Showing papers by "John D. Bozek published in 2007"

A Velocity Map Imaging Spectrometer for Electron-Ion And Ion-Ion Coincidence Experiments With Synchrotron Radiation

Abstract: We have built a velocity imaging (VMI) spectrometer optimized for angle-resolved photoionization experiments with synchrotron radiation (SR) in the VUV and soft X-tay range. The spectrometer is equiped 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 [T.J.B. Eppink and D.H. Parker, Rev. Sci. Instrum. 68 (1997) 3477]provides better focusing of an extended interaction region, which is crucial for most SR applications. Furthermore, the apparatus is equipped with a second micro-channel plate 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 CO2 molecules are presented.

Size effects in angle-resolved photoelectron spectroscopy of free rare-gas clusters

Abstract: 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.

Shape resonances in the absolute K -shell photodetachment of B

Abstract: $K$-shell photodetachment of ${\mathrm{B}}^{\ensuremath{-}}$ 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.

Progress report on the LCLS XFEL at SLAC

Abstract: The Linac Coherent Light Source (LCLS) Project will be an x-ray free-electron laser. It is intended to produce pulses of 800-8,000 eV photons. Each pulse, produced with a repetition frequency of up to 120 Hz, will provide >1012 photons within a duration of less than 200 femtoseconds. The project employs the last kilometer of the SLAC linac to provide a low-emittance electron beam in the energy range 4-14 GeV to a single undulator. Two experiment halls, located 100 m and 350 m from the undulator exit, will house six experiment stations for research in atomic/molecular physics, pump-probe dynamics of materials and chemical processes, x-ray imaging of clusters and complex molecules, and plasma physics. Engineering design activities began in 2003, and the project is to be completed in the middle of 2010. The project design permits straightforward expansion of the LCLS to multiple undulators.

MERLIN — A meV Resolution Beamline at the ALS

Abstract: An ultra‐high resolution beamline is being constructed at the Advanced Light Source (ALS) for the study of low energy excitations in strongly correlated systems with the use of high‐resolution inelastic scattering and angle‐resolved photoemission. This new beamline, given the acronym Merlin (for meV resolution line), will cover the energy range 10–150 eV. The monochromator has fixed entrance and exit slits and a plane mirror that can illuminate a spherical grating at the required angle of incidence (as in the SX‐700 mechanism). The monochromator can be operated in two different modes. In the highest resolution mode, the energy scanning requires translating the monochromator chamber (total travel 1.1 m) as well as rotating the grating and the plane mirror in front of the grating. The resolution in this mode is practically determined by the slits width. In the second mode, the scanning requires rotating the grating and the plane mirror. This mode can be used to scan a few eV without a significant resolution loss. The source for the beamline is a 1.9 m long, 90 mm period quasi periodic EPU. The expected flux at the sample is higher than 1011 photons/s at a resolving power of 5 × 104 in the energy range 16–130 eV. A second set of gratings can be used to obtain higher flux at the expense of resolution.

Quasibound continuum states in SiF4 (D̃A12) photoionization: Photoelectron-vibrational coupling

Abstract: The authors report a fully vibrationally resolved photoelectron spectroscopy investigation of a nonplanar molecule studied over a range of excitation energies. Experimental results for all four fundamental vibrational modes are presented. In each case significant non-Franck-Condon effects are seen. The vibrational branching ratio for the totally symmetric mode ν1+ is found to be strongly affected by resonant excitation in the SiF4+ (DA12) photoionization channel. This is shown to be the result of two distinct shape resonances, which for the first time have been both confirmed by theoretical calculations. Vibrationally resolved Schwinger photoionization calculations are used to understand the vibronic coupling for the photoelectrons, both using ab initio and harmonic vibrational wave functions.

Photo double detachment of CN$^{-}$: Electronic decay from an inner-valence hole in molecular anions

Abstract: The first measurements of inner-valence photodetachment from a molecular negative ion are presented. Experimental and theoretical studies of CN − photodetachment around the 2-electron threshold (25–90 eV) are reported. Included are measured absolute cross sections for CN + production by photo double detachment of CN − , and for C + and N + fragments produced from the dissociation of the excited molecule. The measurements also reveal the signature of inner-valence autoionization, similar to the interatomic Coulombic decay (ICD) phenomenon. This work confirms the predicted effect, which should in general be present for molecular anions, even in very small molecules.

Dynamical and relativistic effects in experimental and theoretical studies of inner-shell photoionization of sodium

Abstract: High-resolution measurements for inner-shell photoionization of Na are presented along with Breit?Pauli R-matrix calculations. The results show excellent agreement, and give insight into the nature and importance of the various relativistic and correlation effects on the inner-shell process.

Vibrationally resolved photoionization dynamics of CF4 in the DA12 state

Abstract: Vibrationally resolved photoelectron spectroscopy of the CF4+ (DA12) state is studied for the first time over an extended energy range, 26.5⩽hν⩽50eV. It is found that the energy dependence of the totally symmetric stretching vibration is qualitatively different from all of the other vibrational modes. Moreover, the vibrational branching ratio curves for all of the symmetry forbidden vibrations are nearly identical. Qualitative arguments are used to show that it is likely that at least two shape resonances are present in the continuum, and that their characteristics, such as energy dependence and spatial localization, are distinctly different.

Size effects in van der Waals clusters studied by spin and angle-resolved electron spectroscopy and multi-coincidence ion imaging

Abstract: We have studied the valence and inner-shell photoionization of free rare-gas clusters by means of angle and spin resolved photoelectron spectroscopy and momentum resolving electron-multi-ion coincidence spectroscopy. The electron measurements probe the evolution of the photoelectron angular distribution and spin polarization parameters as a function of photon energy and cluster size, and reveal a strong cluster size dependence of the photoelectron angular distributions in certain photon energy regions. In contrast, the spin polarization parameter of the cluster photoelectrons is found to be very close to the atomic value for all covered photon energies and cluster sizes. The ion imaging measurements, which probe the fragmentation dynamics of multiply charged van der Waals clusters, also exhibit a pronounced cluster size dependence.

Shape resonances in K-shell photodetachment of small size-selected clusters: Experiment and theory

Abstract: $K$-shell photodetachment of size-selected ${{\mathrm{B}}_{2}}^{\ensuremath{-}}$ and ${{\mathrm{B}}_{3}}^{\ensuremath{-}}$ cluster anions has been measured and calculated. The experimental absolute photodetachment cross sections exhibit bound resonances below threshold and two shape resonances above the $K$-shell threshold. Similar results were obtained for all of the cationic products observed, ${\mathrm{B}}^{+}$ and ${{\mathrm{B}}_{2}}^{+}$ from ${{\mathrm{B}}_{2}}^{\ensuremath{-}}$, as well as ${\mathrm{B}}^{+}$, ${{\mathrm{B}}_{2}}^{+}$, and ${{\mathrm{B}}_{3}}^{+}$ from ${{\mathrm{B}}_{3}}^{\ensuremath{-}}$. The overall agreement between measured and calculated photodetachment cross sections is very good. However, the theoretical study yielded additional bound resonances not observed in the experimental data.