have shown substantial differences in the angular dependence of the photoelectron intensity from Xe clusters compared to free Xe atoms, but their experiment did not provide absolute measurements of the angular distribution parameter.In this paper, we present the first quantitative measurement of the photoelectron angular distribution parameter  ͓18͔ as a function of photon energy and cluster size for any rare-gas cluster system. Our experimental results are supported by multiple scattering calculations, which elucidate the effect of elastic electron scattering on the photoelectron angular distribution.The measurements were carried out with linearly polarized sync...
We have built a velocity map imaging (VMI) spectrometer optimized for angle-resolved photoionization experiments with synchrotron radiation (SR) in the VUV and soft x-ray range.The spectrometer is equipped with four electrostatic lenses that focus the charged photoionization products onto a position-sensitive multi-hit delay line anode. The use of two additional electrostatic lens elements as compared to the standard design of Eppink and Parker [Rev. Sci.Instrum. 68, 3477 (1997)] provides better focusing of an extended interaction region, which is crucial for most SR applications. Furthermore, the apparatus is equipped with a second microchannelplate detector opposite to the VMI spectrometer, enabling electron-ion coincidence experiments and thereby mass-resolved ion spectroscopy independent of the time structure of the synchrotron radiation. First results for the photofragmentation of CO 2 molecules are presented.
K-shell photodetachment of B − has been measured using the collinear photon-ion beamline at the Advanced Light Source, Lawrence Berkeley National Laboratory, as well as calculated using two separate R-matrix methods. The measurement of the absolute photodetachment cross section, as a function of photon energy, exhibits three near-threshold shape resonances due to the 3 S, 3 P, and 3 D final partial waves. A fit to the measured data using three resonance profiles shows good overall qualitative agreement with the three partial wave cross sections calculated using either R-matrix method. However, certain significant and unresolved quantitative discrepancies exist between the experimentally inferred and the calculated resonance profiles.K-shell photodetachment of B − produces a p-wave photoelectron ⑀p departing from a K-shell-vacancy 1s2s 2 2p 2 state of B * that subsequently undergoes Auger decay, producing a second, Auger electron ⑀l and a B + ion; the latter is detected in the present experiment. We note that if 2p 2 ͑ 3 P͒ core rearrangement, via interchannel continuum coupling, is neglected in Eq. ͑1͒, then only the 4 P and 2 P final states of B * are populated.The motivation for this work stems from the fact that K-shell photodetachment cross sections of He − , Li − , and C − showed pronounced structures such as triply excited, Feshbach, and shape resonances. Here we provide additional results on K-shell photodetachment of a different light negative ion whose atomic number lies in between those already explored, thus allowing a systematic study of resonance structure in addition to providing absolute photodetachment cross sections. One clear qualitative difference is that three final partial waves result from the K-shell photodetachment of B − , compared to the single final partial wave in the neighboring Li − ͓4,5,20,26͔ or C − ͓9,18,27͔ ions.
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