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The ${L}_{1}\ensuremath{-}{L}_{2,3}M$ Coster-Kronig spectrum of argon has been measured by using photon excitation. The intensity ratio of $0.30\ifmmode\pm\else\textpm\fi{}0.02$ between the ${L}_{1}\ensuremath{-}{L}_{2,3}{M}_{1}$ main lines and the ${L}_{1}\ensuremath{-}{L}_{2,3}{M}_{2,3}$ lines is extracted from experiment. The experimental results for the ratio and partial decay rates are compared with present and previous theoretical estimates.

Angle-resolved ${M}_{4,5}{N}_{4,5}{N}_{4,5}$ Auger spectra of xenon have been measured near the Xe $3d$ threshold by using synchrotron radiation excitation. The angular anisotropy parameters extracted from the experimental data generally agree with the recent theoretical results based on multiconfiguration Dirac-Fock calculations. The angle-resolved measurements have also been employed in order to study the character of the resonance structures below the ${M}_{4}$ threshold.

The photo-double-ionization (PDI) of Mg to the Mg${}^{2+}$($3{s}^{\ensuremath{-}2}$) state has been studied by photoelectron-photoelectron-coincidence experiments at a photon energy corresponding to the excitation of the 2$p$\ensuremath{\rightarrow}3$d$ resonance. The equal energy sharing (${E}_{1}$ = ${E}_{2}$ = 16.4 eV) as well as the complementary unequal energy (${E}_{1}$\ensuremath{\leftrightarrow}${E}_{2}$ = 10.4\ensuremath{\leftrightarrow}22.4 eV) sharing kinematics have been investigated. From the experimental results without any approximation the symmetrized gerade and ungerade amplitudes have been obtained. The experimental angular correlation patterns as well as the amplitudes are compared to CCC calculations in which the resonant process has been incorporated. The results confirm that the amplitudes of the photo-double-ionization carry the signature of the target radial wave function. The investigation of the triple differential cross sections has been then extended by simulations to a kinematics with the fixed detector at 90${}^{o}$, which can not be studied experimentally by the present setup. These simulations shed light on results of previous measurements on alkaline-earth-metal atoms in this kinematics, which were not consistent with the common understanding of photo-double-ionization derived from He experiments.

The triple-differential cross sections for the ionization of $\mathrm{Kr}(4p)$ and $\mathrm{Xe}(5p)$ by a fast electron where the exiting electrons have energies of 20 eV and 1 keV have been measured and compared with distorted-wave Born calculations. Difficulties were encountered when trying to place the relative data on an absolute scale by extrapolation to the optical limit: an extensive study of this normalization process is presented and it is argued that it is not applicable to the heavier atoms considered here, but can be used effectively for helium targets.

We study interference effects that appear in the cross section due to detection of two indistinguishable electrons in a photoelectron--Auger-electron pair (intrachannel exchange interference), or due to detection of two indistinguishable photoelectron-Auger electron pairs that share the final state but are emitted via two different intermediate states (interchannel exchange interference). For photoinduced Kr ${M}_{4,5}$--${N}_{1}{N}_{23}$ (Xe $N$${}_{4,5}$--${O}_{1}{O}_{23}$) Auger decay, the intrachannel exchange interference is ``switched on'' at 131.3 eV (87.0 eV) and 133.8 eV (91.0 eV) photon impact energy for the ${M}_{5}$ (${N}_{5}$) and ${M}_{4}$ (${N}_{4}$) holes, respectively. The interchannel exchange interference is expected to appear at 132.6 eV (89.0 eV) when the photoelectron ejected from the ${M}_{4}$ (${N}_{4}$) subshell is emitted with the same energy as the Auger electron emitted in the decay of the ${M}_{5}$ (${N}_{5}$) hole, and vice versa, the energy of the photoelectron ejected from the ${M}_{5}$ (${N}_{5}$) subshell equals the energy of the Auger electron emitted in the decay of the ${M}_{4}$ (${N}_{5}$) hole. We use the approach of Vegh and Macek [Phys. Rev. A 50, 4031 (1994)], and Vegh [Phys. Rev. A 50, 4036 (1994)] to model these phenomena in a unified manner and examine their effects in an angle-resolved and angle-integrated coincidence spectra considering also the postcollision interaction (PCI). The previous and the present angle-resolved experimental results, together with the data obtained by a magnetic bottle time-of-flight spectrometer, indicate that in this particular case the exchange interference effects in the angle-integrated cross sections are considerably weaker than in the angle-resolved $(\ensuremath{\gamma},2e)$ cross sections.

The ${\mathrm{Ar}}^{2+}$(3${\mathit{s}}^{2}$3${\mathit{p}}^{3}$nl) satellite states between 73 and 85 eV have been observed directly by threshold photoelectrons coincidence (TPEsCO) spectroscopy. The present study extends the previous observations of these states by optical emission spectroscopies to the region close to the ${\mathrm{Ar}}^{3+}$ ionization. Different features have been discerned in the TPESCO spectrum. A preliminary assignment of these features using the Rydberg formula and values of the quantum defect that are deduced from lower-lying states is proposed.

The state dependence of associative ionization of the ns and nd Rydberg series of Ne atoms with 5

The ${\mathrm{C}}^{+}\ensuremath{-}{\mathrm{H}}^{+}$ channel in the fragmentation of the ethyne dication following inner-shell ionization has been studied by Auger electron-ion-ion coincidence spectroscopy. The ion-ion coincidence map shows a peculiar feature which corresponds to the emission of both the ${\mathrm{H}}^{+}$ and ${\mathrm{C}}^{+}$ ions in the same direction. The analysis of the data, complemented by ab initio calculations, suggests an interpretation in terms of a two-step, asynchronous concerted reaction, in which the charge of the ethynyl intermediate ion localizes on the terminal carbon atom.

The acetylene-vinylidene isomerization and the dissociation of the ${\text{C}}_{2}\text{H}_{2}{}^{2+}$ dication formed in the inner-shell ionization by electron impact have been studied via Auger-electron--ion and Auger-electron--ion--ion coincidence experiments. The occurrence of the isomerization is proved by measuring the production of the $\text{CH}_{2}{}^{+}$ fragment ion as well as by detecting the ${\text{C}}^{+}\ensuremath{-}\text{CH}_{2}{}^{+}$ ion pair in coincidence. The state selectivity of the technique shows that isomerization mainly takes place in the three $1{\ensuremath{\pi}}_{u}^{\ensuremath{-}2}$ lower states of the acetylene dication.

The dynamics of core ionization by electron impact is investigated through the measurement of the triply differential cross section of the C {sigma}1{ital s} orbital in the molecule C{sub 2}H{sub 2}. The ({ital e},2{ital e}) experiments have been performed under asymmetric conditions and at small scattering angles, with a scattered electron energy of 1500 eV and low energies of the ejected electrons (9.6 and 41.0 eV). The measured angular distributions are characterized by large-size recoil lobes, breaking of the symmetry around the momentum-transfer direction, and unusual deviations of the maxima of the recoil peaks towards smaller deflection angles. In the ({ital e},2{ital e}) energy spectrum a shift is observed in the position of the C {sigma}1{ital s} peak with respect to the expected value as measured by x-ray photoelectron spectroscopy. The amplitude of the shift amounts to 0.46{plus minus}0.23 eV at 9.6 eV excess energy, and it is too large to be explained only in terms of postcollision interactions.

The photoionization of $\mathrm{Ar}\phantom{\rule{0.3em}{0ex}}2{p}_{3∕2}$ has been investigated by Auger electron\char21{}photoelectron coincidence experiments. The results have shown that the basic quantities that define the photoionization, i.e., the dipole matrix elements, can be extracted from these experiments and all the observables of the process, apart from the sign of the circular dichroism, can be predicted. The analysis also proved that a description of the process in terms of LSJ approximation is satisfactory.

The resonant double photoionization of Ne has been studied via an electron-electron coincidence experiment. A complete set of measurements was performed at different mutual angles when the photoelectron and Auger-electronpairs have nearly equal energy. The overall experimental energy resolution, narrower than the natural linewidth of the intermediate state, has allowed us to observe interference effects due to the indistinguishability of the two electrons. The observed energy and angular distribution exhibit a strong interplay of postcollisional interaction and interference effects. Calculations based on the model developed by Sheinerman and Schmidt [J. Phys. B 30, 1677 (1997)] describe the main trend of the experimental observations.

The high flux of the gas-phase beamline at the Elettra storage ring, Trieste (Italy), and the efficiency of a multicoincidence end station have been combined to measure the triple-differential cross section (TDCS) for the double photoionization (DPI) of He at excess energies $E=20$ and 80 eV. The TDCS has been measured in the equal-energy-sharing kinematics, in which the kinetic energies of the two photoelectrons are equal ${(E}_{1}{=E}_{2}=E/2).$ The experiments are compared with the predictions of a many-body Coulomb wave method and a convergent close-coupling calculation. The symmetrized gerade amplitudes of the He (DPI) extracted by the experiments as well as by the two calculations have been fitted by a Gaussian function. The results support the validity of the parametrization of the gerade amplitude via only one parameter, the full width at half maximum ${\ensuremath{\vartheta}}_{1/2},$ of the Gaussian function, up to 80 eV above threshold.

The ({ital e},2{ital e}) energy-separation spectrum of the 1{pi}{sub {ital u}} orbital of acetylene has been measured in coplanar asymmetric kinematics for several values of the incident electron energy loss, {Delta}{ital E}. In the {Delta}{ital E} region dominated by resonant processes, a shift of the 1{pi}{sub {ital u}} peak with respect to the value measured by photoelectron spectroscopy has been observed. This finding has been attributed to the change of the relative intensities of the vibrational components of the electronic transition induced by resonant processes. Moreover, a feature assigned as a molecular autoionizing transition is observed in an ({ital e},2{ital e}) energy-separation spectrum.

The Ar 3{ital s} satellite spectrum has been studied by means of asymmetric ({ital e},2{ital e}) experiments at 1-keV incident energy and in three different kinematical conditions. The intensities of the main satellite lines relative to the intensity of the 3{ital s} main line (29.3 eV) have been determined. The three chosen kinematical conditions allow for studying the dependence of the relative intensity on the momentum transfer in a region which is intermediate between the binary and dipolar regimes. The discrepancy between the relative intensity of the transition to the 3{ital d} {sup 2}S ion state measured by ({ital e},2{ital e}) and high-energy photoionization experiments is confirmed. Sizeable contributions from transitions not belonging to the {sup 2}S manifold and a kinematical dependence of the peaks centered at 36.5 and 41.3 eV have been observed.