Search

We present a comparison of the photoionization dynamics of the 4d shell of XeF

The ionic fragmentation following B 1s and C 1s excitation of three isomeric carborane cage compounds [closo-dicarbadodecaboranes: orthocarborane (1,2-C2B10H12), metacarborane (1,7-C2B10H12), and paracarborane (1,12-C2B10H12)] is compared with the energetics of decomposition. The fragmentation yields for all three molecules are quite similar. Thermodynamic cycles are constructed for neutral and ionic species in an attempt to systemically characterize single-ion closo-carborane creation and fragmentation processes. Lower energy decomposition processes are favored. Among the ionic species, the photon-induced decomposition is dominated by BH+ and BH2(+) fragment loss. Changes in ion yield associated with core to bound excitations are observed.

We have measured the production of both Ly alpha and H alphafluorescence from atomic H and D for the photodissociation of H2 and D2by linearly polarized photons with energies between 24 and 60 eV. In thisenergy range, excited photofragments result primarily from the productionof doubly excited molecular species which promptly autoionize ordissociate into two neutrals. Our data are compared with ab initiocalculations of the dissociation process, in which both doubly excitedstate production and prompt ionization (nonresonant) channels areconsidered. Agreement between our experimental data and that of earlierwork, and with our theoretical calculations, is qualitative atbest.

When exposed to ultraintense x-radiation sources such as free electron lasers (FELs) the innermost electronic shell can efficiently be emptied, creating a transient hollow atom or molecule. Understanding the femtosecond dynamics of such systems is fundamental to achieving atomic resolution in flash diffraction imaging of noncrystallized complex biological samples. We demonstrate the capacity of a correlation method called "partial covariance mapping'' to probe the electron dynamics of neon atoms exposed to intense 8 fs pulses of 1062 eV photons. A complete picture of ionization processes competing in hollow atom formation and decay is visualized with unprecedented ease and the map reveals hitherto unobserved nonlinear sequences of photoionization and Auger events. The technique is particularly well suited to the high counting rate inherent in FEL experiments.

The silicon 2p photoelectron spectra of eleven molecules: SiH 4 , Si(CH 3 ) 4 , C 2 H 5 SiH 3 , (CH 3 ) 2 SiH 2 , (CH 3 ) 3 SiH, SiF 4 , SiCl 4 , (CH 3 ) 3 SiCl, (CH 3 ) 3 SiI, Cl 2 SiH 2 , and CH 3 SiF 3 , have been recorded at high experimental resolution. The spectra all exhibit broadening, and in some cases resolved fine structure resulting from the excitation of vibrations in the core-ionized molecules. Extensive use is made of the equivalent-cores approximation to aid in the interpretation of the observed vibrational structure

High-resolution variable-energy photoelectron spectra are reporoted for the upper valence band region of CpPtMe 3 (Cp=η 5 −C 5 H 5 ) and Me 2 Pt(COD) (COD=1,5-cyclooctadiene) for photon energies between 21 and 100 eV. Xα−SW results on the CpPtMe 3 valence levels are also reported as the basis for the spectral assignment. Experimental and theoretical braqnching ratios are in good agreement with the Xα-SW assignment but are inconsistent with the previous Fenske-Hall assignment for the CpPtMe 3 valence ionizations. Contrary to the Fenske-Hall assignment, our present results strongly indicate that the nonbonding Pt 5d orbitals are not the HOMO's and have higher ionization potentials than several Pt−L orbitals

Ultrafast molecular dynamics can be studied using x-rays from both synchrotrons sources and x-ray free electron lasers. Synchrotron studies are limited by the 10-100 ps duration pulses to processes where the Auger lifetime can be used to probe dynamics initiated by excitation of an inner-shell electron to an antibonding orbital. The short pulses produced by x-ray free electron lasers offer the opportunity to study molecular dynamics directly with pump-probe techniques. A two-mirror x-ray split and delay device has been developed for x- ray pump - x-ray probe experiments at the soft x-ray AMO instrument at the LCLS. The device operates over a photon energy range of 250-1800 eV with a variable delay of up to 200 femtoseconds with 0.1 fs resolution.

We discuss recent experiments that study the transfer of angular momentum from a projectile to the residual target in collisions between the simple diatomic molecules H2 and N2 and spin‐polarized electrons or circularly‐polarized photons. We observe the fluorescence of both the atomic fragments and excited molecular states, and measure the circular polarization fraction of this light, P3. The incident electron energies range from 10 to 100 eV; the incident photon energies from 33 to 38 eV.

We have used spin-resolved electron spectroscopy with circularly polarized synchrotron radiation to delineate the electronic structure of the hydrogen sulfide and carbonyl sulfide molecules and their photoemission dynamics. Specifically, inner-shell spin-polarization measurements of the molecular field split components of the S 2p photolines were carried out to reveal effects of the different molecular environments. The validity of simple atomic models to explain the results is discussed.

The use of high‐spectral resolution VUV‐photon beams in atomic photoionization experiments has significantly increased over the past few years, owing to the number of operating third generation synchrotron radiation sources and associated high‐resolution beam lines. For laser‐excited atoms, however, the number of experiments is still low, because of the difficulties in combining the use of these two widely‐different photon sources. In the following, we will present the results of high‐resolution photoionization experiments performed by combining a cw dye laser and the photon beam available at the 10.0.1 station of the Advanced Light Source.

Detailed analysis of the highest occupied molecular orbital band shape in the photoelectron spectrum of gaseous C60 reveals a dynamic Jahn-Teller effect in the ground state of C60(+). The direct observation of three tunneling states asserts a D(3d) geometry for the isolated cation, originating from a strong vibronic coupling. These results show that the ionic motion plays an important role in the electron-phonon interaction.

High-resolution $K$-shell photodetachment measurements of ${\mathrm{He}}^{\ensuremath{-}}$ giving rise to ${\mathrm{He}}^{+}$ ions have been performed using a merged synchrotron vacuum ultraviolet photon-ion beam technique. The measurements on this fundamental negative ion display dramatic structure differing substantially, qualitatively and quantitatively, from the corresponding process in neutral atoms and positive ions, owing to the dominance of correlation in both initial and final states of ${\mathrm{He}}^{\ensuremath{-}}$. In addition, this experimental investigation provides an unambiguous test of two independent theoretical calculations that report serious discrepancies and shows excellent agreement with one of them.

High-resolution K-shell photodetachment measurements of He- giving rise to He+ ions have been performed using a merged synchrotron vacuum ultraviolet photon-ion beam technique. The measurements on this fundamental negative ion display dramatic structure differing substantially, qualitatively and quantitatively, from the corresponding process in neutral atoms and positive ions, owing to the dominance of correlation in both initial and final states of He-. In addition, this experimental investigation provides an unambiguous test of two independent theoretical calculations that report serious discrepancies and shows excellent agreement with one of them.

High-resolution absolute experimental measurements and two independent theoretical calculations were performed for photoionization of O+ ions from the 2P(o) and 2D(o) metastable levels and from the 4S(o) ground state in the photon energy range 30-35.5 eV. This is believed to be the first comparison of experiment and theory to be reported for photoionization from metastable states of ions. While there is correspondence between the predicted and measured positions and relative strengths of the resonances, the cross-section magnitudes and fine structure are sensitive to the choice of basis states.

An experimental K-shell photodetachment study of Li(-) giving rise to doubly photoionized Li(+) ions has been carried out at the Advanced Light Source, using a collinear photon-ion beam apparatus. The experiment reveals dramatic structure, differing substantially both qualitatively and quantitatively from the corresponding processes above the 1s ionization threshold in Li and Li(+), as predicted by our enhanced R-matrix calculation. The experimental/theoretical comparison shows good agreement over some of the photon energy range, and also reveals some puzzling discrepancies.

Oscillator strength spectra in the region of B 1s and C 1s excitation of three isomeric carborane cage compounds [closo-1,2-orthocarborane, closo-1,7-metacarborane, closo-1,12-paracarborane (C2B10H...

We present time-resolved femtosecond photoelectron momentum images and angular distributions of dissociating, laser-aligned 1,4-dibromobenzene (C6H4Br2) molecules measured in a near-infrared pump, soft-x-ray probe experiment performed at an x-ray free-electron laser. The observed alignment dependence of the bromine 2p photoelectron angular distributions is compared to density functional theory calculations and interpreted in terms of photoelectron diffraction. While no clear time-dependent effects are observed in the angular distribution of the Br(2p) photoelectrons, other, low-energy electrons show a pronounced dependence on the time delay between the near-infrared laser and the x-ray pulse. Keywords: free-electron laser, photoelectron diffraction, molecular-frame photoelectron angular distributions, laser alignment, femtosecond pump–probe