We report new measurements of ${\mathrm{Kr}}^{q+}$ photo-ions, coincident with $K\ensuremath{\alpha}$ or $K\ensuremath{\beta}$ fluorescence as incident-photon energy is swept through the Kr $K$-shell threshold. From the branching ratios just above threshold, we obtain measurements of the ion charge-state probabilities for decay from the Kr $[2p]$ and Kr $[3p]$ states. In the threshold region, we observe both resonant enhancement and depletion of the branching ratios. By analyzing this behavior in light of theory, we extract sticking probabilities, which we feel are a useful set of parameters for investigating the general relationship between cascade decay from resonant and nonresonant hole states. A simplified theoretical model is employed to calculate these probabilities for the ${\mathrm{Kr}}^{2+}$ and ${\mathrm{Kr}}^{3+}$ cases.

Near inner-shell absorption edges, Auger and fluorescence spectra which characterize the first step of a complex cascade process exhibit properties which are well described by radiationless and radiative resonant Raman scattering theory. We present comparisons of our recent data and theory for Auger decay of argon K vacancies, xenon L vacancies, and of fluorescence decay of xenon L vacancies. A theoretical unification of Auger decay and fluorescence decay is presented which clarifies the similarities and differences between the two processes.

Double K-shell photoionization of Ne at 5000 eV was observed by recording the KK-KLL Auger-electron hypersatellite spectrum. The measured Auger spectrum is compared with the results of multiconfiguration Dirac-Fock calculations. Shake calculations are used to identify likely multivacancy states produced by photoexcitation or ionization in addition to double-K vacancies, and their calculated Auger spectra are compared with the measured spectrum. The measured relative intensities of hypersatellite and diagram Auger lines are combined with experimental and theoretical determinations of branching ratios from single- and double-K vacancies into final states to determine the ratio of double-to-single K-shell photoionization cross sections to be 0.32(4)%. This ratio is much larger than the calculated high-energy-limit ratio and indicates a large contribution of dynamic electron correlation.

We have studied double K-shell photoionization of Ne and Mo (Z = 10 and 42) at the Advanced Photon Source. Double K-vacancy production in Ne was observed by recording the KK-KLL Auger hypersatellite spectrum. Comparison is made with calculations using the multiconfiguration Dirac-Fock method. For Mo, double K-vacancy production was observed by recording the K{alpha}, {beta} fluorescence hypersatellite and satellite x rays in coincidence. From the intensities of the Auger or x-ray hypersatellites relative to diagram lines, the probabilities for double K-vacancy production relative to single K-vacancies were determined. These results, along with reported measurements on other atoms, are compared with Z-scaling calculations of the high-energy limits of the double-to-single K-shell photoionization ratio.

Measurements of Krq+ yields in coincidence with K-shell fluorescence, as incident x-ray energy is varied across the K-shell threshold, are reported. Near threshold, we observe slight variations in the branching ratios as a function of energy, which are connected with the different behaviors of the flux-normalized partial yields for each q. The lower-q yields show a resonance peak near threshold superimposed on a smoothly rising edge, whereas the higher-q yields show only a smooth rise. A simple model is developed which accounts for these features, incorporating both the threshold photoexcitation and the cascade behavior of the spectator electrons.

We present measurements of projectile angular differential cross sections, d sigma/d theta, and mean projectile energy gain or loss, Delta E-mean, as functions of the number s of electrons stabilized on the projectile in 16- and 26.4-keV Ar8+ + C-60 --> Ar(8-s)+ + C-60(r+) + (r-s)e(-) collisions. These results are discussed in view of two models of the electronic response of C-60. In the infinitely conducting sphere model the charge mobility is sufficiently high in order to average out all effects of localization of individual charge carriers. In the movable-hole model "positive holes" are assumed to be localized as point charges in their equilibrium positions on the "molecular surface" within thr times (down to 10(-16) s) between sequential over-the-barrier electron transfers. The two sets of predictions for theta are close for r less than or equal to 8, and for r less than or equal to 5 they are also in agreement with experimental results indicating ultrafast electronic response of ionized C-60. For r > 5, both models underestimate theta and therefore we have developed Monte Carlo calculations for close collisions with individual carbon atoms in C-60. The energy gain first increases with s, has a flat maximum around s = 4 and yields mean energy loss Delta E-mean = -20 +/- 5 eV for s = 7. The measured fragmentation spectra theta(s) and Delta E-mean(s) may be partially rationalized by combining each of the two smooth-sphere models with the Monte Carlo calculations for close collisions.

A method devised to measure separate absolute rates for deexcitation of metastable hydrogenic atoms and ions via radiative (R) and non-radiative (NR, e.g. Penning ionization (PI)) processes has first been applied to 6.6 keV He+(2s) collisions in Ar. Preliminary results of this new method were presented at the 1997 Budapest ISIAC conference by Henning Schmidt; subsequently, refined results have been published in the June 1998 issue of Phys. Rev. A, Rapid Communications, p. R4082. Additional data for the same projectiles have now been acquired for He and Xe targets. Preliminary comparisons of the R and NR rates for these new targets with the now final results for Ar as well as comparisons of the corresponding branching ratios are the primary focus of this paper. (C) 1999 Elsevier Science B.V. All rights reserved.

The present status of double photoionisation studies is comprehensively reviewed. Recent findings are described which shed some light on the transition from the correlated motion of the two electrons near threshold to the shakeoff-like behaviour at higher photon energies. For extremely high photon energies, where Compton scattering becomes the dominant process, new results for the He2+/He+ ratio between 6 and 120 keV are presented. The results confirm the prediction of Bergstrom et al. that the ratio reaches an intermediate maximum between 12 and 15 keV, before declining towards the asymptotic limit. Furthermore, this asymptotic limit seems not to be reached even at energies as high as 120 keV.

We introduce a novel angle-resolving electron-spectrometer system for coincidence studies of gas-phase targets using the timing structure of synchrotron radiation. The experimental setup, mainly consisting of a cylindrical mirror analyzer with a position sensitive detector and an electron time-of-flight spectrometer is particularly useful in cases where low-kinetic-energy electrons are measured in coincidence with high-kinetic-energy electrons. The experimental method and corresponding electronics are discussed. As an example, we present argon LMM Auger electron spectra taken in coincidence with KL2,3L2,3 and KL1L2,3 Auger electrons after resonant 1s→4p photoexcitation.

The present status of double photoionisation studies is comprehensively reviewed. Recent findings are described which shed some light on the transition from the correlated motion of the two electrons near threshold to the shakeoff-like behaviour at higher photon energies. For extremely high photon energies, where Compton scattering becomes the dominant process, new results for the He2+ /He+ ratio between 6 and 120 keV are presented. The results confirm the prediction of Bergstrom et al. that the ratio reaches an intermediate maximum between 12 and 15 keV, before declining towards the asymptotic limit. Furthermore, this asymptotic limit seems not to be reached even at energies as high as 120 keV.

An ion time-of-flight (TOF) spectrometer was used to measure Ar photoion charge-state distributions near the K-shell threshold (hν=3206.3 eV). When photons interact with argon atoms producing 1s holes, the atoms typically decay by KLL Auger-electron emission so that two holes in the L-shell are created; the atoms can further decay by LMM Auger electron emission. The result is a cascade-like decay process. When cascade decays take place, the final photoion charge-state depends on the decay path. Previous measurements performed by our group and others showed large discrepancies in the lowest and highest charge fractions measured. The measured charge-state fractions, however, are sensitive to several experimental parameters that can adversely affect the values of the charge-state fractions. We therefore performed a quantitative study of the variation of the measured charge-state fractions with the experimental parameters. Branching ratios for several decay processes were obtained from measurements of the photoion charge-state fractions as a function of the photon energy as well as the fluorescence yield, which is now in much better agreement with other determinations than was the case heretofore.

A method is presented to measure separate cross sections for collisional deexcitation of metastable ions via radiative and non-radiative processes. The principle of the experiment is to first determine the total collisional deexcitation cross section in an attenuation measurement. After this the non-radiative part is determined separately in a measurement where the ionized target atom (unique to the non-radiative process) is detected. Here, we discuss recently published results on deexcitation of metastable He+(2s) ions colliding with Ar at 1.65 keV/amu (1,2) as well as preliminary results for Xe and H-2 targets. In all cases we find that the dominating contribution from radiative deexcitation agrees with the result of a semi-classical calculation of the 2s-2p mixing driven by the induced dipole field of the target atom or molecule modified by taking competing processes, for which we measured the cross sections, into account. Further, the observed time-of-flight spectra with the molecular target H-2 are discussed and evidence of low-energy dissociation is presented.

A method is presented to measure separate cross sections for collisional deexcitation of metastable ions via radiative and non-radiative processes. The principle of the experiment is to first determine the total collisional deexcitation cross section in an attenuation-like measurement. After this the non-radiative part is determined separately in a measurement where the ionized target atom (unique to the non-radiative process) is detected. Here we discuss recently published results on deexcitation of metastable He+(2s) ions colliding with Ar at 1.65 keV/amu (H T Schmidt et al 1998 Phys. Rev. A 57 R4082 and 1998 Phys. Rev. A 58) as well as preliminary results for a Xe target. In both cases we find that the dominating contribution from radiative deexcitation agrees with the result of a semi-classical calculation of the 2s-2p mixing driven by the induced dipole field of the target atom modified by taking competing processes, for which we measured the cross sections, into account. Finally the extension to H-like projectile ions in higher charge states is discussed.

In an exploratory feasibility study we have measured the angular correlation between Auger electrons that were emitted in a cascadelike decay process after resonant photoexcitation, using synchrotron radiation from the Brookhaven National Synchrotron Light Source. While the monochromator was tuned to the argon Is --> 4p resonance (3203.5 eV) we recorded Ar LMM Anger electrons in coincidence with KL2,3L2,3,KL1L2,3, and KL2,3M1,2,3 Auger electrons. We found different nonisotropic angular correlations between distinct energy regions of the LMM group of Auger lines and the KL2,3L2,3 Auger electrons, while for other kinetic energies the LMM Auger electrons exhibit isotropy. Because the KLL and LMM Auger energies are so different, we believe that the nonisotropic angular correlation observed is due to an alignment effect rather than a dynamical postcollision interaction effect. [S1050-2947(99)07801-4].

A method devised to measure separate absolute rates for deexcitation of metastable hydrogenic atoms and ions via radiative and nonradiative processes is applied in a pilot study of 6.6-keV He+(2s)-Ar -->He+(1s)-... collisions. An absolute total deexcitation cross section sigma(de)(tot)= (7.6 +/- 1.2)X 10(-16) cm(2) was measured by attenuation. The cross section for nonradiative deexcitation was then measured independently. A radiative branching ratio R-BR=(70 +/- 8)% was determined, without requiring absolute photon detection calibrations. The measured radiative deexcitation cross section agrees with the result of a semiclassical calculation when competing capture and ionization precesses are taken into account.

Ion time-of-flight mass spectroscopy was used to study the relaxation dynamics of HCl following photoexcitation in the vicinity of the Cl K threshold ({approx}2.8 keV). Detailed observations of molecular fragmentation mediated by postcollision interaction between a photoelectron and an Auger electron are presented, evidenced by the recapture of Cl K photoelectrons by either Cl{sup n+} or H{sup +} dissociation fragments. {copyright} {ital 1998} {ital The American Physical Society}

The relaxation dynamics of HCl, DCl, H2S, and D2S following photoexcitation in the vicinities of the Cl and 8 K-shell thresholds (similar to 2.8 keV for Cl, similar to 2.5 keV for S) were studied by means of ion time-of-flight mass spectroscopy. In all cases; the onset of pre-edge core-shell photoionization precedes the formation on resonance of a significant amount of neutral hydrogen as well as postcollision-interaction effects above threshold. Examination of the width of the H+ peak in spectra taken with the analyzer parallel and perpendicular to the polarization vector of the incident light indicates that on resonance, the photofragmentation asymmetry parameter, beta, is approximately two for HCl, and is clearly positive for H2S. [S1050-2947(98)08211-0].

In this work we present a method developed to measure separately the radiative and nonradiative contributions to the cross section for collisional deexcitation of metastable hydrogenlike ions. We present the results of the first experiment applying this method, where deexcitation of metastable He+(2s) ions in collisions with argon atoms is considered. First the total deexcitation cross section for 6.6 keV He-4(+)(2s) ions is measured absolutely in a beam-gas attenuation experiment. Then the absolute cross section for nonradiative deexcitation is measured separately by coincident detection of projectiles keeping their initial charge state 1+ and recoil ions (formed only in nonradiative deexcitation events). Further, we discuss a semiclassical calculation of the radiative deexcitation cross section, in which the influence of competing electron-capture processes is taken into account in a semiempirical fashion. The result is in agreement with the measured radiative deexcitation cross section of (5.4+/- 1.3)x 10(-16) cm(2), which amounts to (70+/- 8)% Of the total deexcitation cross section. [S1050-2947(98)04110-9].

Time-of-flight mass spectroscopy was used to study the relaxation dynamics of HCl following photoexcitation in the vicinity of the Cl K edge (similar to 2.8 keV) using monochromatic synchrotron radiation. At the lowest resonant excitation to the 6 sigma* antibonding orbital, almost half of the excited molecules decay by emission of a neutral H atom, mostly in coincidence with a highly charged Cln+ ion. The present work demonstrates that neutral-atom emission can be a significant decay channel for excited states with very short lifetimes (1 fs).