Absolute cross sections for the K-shell photoionization of ground-state Li-like boron [B$^{2+}$(1s$^2$2s $^2$S)] ions were measured by employing the ion-photon merged-beams technique at the Advanced Light Source synchrotron radiation facility. The energy ranges 197.5--200.5 eV, 201.9--202.1 eV of the [1s(2s\,2p)$^3$P]$^2$P${\rm ^o}$ and [1s(2s\,2p)$^1$P] $^2$P${\rm ^o}$ resonances, respectively, were investigated using resolving powers of up to 17\,600. The energy range of the experiments was extended to about 238.2 eV yielding energies of the most prominent [1s(2$\ell$\,n$\ell^{\prime}$)]$^2$P$^o$ resonances with an absolute accuracy of the order of 130 ppm. The natural linewidths of the [1s(2s\,2p)$^3$P] $^2$P${\rm ^o}$ and [1s(2s\,2p)$^1$P] $^2$P${\rm ^o}$ resonances were measured to be $4.8 \pm 0.6$ meV and $29.7 \pm 2.5$ meV, respectively, which compare favourably with theoretical results of 4.40 meV and 30.53 meV determined using an intermediate coupling R-matrix method.

Absolute cross sections for K-shell photoionization of Be-like B+ ions were measured employing the photon-ion merged-beam technique at the Advanced Light Source in Berkeley. The results are found to be in fair agreement with R-matrix calculations using L-S coupling.

Cross sections for single photoionization of Fe3+, Fe5+, and Fe7+ ions were measured at spectral resolutions of 0.04, 0.15, and 0.13 eV, respectively, by merging mass-and charge-selected ion beams with a beam of monochromatized synchrotron undulator radiation. The measurements span photon energy ranges beginning at the ionization thresholds and extending several tens of electron volts to include the most important resonant contributions due to 3p-nd transitions to autoionizing states. The photoion yield spectra are characterized by narrow resonances and also broad features in Fe3+ and Fe5+ that are believed to result from unresolved fast super-Coster-Kronig transitions following 3p-3d excitation. Absolute photoionization cross-section measurements were also performed using ion beams containing undetermined fractions of ions in their ground and metastable states. A Rydberg series attributed to 3p-nd transitions from the (2)G(1) metastablestate in Fe3+ was identified. The data are compared with recently published measurements on Fe3+ and Fe5+ using a similar technique at lower spectral resolution.

Absolute cross sections for the K-shell photoionization of ground-state Li-like boron [B2+(1s(2)2s S-2)] ions were measured by employing the ion-photon merged-beams technique at the Advanced Light Source synchrotron radiation facility. The energy ranges 197.5-200.5 eV, and 201.9-202.1 eV of the [1s(2s2p)P-3]P-2(o) and [1s(2s2p)P-1] P-2(o) resonances, respectively, were investigated using resolving powers of up to 17 600. The energy range of the experiments was extended to about 238.2 eV yielding energies of the most prominent [1s(2l nl')] Po-2 resonances with an absolute accuracy of the order of 130 ppm. The natural linewidths of the [1s(2s2p)P-3] Po-2 and [1s(2s2p)P-1] P-2(o) resonances were measured to be 4.8 +/- 0.6meV and 29.7 +/- 2.5meV, respectively, which compare favourably with theoretical results of 4.40 meV and 30.53 meV determined using an intermediate-coupling R-matrix method.

Absolute cross sections for the K-shell photoionization of ground-state Li-like carbon [C3+(1s(2)2s(2)S)] ions were measured by employing the ion-photon merged-beams technique at the Advanced Light Source. The energy ranges 299.8-300.15 eV, 303.29-303.58 eV and 335.61-337.57 eV of the [1s(2s2p)P-3]P-2, [1s(2s2p)P-1]P-2 and [(1s2s)S-3 3p]P-2 resonances, respectively, were investigated using resolving powers of up to 6000. The autoionization linewidth of the [1s(2s2p)P-1]P-2 resonance was measured to be 27 +/- 5meV and compares favourably with a theoretical result of 26 meV obtained from the intermediate coupling R-matrix method. The present photoionization cross section results are compared with the outcome from photorecombination measurements by employing the principle of detailed balance.

Absolute cross-section measurements for resonant double photoexcitation of Li+ ions followed by autoionization have been performed at high resolution in the photon energy range from 148 eV, just below the (2s2p, 2(0, 1)n+) resonance, to 198 eV (the region of the double ionization threshold). The measurements have been made using the photon–ion merged-beam endstation at the Advanced Light Source, Lawrence Berkeley National Laboratory, USA. The absolute cross-section measurements show excellent agreement with theoretical results from the R-matrix plus pseudo-state (RMPS) method. Comparisons between theory and experiment for the Auger resonance energies, autoionization linewidth (Γ) and the Fano line profile index q for several members of the principal (2snp, 2(0, 1)n+) and (3snp, 3(1, 1)n+) Rydberg series found in the photoionization spectra for the 1Po symmetry show satisfactory accord.

Photoion-pair formation in H2O and D2O has been studied using synchrotron radiation within the energy range 15-50 eV. Appearance potentials for the formation of O-, OH- and H- have been determined and coincide with their thermochemical thresholds. Above threshold it is shown that superexcited states play an important role in the polar photodissociation process. Substantial isotope effects have been observed.

A Reply to the Comment by A. V. Korol and A. V. Solov’yov.Received 21 December 2006DOI:https://doi.org/10.1103/PhysRevLett.98.179602©2007 American Physical Society

Photoionization spectra have been recorded in the $4s$, $4p$, and $3d$ resonance regions for the Kr I isoelectronic sequence using both the dual laser produced plasma (DLP) technique (at DCU) to produce photoabsorption spectra, and the merged ion beam and synchrotron radiation technique (at ASTRID) to measure absolute photoionization cross sections. Profile parameters are compared for the $4s\text{\ensuremath{-}}np$ resonances of ${\mathrm{Rb}}^{+}$ and ${\mathrm{Sr}}^{2+}$. Many $4p\ensuremath{\rightarrow}ns$, $md$ transitions are identified with the aid of Hartree-Fock calculations, and consistent quantum defects are observed for the various $ns$ and $md$ Rydberg series. Absolute single and double photoionization cross sections recorded in the $3d$ region for ${\mathrm{Rb}}^{+}$ and ${\mathrm{Sr}}^{2+}$ ions show preferential decay via double photoionization. This is only the second report to our knowledge where both the DLP technique and the merged-beam technique have been used simultaneously to record photoionization spectra, and the advantages of both techniques (i.e., better resolution in the case of DLP and values for absolute photoionization cross sections in the case of the merged-beam technique) are highlighted.

Time-of-flight mass spectrometry was used to investigate fragmentation and energy transfer processes in water by C ions at the distal part of the Bragg peak. Measurements of the positive ion fragments from ionization, electron capture, electron loss, transfer-loss and loss-ionization channels have allowed us for the first time (a) to obtain a quantitative determination of the energy lost by C ions in water and (b) to show that total water fragment ion production has a much flatter profile with projectile energy than would be expected if the water radical formation was assumed to follow the energy-loss profile obtained from available stopping power models.

The electron impact dissociative double-ionization cross sections for H2O between 45 and 1500 eV have been measured using time of flight mass spectrometry. The energy dependence of the H++OH+ and H++O+ ion pair production cross sections indicate that Auger-like autoionization following a vacancy in the 2a(1) molecular orbital is the main double ionization channel at high velocities. In contrast to expectation, these findings show that dissociation through the H2O2+ precursor state is a significant process at high collision energies. Knowledge of this process is vital as it has a direct affect on the production of important molecular species, such as H-2, during water radiolysis. Branching ratios of the various fragments produced following both autoionization and double ionization have also been obtained.

Absolute photoionization cross sections for Kr5+ were measured in the photon energy range 74-175 eV using synchrotron radiation. For comparison, a detailed energy scan of the electron-impact ionization cross section was made in the same energy range and normalized to previously published absolute measurements. The Flexible Atomic Code and Cowan atomic structure code were used to calculate energy levels, excitation energies, and oscillator strengths for 3d -> np, 3d -> nf, and 4s -> np autoionizing transitions from the ground and metastable states. Within the experimental uncertainty, oscillator strengths determined from the photoionization measurements are in agreement with both calculations. Excitation-autoionization and resonant excitation-double-autoionization features are evident in the electron-impact ionization cross section.

The paths and mechanisms leading to fragmentation of multiply charged molecules are still not well known. Multiply charged molecules can remain intact, or fragment via evaporation – eliminating light neutral atoms such as H0 or via fission – ejecting one H+, or they can breaking up into two or more charged species [S.W.J. Scully, J.A. Wyer, V. Senthil, M.B. Shah, E.C. Montenegro, Phys. Rev. A 71 (2005) 030701(R)]. Small molecules, such as water and methane, are unstable after two or more electron removal. In this work we present experimental results of fragmentation of doubly charged water molecules by 30–1500eV electrons. We show that, at low energies, doubly charged water essentially undergoes fission but, as the electron energy increases, the complete break-up of water becomes progressively dominant. The contribution to double ionization from auto-ionization [S.W.J. Scully, J.A. Wyer, V. Senthil, M.B. Shah, E.C. Montenegro, Phys. Rev. A 73 (2006) 040701(R)] of singly charged water molecules is discussed.