Ahmad H. Farhat

High-resolution angle-resolved measurements in atoms and molecules using advanced photoelectron spectroscopy at the ALS

Abstract A two-dimensional photoelectron imaging technique using time-of-flight spectrometry and a high-resolution SES-200 electron energy analyzer have been used with photons from the atomic and molecular undulator beamline of the Advanced Light Source to study the structure and dynamics of atoms and molecules. The analyzers are rotatable around the direction of the photon beam, allowing angular-dependent studies to be performed. Results on the photoexcitation and decay mechanisms of doubly excited resonances in neon illustrate the importance of spin-orbit effects for such a light atom, since significant breakdown of LS-coupling was measured for such a light atom . Also, fully vibrationally resolved Auger electron spectra measured under Resonant Raman conditions following the C 1s→π* excitation in a diatomic molecule, CO, is presented which allows us to probe the core excited states, their excitation and de-excitation dynamics with unprecedented resolution.

Angular distribution of the Ne autoionization resonances: experimental and theoretical study

We have used angle-resolved photoelectron spectroscopy and improved R-matrix calculations to study the (n = 3 - 5) autoionizing resonances and the doubly excited resonance in the photon energy range between 44 and 48.5 eV. The photoelectrons were detected at different angles using three time-of-flight spectrometers. The measurements were obtained utilizing the 18 meV photon energy resolution provided by an undulator coupled with a spherical grating monochromator of the Advanced Light Source. Angular distribution parameters of these resonances have been parametrized using a model by Kabachnik and Sazhina and have been compared with our R-matrix calculations which are similar to those of Burke and Taylor. We find that the calculations agree quite well with the measurements. Our analysis of the cross section shape shows no strong n dependence of the q and parameter, while the width varies as the quantum defect theory predicts. Also, the differential cross section parameters agree well with previous experiments carried out for the lower resonances.

Angle resolved resonant raman Auger spectroscopy of the Xe 4p→6p transition

We studied the angular distributions and decay rates of the Xe 4d5/2→6p resonant Auger lines using the high resolution and high flux of undulator beamline 9.0.1 at the Advanced Light Source. The electron spectra were recorded by two time-of-flight (TOF) spectrometers with energy resolutions as low as to 43 meV. This made it possible to determine the angular distribution parameters β of almost all possible final ionic 5p4(3P,1D,1S)6p states. A variation of the β parameter in an energy scan over the resonance shows evidence for a possible interference of the resonant with the nonresonant path way to the 5p4(3P)6p(2P3/2) final state.

Precision Angle Resolved Autoionization Resonances in Ar and Ne

We have used angle-resolved photoelectron spectroscopy and improved R-matrix calculations to study the Ar 3s23p6→3s3p6np (n=4–16) and Ne 2s22p6→2s2p6np (n=3–5) autoionization resonances, and the Ne 2s22p6→2p43s3p doubly excited resonance. The photoelectron angular distribution parameters β have been parameterized according to the prescription of Kabachnik and Sazhina (1) and have been compared with our R-matrix calculations and those of Taylor (2). We have also analyzed the cross section shape for the resonances, which agree well with previous experiments done for the lower resonances.

High Resolution Photoionization and Excitation Using Third Generation Synchrotron Radiation Sources

Atomic and molecular structure and dynamics can now be explored with unprecedented resolution with third generation synchrotron radiation sources. Insertion devices, like undulators and wigglers, coupled with well designed monochromators, make synchrotron radiation a powerful tool since it can offer tunability, intensity, polarization and time structure.