Argon-photoion–Auger-electron Coincidence Measurements Following K-shell Excitation by Synchrotron Radiation

Argon photoion spectra have been obtained for the first time in coincidence with K-LL and K-LM Auger electrons, as a function of photon energy. The simplified charge distributions which result exhibit a much more pronounced photon-energy dependence than do the more complicated noncoincident spectra. In the near-K-threshold region, Rydberg shakeoff of np levels, populated by resonant excitation of K electrons, occurs with significant probability, as do double-Auger processes and recapture of the K photoelectron through postcollision interaction

Measurement of the Ratio of Double-to-Single Photoionization of Helium at 2.8 keV Using Synchrotron Radiation

We report the first measurement of the ratio of double-to-single photoionization of helium well above the double-ionization threshold. Using a time-of-flight technique, we find He++/He+=1.6±0.3% at hν=2.8 keV. This value lies between calculations by Amusia (2.3%) and by Samson, who predicts 1.2% by analogy with electron-impact ionization cross sections of singly charged ions. Good agreement is obtained with older shake calculations of Byron and Joachain, and of Åberg, who predict 1.7%

High-energy Behavior of the Double Photoionization of Helium from 2 to 12 keV

We report the ratio of double-to-single photoionization of He at several photon energies from 2 to 12 keV. By time-of-Aight methods, we find a ratio consistent with an asymptote at 1.5%±0.2%, essentially reached by h v≈4 keV. Fair agreement is obtained with older shake calculations of Byron and Joachain [Phys. Rev. 164, 1 (1967)], of Aberg [Phys. Rev. A 2, 1726 (1970)], and with recent many-body perturbation theory (MBPT) of Ishihara, Hino, and McGuire [Phys. Rev. A 44, 6980 (1991)]. The result lies below earlier MPBT calculations by Amusia et al. [J. Phys. B 8, 1248 (1975)] (2.3%), and well above semiempirical predictions of Samson [Phys. Rev. Lett. 65, 2861 (1990)], who expects no asymptote and predicts ơ(He2+)/ơ (He+) =0.3% at 12 keV

Auger-electron–Photoion Coincidence Measurements of Atoms and Molecules Using X-ray Synchrotron Radiation

As the availability of intense beams of hard X-rays from synchrotron-radiation sources has increased, interest in the fundamental interactions of X-rays with deep core electrons in free atoms and molecules has undergone a resurgence. These powerful X-ray sources have led to more highly differential measurements, providing ever more detailed information about atomic and molecular structure and dynamics. One such improvement in measurement technique is the application of electron—ion coincidence spectroscopy, which only recently has been extended to studies of deep core levels. A collaborative research program focusing on the dynamics of photoionization and Auger decay following hard X-ray absorption by atoms and small molecules, as well as subsequent fragmentation in the molecular case, has been ongoing at the National Synchrotron Light Source for the past few years. This report summarizes the capabilities of the program by reviewing results on argon near the K edge, and discusses briefly newer measurements on xenon and a few sulfur- and chlorine-containing molecules

Photoion–Auger-electron Coincidence Measurements Near Threshold

The vacancy cascade which fills an atomic inner-shell hole is a complex process which can proceed by a variety of paths, often resulting in a broad distribution of photoion charge states. We have measured simplified argon photoion charge distributions by requiring a coincidence with a K-LL or K-LM Auger electron, following K excitation with synchrotron radiation, as a function of photon energy, and report here in detail the argon charge distributions coincident with K-L1L23 Auger electrons. The distributions exhibit a much more pronounced photon-energy dependence than do the more complicated non-coincident spectra. Resonant excitation of the K electron to np levels, shakeoff of these np electrons by subsequent decay processes, double-Auger decay, and recapture of the K photoelectron through postcollision interaction occur with significant probability