J. B. West

Triply differential photoelectron studies of non-Franck–Condon behavior in the photoionization of acetylene

Vibrational branching ratios and photoelectron angular distributions for alternative vibrational levels of C2H2+X 2Πu have been measured in the range 13 eV⩽hν⩽25 eV using synchrotron radiation. Below hν∼16 eV, these data exhibit strong non‐Franck–Condon effects, namely, wavelength‐dependent vibrational branching ratios and vibrational‐state‐dependent photoelectron asymmetry parameters. Moreover, enhanced excitation of bending modes of the ion is observed below hν∼16 eV, in addition to the C–C stretch mode, which is the only mode readily observed in photoelectron spectra of C2H2 at shorter wavelengths, e.g., at the He I (21.2 eV) resonance line. The non‐Franck–Condon behavior is attributed to resonant photoionization processes, whose identification is discussed in the framework of several recent theoretical and experimental studies on acetylene and related molecules.

The performance of a 5 metre normal incidence monochromator at the Daresbury Laboratory Synchrotron Radiation Source

Abstract The characteristics and performance of a 5 metre normal incidence monochromator used for gas phase experiments are given, together with a description of the pre- and post-monochromator focusing optics. Measurements have been performed to determine the absolute photon flux output and the contribution from higher orders of radiation. Highlights are given of the experimental programme in photochemistry and photophysics.

Soft x‐ray beam line for surface EXAFS studies in the energy range 60≤hν≤11 100 eV at the Daresbury SRS

The characteristics of the beam line used for surface EXAFS (extended x‐ray absorption fine structure) studies at the Daresbury Synchrotron Radiation Source (SRS) are described. Monochromatic photons are available in the energy range 60≤hν≤11 100 eV. This has been achieved using a monochromator with a combination of crystal and grating optics. The radiation is focused through the monochromator by a toroidal premirror at 0.5° grazing angle. Three pairs of crystals and a plane grating with focusing mirror are available which can be interchanged under ultrahigh vacuum (UHV). The plane grating/mirror monochromator has two ranges of zero‐order angles, 6.4° and 2°, giving photon energy ranges of 60–550 eV and 600–2000 eV, respectively, with a 1200 l mm−1 grating. The crystals in use are InSb(111), Ge(111), and Ge(220) having photon energy ranges of 1745–7360 eV, 2000–8430 eV, and 3260–11 100 eV. The performance of the gratings and crystals in respect to intensity, resolution, scattered light, and higher orders ...

0.1–10 keV soft X-ray beamline for surface EXAFS studies at the Daresbury SRS

Abstract The characteristics of a 0.1–10 keV beamline used for SEXAFS studies are described.

The angular distribution parameters of argon, krypton and xenon for use in calibration of electron spectometers

Abstract Measurements are presented of the angular distribution parameter for the photo ejected valence electrons of argon, krypton and xenon, from threshold to hv ≈25 eV. The experimental arrangement at the NBS (SURF II) storage ring is described and the method of data analysis discussed. The results are compared with other experimental data and theoretical predictions.

Wavelength and vibrational‐state dependence of photoelectron angular distributions. Resonance effects in 5 σ photoionization of CO

Photoelectron angular distribution were studied for the photoionization of CO for v=0−3 in the range 16–26 eV. (AIP)

The 4d, 5s and 5p partial photoionization cross sections of xenon above the 4d threshold

The partial cross sections for the 4d, 5s and 5p electrons in xenon have been measured between 60 and 135 eV. The results indicate the importance of electron correlation effects between the 4d inner shell and the 5s and 5p outer subshells.

Measurement of the Absolute Photoionization Cross Section of C+ near Threshold

The absolute photoionization cross section of the ground-state C+ ion has been measured from the ionization threshold at 24 eV to 31 eV using a merged ion-photon beam setup with synchrotron radiation from an undulator. The experimental results have been compared with nonrelativistic theoretical data from the Iron Project and the agreement is generally good in the near-threshold region, but differences in the magnitude of the continuum cross section of 15%-35% at higher energies and several deviations in the resonance structure are observed. The present measurements are a part of an experimental program to test the extensive theoretical photoionization cross section calculations performed within the framework of the Opacity Project and the Iron Project.

Absolute photoionization cross section of K+ ions from the 3p to the 3s threshold

The absolute photoionization cross section of ground state K+ ions has been measured from the 3p threshold to above the 3s threshold (31-49 eV) by VUV radiation using a merged ion-photon beam set-up. The cross section, which reaches a maximum value of 32 Mb ±10% at 31.8 eV and then decreases with energy, is in overall good agreement with absolute measurements by Peart and Lyon, but the present investigation also reveals the 3s3p6np 2P (n = 4-6) resonances and eliminates a previously unassigned structure at 35.5 eV. The energies, widths and shape parameters of the autoionizing 2P resonances are determined and compared with recent data from dual-laser plasma measurements and with theoretical predictions. The present experimental set-up will allow absolute photoionization cross sections 0.1 Mb to be measured, making it possible to measure the absolute photoionization cross sections of ions of astrophysical importance.

Angular distribution and photoionization cross section measurements on the 3p and 3s subshells of argon

The photoelectron angular distribution parameter has been measured in the energy region 0-34 eV above threshold for argon 3p electrons, using a synchrotron light source. The cross section of the argon 3s subshell has also been measured in the 0-20 eV region above its threshold. Both sets of data are compared with recent theoretical calculations.