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The slowest reaction in the CNO cycle $^{14}\mathrm{N}(p,\ensuremath{\gamma})^{15}\mathrm{O}$ has been studied by populating the ${E}_{p}^{\mathrm{lab}}=278$ keV (${E}_{\mathrm{c}.\mathrm{m}.}^{r}=259$ keV) proton capture resonant state of $^{15}\mathrm{O}$ at 7556 keV. The strength of the resonance ($\ensuremath{\omega}\ensuremath{\gamma}$) has been determined from the experimental data. The level lifetime of the subthreshold resonant state at ${E}_{x}=6792$ keV, as well as the lifetimes of the 5181 keV and 6172 keV states, have been measured using the Doppler shift attenuation method (DSAM). The structural properties of the nucleus $^{15}\mathrm{O}$, such as the level energies, transition strengths, level lifetimes, and spectroscopic factors, have been calculated theoretically by using the large basis shell model, which agrees reasonably well with the present as well as the previous experimental data.

Using electron spectroscopy technique, we measure the absolute double-differential cross sections (DDCSs) of electrons emitted in collisions of 200-keV protons on He, CH4, and O2 and 5.5-MeV/u bare C ions colliding on O2. The emitted electrons are measured in the energy range from 1 to 400 eV for He and CH4 targets and up to 600 eV for O2 to include the K-LL Auger line of oxygen. The electrons are detected over different emission angles varying from 20∘ in the extreme forward direction to 160∘ in the backward direction. The single-differential cross section (SDCS) and total cross section are deduced from the measured DDCSs spectra for all the targets. The DDCS and SDCS are compared with the state-of-the-art continuum distorted wave-eikonal initial state (CDW-EIS) theoretical model. The CDW-EIS model provides excellent agreement with the oxygen data at MeV energy, whereas the deviation in the case of keV energy is substantial, in spite of having nearly the same perturbation strength. The forward-backward angular asymmetry shows a saturation effect in the case of keV energy protons but no such signature is observed for the high-energy collision. A systematic analysis reveals that the asymmetry at low electron energy is sensitive to the associated atomic or molecular structure and is in close agreement with the theory. Fil: Roy Chowdhury, Madhusree. International Centre of Theoretical Science. Tata Institute of Fundamental Research; España Fil: Mandal, A.. International Centre of Theoretical Science. Tata Institute of Fundamental Research; España Fil: Bhogale, A.. International Centre of Theoretical Science. Tata Institute of Fundamental Research; España Fil: Bansal, H.. International Centre of Theoretical Science. Tata Institute of Fundamental Research; España Fil: Bagdia, C.. International Centre of Theoretical Science. Tata Institute of Fundamental Research; España Fil: Bhattacharjee, S.. International Centre of Theoretical Science. Tata Institute of Fundamental Research; España Fil: Monti, Juan Manuel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina Fil: Rivarola, Roberto Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina Fil: Tribedi, Lokesh C.. International Centre of Theoretical Science. Tata Institute of Fundamental Research; España

Double-differential cross section (DDCS) for electrons ejected in collisions of fast ${\mathrm{C}}^{6+}$ and ${\mathrm{Si}}^{13+}$ projectiles, with a ${\mathrm{H}}_{2}\mathrm{O}$ vapor target, were measured. The electrons were detected over an energy range of 1--600 eV and an angular range of ${20}^{\ensuremath{\circ}}$--${160}^{\ensuremath{\circ}}$. The obtained DDCS spectra, for both the ions, were compared with the CDW-EIS model. Occasional reference has been made to the DDCS data for the case of 3.75-MeV/u ${\mathrm{O}}^{8+}$ colliding on the same molecule for an overall comparison. A reasonable agreement with theoretical results was seen for the case of ${\mathrm{C}}^{6+}$ and ${\mathrm{O}}^{8+}$ projectiles. However, between ${\mathrm{C}}^{6+}$ and ${\mathrm{O}}^{8+}$ projectiles, the deviation from theory is larger for the case of the carbon projectile. Substantial deviation starts to show up for the case of the ${\mathrm{Si}}^{13+}$ projectile. By numerical integration of the DDCS data, the single-differential cross section (SDCS) and total cross section (TCS) were obtained and compared with theoretical models. The present TCS data along with the other available data for $p$, $\mathrm{He}$, and $\mathrm{C}$ ions were plotted together. A clear and gradual deviation from the Bethe-Born predicted ${q}^{2}$ scaling was observed, where $q$ is the projectile charge state. From all the data we find TCS varies as ${q}^{n}$ where $n$ = 1.7 $\ifmmode\pm\else\textpm\fi{}$ 0.1. The provided data set will be valuable in order to help model the radiation damage in hadron therapy, particularly in the Bragg peak region.

We study multiple ionization and subsequent dissociation of nitrous oxide $({\mathrm{N}}_{2}\mathrm{O})$ in collisions with 1 a.u. ${\mathrm{Ar}}^{8+}$ and ${\mathrm{Xe}}^{15+}$ ions. The experiments are performed by using a recoil ion momentum spectrometer (RIMS) equipped with a position- and time-sensitive detector which allows the measurement of the momenta of fragment ions in coincidence. By measuring the momentum vectors of the recoiling fragment ions various important parameters, such as kinetic energy release and those related to molecular structure prior to fragmentation, have been derived. Furthermore, the projectile-charge-state dependence of the fragmentation dynamics of ${\mathrm{N}}_{2}\mathrm{O}$ is investigated and a very mild dependence has been noticed in a few channels. In addition, we also study the concerted and sequential mechanisms in the three-body decay of ${\mathrm{N}}_{2}{\mathrm{O}}^{q+}$ (where $q\phantom{\rule{4pt}{0ex}}\ensuremath{\le}7)$. It has been observed that ${\mathrm{N}}_{2}{\mathrm{O}}^{q+}$ breaks up mainly in a concerted manner except for the ${\mathrm{N}}_{2}{\mathrm{O}}^{3+}\ensuremath{\rightarrow}{\mathrm{N}}^{+}+{\mathrm{N}}^{+}+{\mathrm{O}}^{+}$ (1,1,1) and ${\mathrm{N}}_{2}{\mathrm{O}}^{4+}\ensuremath{\rightarrow}{\mathrm{N}}^{2+}+{\mathrm{N}}^{+}+{\mathrm{O}}^{+}$ (2,1,1) channels. For both these channels, the presence of an intermediate rotating ${\mathrm{NO}}^{2+}$ has been identified. Furthermore, by using Dalitz plot analysis, we have been able to separate various mutually mixed channels of highly charged ${\mathrm{N}}_{2}{\mathrm{O}}^{q+}$.

We report on the measurement of double-differential distribution of soft electron emission from ${\text{C}}_{60}$ fullerene, induced by a fast-moving Coulomb field of 76 MeV energy bare fluorine ions. A broad ``plasmon-electron'' peak, riding on the Coulomb-ionization continuum, is observed due to the deexcitation of the giant dipole plasmon resonance state in ${\text{C}}_{60}$. The angular distribution of the plasmon electrons goes through a dip around ${90}^{\ensuremath{\circ}}$, which is contrary to that observed in ion-atom collisions measured in situ, indicating the alignment of the induced dipole moment along the projectile beam direction. A model based on the photoelectron angular distribution which is modified due to the ion-induced postcollisional interaction provides an excellent agreement with the observed asymmetric distribution. The distribution smoothly changes from a dip at ${90}^{\ensuremath{\circ}}$ to a peak with the variation of ejected electron energy indicating transition from a collective plasmon behavior of the whole system to a single ion-atom interaction. The single-differential cross section was also derived, which preserves the signature of the collective excitation.

We have measured the absolute double-differential cross sections (DDCS) for electron emission in ionization of He and Ne atoms under the impact of 6-MeV/u C6+ ions. Data were collected between 1 and 500 eV for He, while for Ne this range was extended up to 1000 eV. The angular ranges covered in the experiment are 30∘ to 150∘ and 20∘ to 160∘ for He and Ne, respectively. The DDCS spectra are compared with the prior and the post forms of the state-of-the-art continuum-distorted-wave eikonal-initial-state model. Both the theoretical models show very good agreement with the energy and angular distributions of the DDCS in the case of He. For Ne, at low energies both are going together and matching very well with the data. In the high-energy region, at extreme forward and backward angles, although both the forms are underestimating the experimental data to some extent, the prior form shows much better agreement compared to the post form. This post-prior discrepancy is attributed to the influence of dynamic screening, on the ionized one, produced by the electrons remaining in the target. The single differential cross sections (SDCS) in emission angle (dσdΩe) and electron energy (dσdεe) are deduced by integrating the electron DDCS spectra. While excellent agreement is obtained for the dσdεe spectrum, the dσdΩe provides a further sensitive test to the adequacy of the theoretical model employed. The total cross section obtained from the SDCS spectra is about 11% higher than the prior model for He and about 6% lower for Ne. To get the quantitative picture of the two-center effect, the forward-backward angular asymmetry parameter has been deduced as a function of velocity of the ejected electrons. For both the targets, it is very well reproduced by both the forms of the theory. For the Ne target, K-LL Auger angular distribution has also been studied, which shows small asymmetry caused by multiple vacancies in the L shell along with the K-shell vacancy. Fil: Biswas, Shubhadeep. International Centre Of Theoretical Science. Tata Institute Of Fundamental Research; España Fil: Misra, D.. International Centre Of Theoretical Science. Tata Institute Of Fundamental Research; España Fil: Monti, Juan Manuel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentina Fil: Tachino, Carmen Alejandra. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentina Fil: Rivarola, Roberto Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentina Fil: Tribedi, L. C.. International Centre Of Theoretical Science. Tata Institute Of Fundamental Research; España

Strong evidence has been found for enhanced multiple electron capture into 46 MeV/u Pb81+ with a significant contribution from the entrance surface of thin carbon foils. Capture of up to five electrons has been observed. The multiple electron capture yield is found to increase with decreasing target thickness for thin targets. A simple model describing the data and showing the importance of capture from surfaces is discussed. Further evidence is found for a pronounced asymmetry between electron capture at the entrance and the exit surfaces. Absolute yields for multiple electron capture and projectile ionization are presented. The experimental total cross sections for single capture and ionization agree well with theory.

The saturation effect in the excitation cross sections has been investigated for ${\mathrm{Si}}^{12+}$ ions colliding with gas target atoms (atomic number ${Z}_{t}$ between 2 and 54) in the (8--12)-a.u. velocity range, both experimentally and theoretically. The experimental excitation cross sections have been deduced from the measured Ly x rays following the decay of the excited atoms. Theoretical calculations have been performed using the first Born approximation, the symmetric eikonal continuum distorted wave (SECDW) approximation, and the Schwinger variational principle (SVP). Similar to the earlier observations on the heavier ion species, the saturation has been found in both the experimental and the SVP cross sections in the entire velocity range. The SECDW approximation, however, has been found to underestimate the cross sections at large values of ${Z}_{t}$ in the presently investigated systems. In the light of these findings, the role of higher-order effects within different theoretical models has been reviewed and the possible interaction mechanisms have been discussed.

We present an experimental determination of the 2p3dð 1 P o Þ! 1s3dð 1 D e Þ x-ray line emitted from He-like Si, S, and Cl projectile ions, excited in collisions with thin carbon foils, using a high-resolution bent-crystal spectrometer. A good agreement between the observation and state-of-the-art relativistic calculations using the multiconfiguration Dirac-Fock formalism including the Breit interaction and QED effects implies the dominance of fluorescent decay over the autoionization process for the 2p3dð 1 P o Þ state of He-like heavy ions. This is the first observation of the fluorescence-active doubly excited states in He-like Si, S, and Cl ions.

We have measured the double-differential cross sections (DDCS) for electron emission in ionization of H2O molecules under the impact of 4.5-MeV/ uO 8+ ions. The data were collected between 1 and 600 eV, in an angular range of 20 ◦ –150 ◦ by using an electrostatic hemispherical analyzer. In the experiment we used the H2O vapor in a static gas condition which allowed us to deduce the absolute value of the cross sections. The single-differential cross sections (SDCS) and the total cross sections have also been obtained. The DDCS as well as the SDCS spectra are compared with the continuum-distorted-wave eikonal-initial-state (CDW-EIS) calculations for both the prior as well as the post forms of the scattering matrix. The initial state is represented within the complete neglect of differential overlap approximation, where the molecular orbitals are expressed in terms of atomic orbitals of the atomic constituents. The overall agreement with the CDW-EIS model is quite good as far as the energy dependence is concerned. The prior form of the model is found to provide a better understanding of the data compared to the post version. In particular, excellent agreement between the theory and experiment has been observed for the angular distribution data at forward angles.

In this paper recoil-ion production cross sections are presented differential in recoil-ion longitudinal momentum and electron emission angle in ionization of atomic and molecular hydrogen by bare ion projectiles. A new formulation for constructing these double differential distributions from the measured electron double differential cross sections is used. A novel feature is the separation of two different branches of the recoil-ion longitudinal momentum distribution corresponding to soft and hard collision mechanisms of ionization. The single differential distributions have also been derived. {copyright} {ital 1996 The American Physical Society.}

We have measured the double differential cross sections (DDCS) ({ital d}{sup 2}{sigma}/{ital d}{var_epsilon}{sub {ital ed}}{Omega}{sub {ital e}}) of low-energy electron emission in the ionization of H{sub 2} bombarded by bare carbon ions of energy 30 MeV. The energy and angular distributions of the electron DDCS have been obtained for 12 different emission angles and for electron energies varying between 0.1 and 300 eV. We have also deduced the single differential and total ionization cross section from the measured DDCS. The data have been compared with the predictions of first Born approximations and the CDW-EIS (continuum distorted wave{endash}eikonal initial state) model. The CDW-EIS model provides an excellent agreement with the data. {copyright} {ital 1996 The American Physical Society.}

The single and double {ital K}-shell vacancy fractions for swift O and Si ions inside Fe and Gd foils have been measured using the probe layer technique. The measurements are carried out at velocities varying from 7.5 to 13.5{ital v}{sub 0} for Si ions and at a velocity of 7.8{ital v}{sub 0} for oxygen ions ({ital v}{sub 0}={alpha}{ital c} where {alpha} is the fine-structure constant and {ital c} is the speed of light). It is shown that all such available data for light ions fall on a smooth curve when plotted against the reduced velocity of the ion. These values are used along with the existing transient magnetic field data to derive the electron spin polarization acquired by the ions traveling inside ferromagnetic hosts. The degree of polarization is shown to decrease with the atomic number of the ions. This observation is, however, in disagreement with recent theoretical calculations.

Fast highly charged C and O ion-induced total ionization of an RNA base molecule, uracil (C${}_{4}$H${}_{4}$N${}_{2}$O${}_{2}$, $m=112$ amu), has been investigated in a wide energy range of keV to MeV. A combined study of the collision products using a time-of-flight mass spectrometer and an electron spectrometer allows one to determine absolute total ionization cross sections (TCSs). Experimental measurements of TCSs are compared to theoretical predictions performed in the classical trajectory Monte Carlo and classical over-barrier (CTMC-COB) and quantum mechanical (Continuum Distorted Wave with Eikonal Initial State and first-order Born with correct boundary condition) frameworks. The overall energy dependence of the TCSs is approximately reproduced by the models, especially well in the high energy range. The CTMC-COB model provides an excellent agreement for the high-energy data. The projectile charge-state $q$ dependence of TCSs deviates from the well-known quadratic behavior in ion-atom collisions.

Single and double [ital K]-shell vacancy production and [ital K]-[ital K] electron-transfer cross sections have been measured in the limit of zero target thickness for Fe and Ni induced by 1.25--4.70 MeV/amu [sup 28]Si and [sup 32]S ions. The fluorescence yield [omega][sub [ital k]] for Fe and Ni at x-ray emission was determined from the measured energy shifts of target [ital K] x rays, the intensity ratios of [ital K][beta] and [ital K][alpha] and using the statistical scaling procedure of Larkins [J. Phys. B 4, L29 (1971)]. The single-electron-transfer cross sections are compared with the atomic-orbital-expansion model (AO) of Fritsch and Lin [Phys. Rep. 202, 1 (1991)], and the semiempirical perturbed-stationary-state (PSS) approach of Lapicki and McDaniel [Phys. Rev. A 22, 1896 (1980)]. The AO calculations show excellent agreement with the single [ital K]-[ital K] transfer cross sections. This model also explains the double [ital K]-[ital K] transfer data quite well although it underestimates slightly the measured values. The direct [ital K]-shell ionization cross section data are compared with the ECPSSR (perturbed-stationary-state theory with energy-loss, Coulomb deflection, and relativistic corrections) calculations.

Single and double [ital K]-[ital K]-electron transfer and the [ital K]-shell-vacancy production cross sections for Ti bombarded by [sup 28]Si and [sup 32]S beams at energies varying between 1.25 and 4.7 MeV/amu have been determined, in the limit of zero target thickness, from the thickness variation of the Ti [ital K] x-ray yield with ion beams having 0, 1, or 2 vacancies in their [ital K] shell. From the measured energy shift and the intensity ratios of [ital K][alpha], [ital K][beta] lines, the average number of 2[ital p] and 3[ital p] vacancies were determined from which the average value of the fluorescence yield [omega][sub [ital K]] was deduced using the statistical scaling procedure of Larkins [J. Phys. B 4, L29 (1971)]. The single-electron transfer cross sections are compared with the calculations of Brinkman and Kramer as formulated by Nikolaev [Sov. Phys. JETP 24, 847 (1967)] and those of Lapicki and McDaniel [Phys. Rev. A 22, 1896 (1980)] and the modified two-center atomic-orbital-expansion model (AO+) of Fritsch and Lin [J. Phys. B 15, 1255 (1982)]. The double-electron transfer cross sections are also compared with the AO+ predictions. The [ital K]-shell-vacancy production data are compared with calculations based on the perturbed-stationary-state approachmore » with the Coulomb-deflection, binding-energy, and relativistic corrections (ECPSSR) included. In addition, the projectile cross sections for single-electron capture and loss have also been deduced at a few energies.« less

We study the Bethe binary-encounter (BE) region in the ejected-electron double-differential emission spectrum of ${\mathrm{H}}_{2}$ and He targets in collisions with 8-keV electrons. We compare the absolute cross sections for these isoelectronic systems at high emission energies. The experimental data are analyzed in terms of a state-of-the-art theoretical model based on a two-effective-center approximation. In the case of the ${\mathrm{H}}_{2}$ molecule the binary peak in the double-differential cross sections (DDCS) is enhanced due to the two-center Young-type interference. The observed undulation in the DDCS ratio is explained in terms of the combined contributions of the Compton profile mismatch and the interference effect. The influence of the interference effect is thus observed for higher-energy electrons compared to most of the earlier studies which focused on low-energy electrons produced in soft collisions.

We have measured absolute cross sections for single, double, triple, and quadruple ionization of ${\mathrm{C}}_{60}$ in collisions with 3 MeV/u C, F, and Si projectile ions at various projectile charge states. The experiment was performed using the recoil-ion time-of-flight technique. Projectile charge state dependence of the ionization yields was compared mainly with a model based on the giant dipole plasmon resonance (GDPR). In some cases, the continuum-distorted-wave--eikonal-initial-state (CDW-EIS) model which is normally applied for ion-atom collisions was also used as a reference. An excellent qualitative agreement between the experimental data for single and double ionization and the GDPR model predictions was found for all projectile charge states.

Angular distribution of the $L$ x-ray intensities in Au and Bi induced by 12--55-MeV carbon ions has been measured. The ${L}_{\ensuremath{\alpha}}$, ${L}_{\ensuremath{\beta}}$, and ${L}_{\ensuremath{\gamma}}$ x-ray intensities were found to be isotropic within experimental uncertainty. The alignment parameter ${A}_{20}$ of the ${L}_{3}$ ($2{p}_{3/2}$) subshell was deduced from the measured anisotropy parameter $\ensuremath{\beta}$ value of the well-resolved ${L}_{l}$ line, obtained from the angular distribution of the ${I}_{Ll}/{I}_{L\ensuremath{\alpha}}$, ${I}_{Ll}/{I}_{L\ensuremath{\beta}}$, and ${I}_{Ll}/{I}_{L\ensuremath{\gamma}}$ x-ray intensity ratios. The measured ${A}_{20}$ values have been compared with those obtained using theoretical models that involve the plane-wave Born approximation; projectile's energy loss and its Coulomb deflection from the straight-line trajectory, perturbed-stationary-state, and relativistic effects (ECPSSR); and ECPSSR with the intrashell effect.

The spectral distribution of the $1s2s\text{ }^{1}S_{0}\ensuremath{\rightarrow}1{s}^{2}\text{ }^{1}S_{0}$ two-photon decay of He-like tin was measured using a novel approach at the gas-jet target of the ESR storage ring. Relativistic collisions of Li-like projectiles with low-density gaseous matter have been exploited to selectively populate the desired $1s2s$ state. Compared to conventional techniques, this approach results in a substantial gain in statistical and systematic accuracy, which allowed us to achieve for the first time a sensitivity to relativistic effects on the two-photon decay spectral shape as well as to discriminate the measured spectrum for Sn from theoretical shapes for different elements along the He-isoelectronic sequence.

Frequency doubling in interference oscillations in fast-ion-induced electron emission spectrum from H2 is investigated. Experimentally observed oscillatory structure is well explained by a model calculation based on the rescattering of emitted electron from the second H center. The second-order contribution is found to be as large as 10%. The doubling of oscillation frequency is found out to be independent of angle of observation. Derived analytical expression for the double differential cross section ratio including the first- and secondorder interference terms, fits the observed oscillatory structure quite well. The present analysis is in broad agreement with the earlier observations by Stolterfoht et al.

The Young-type interference arising due to the spatial coherence has been investigated in the electron emission spectrum from fast electron impact ionization of the inversion symmetric homonuclear diatomic molecule ${\mathrm{H}}_{2}$. The evidence of the interference effect in the angular distribution of the double differential spectrum of the secondary electron is found. The signature of constructive interferences has been identified in the soft-collision regions as well as in binary encounters. The observed oscillation in the forward-backward asymmetry parameter is explained in terms of the Cohen-Fano-type interference coupled with the angular dependence of oscillation frequency. A comparative study indicates a marked difference between the angular asymmetry in the case of fast heavy ion $({\mathrm{F}}^{9+})$ and electron collisions with ${\mathrm{H}}_{2}$ at a similar velocity.

We report the energy and angular distribution of electron double differential cross sections (DDCS) in collision of $6\text{\ensuremath{-}}\mathrm{MeV}∕\mathrm{u}\phantom{\rule{0.3em}{0ex}}{\mathrm{C}}^{6+}$ ions with molecular hydrogen. We explain the observed distributions in terms of the two-center effect and the Young-type interference effect. The secondary electrons having energies between 1 and $1000\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ are detected at about 10 different emission angles between $30\ifmmode^\circ\else\textdegree\fi{}$ and $150\ifmmode^\circ\else\textdegree\fi{}$. The measured data are compared with the state-of-the-art continuum distorted wave-eikonal initial state and the first Born model calculations which use molecular wave function. The single differential cross sections are derived and compared with the theoretical predictions. The oscillations due to the interference effect are derived in the DDCS ratios using theoretical cross sections for the atomic H target. The effect of the atomic parameters on the observed oscillations is discussed. An evidence of interference effect has also been shown in the single differential cross section. The electron energy dependence of the forward-backward asymmetry parameter shows a monotonically increasing behavior for an atomic target, such as He, which could be explained in terms of the two-center effect only. In contrast, for the molecular ${\mathrm{H}}_{2}$ the asymmetry parameter reveals an oscillatory behavior due to the Young-type interference effect superimposed with the two-center effect. The asymmetry parameter technique provides a self-normalized method to reveal the interference oscillation which does not require either a theoretical model or complementary measurements on the atomic H target.

We use the forward-backward angular asymmetry in the electron emission cross sections in fast ion impact ionization of H2 as a probe of the inversion symmetric coherence in homonuclear diatomic molecules. The electron energy dependence of the asymmetry parameter for H2 exhibits oscillatory structure due to Young-type interference in contrast to atomic targets such as He. The asymmetry parameter technique provides a selfnormalized method to reveal the interference oscillation independent of theoretical models and complementary measurements on atomic H target. DOI: 10.1103/PhysRevA.74.060701 PACS numbers: 34.50.Fa, 34.50.Gb Angular distribution of various types of radiations particles and photons is known to be quite sensitive to various effects associated with different physical processes in atomic, nuclear, plasma physics and other branches of physics. In fast ion-atom ionization, the long range Coulomb interaction of the final state electrons with the target and the projectile ions influences the evolution of the electron wave function and thereby the angular distribution of electron emission. Such two-center effect is known to cause a large forward-backward asymmetry 1‐4 in the electron emission spectrum. The electron emission spectrum from the simplest diatomic molecule H2 manifests yet another important aspect of interference 5 in ion-atom ionization besides the wellknown mechanisms such as soft collision, two-center effect and binary encounter 1‐4,6‐8. Since the two indistinguishable H atoms in the H2 molecule may be considered as the coherent emission sources of phase coupled electrons in a large impact parameter collision, their contributions add coherently and an interference effect should be observed. Therefore, the electron emission from H2 may be viewed as a natural coherent system which is similar to Young’s double slit interference phenomenon 5. We demonstrate here that the additional mechanism of Young-type interference plays a major role in the angular asymmetry of electron double differential cross section DDCS and asymmetry parameter itself would be a sensitive test to study the interference for a diatomic molecular target. Following the initial theoretical studies on the interference effect in electron scattering 9 and photoionization 5, very recently the evidence of Young-type interference was found in the fast-ion collisions with H2 10‐12. Ideally one would have expected an oscillation in the DDCS spectrum due to interference. But a steep fall of the DDCS by about four or five orders of magnitude see below does not allow one to observe the oscillation directly. The oscillations, thereby, were observed in the DDCS ratios H2-to-2H which was explained due to the interference. However, the experiments using H are rare due to the experimental constraint and oscillations in the DDCS ratios were observed 4,12 in such experiment with H. Theoretical DDCS for atomic, or effective atomic H have also been employed 10,11 in the absence of an atomic H target. In such cases, the shapes of the oscillations are sensitive to the atomic parameters such as the effective atomic number Zeff which is model dependent. So far oscillations have not been observed based on the H2 DDCS data only. In this work we present a method based on the analysis of the asymmetry parameter which is independent of normalization procedures and permits therefore to observe the interference effect directly in the H2 data only. By eliminating the need for the data on atomic H is an important step forward in the study of this

Absolute $K$-shell ionization cross sections of Sb, Au, and Bi have been measured in collisions with highly charged C, O, and S ions having energies between 2 and $6.25\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}∕u$. The data are presented along with the earlier results with F and Si ions as projectiles. The measured data have been compared with theoretical models based on the semiclassical approximation including relativistic effect and perturbed stationary state approximation including the corrections for energy loss, Coulomb deflection, and relativistic effects. The data were analyzed in term of relativistic effect on $K$-ionization: its dependence on the projectile atomic number, target atomic number and projectile energy. The present data set along with our recently measured similar data for F and Si ions show that the relativistic effect increases with projectile atomic number and decreases with projectile velocity. A comparison of the experimental data and relativistic calculations, both normalized to nonrelativistic model, show the inadequacy of the theoretical models in describing the relativistic correction on $K$-shell ionization. For example, the theoretical predictions agree with experimental findings for O ions on Bi while the same theoretical predictions deviate from the experimental results by about a factor of 2 for Si and S on Bi, the theory being higher.

Measurement on the $M$-shell x-ray production cross sections are reported for Pb and Bi induced by the highly charged F (only on Pb) and C ions (on both). The energy of the incident ions was varied between 20 and $102\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$. We have derived the absolute cross sections for $M\ensuremath{\alpha}\ensuremath{\beta}$ and $M\ensuremath{\gamma}$ lines as well as the total $M$ x-ray cross sections. The measured cross sections are compared with available theoretical calculations, namely, the ECPSSR based on the perturbed-stationary state approximation including the effects due to the increased binding energy, Coulomb deflection $(C)$, energy-loss $(E)$, and relativistic $(R)$ wave function. The energy shifts and the intensity ratios of $M\ensuremath{\alpha}\ensuremath{\beta}$ and $M\ensuremath{\gamma}$ lines are shown to depend on the projectile atomic number. The $M\ensuremath{\gamma}$ x-ray cross sections are unusually higher compared to the ECPSSR prediction that is primarily attributed to a dramatic enhancement in the ${M}_{3}$-subshell fluorescence yield owing to multiple vacancies in $N$ subshells. This enhancement has been quantified and it is shown that the enhancement depends on the projectile atomic number. In addition we have derived the x-ray cross section arising due to $M\text{\ensuremath{-}}K$-shell electron transfer from a study of a charge state dependence of $M$-shell x-ray yields.

Direct evidence of the interference effect in the electron emission spectra from ionization of molecular hydrogen in collisions with bare C and F ions at relatively low collision energies is presented. Oscillations due to the interference are deduced by comparing the measured double differential cross sections of the electrons emitted from molecular hydrogen to those emitted from atomic hydrogen, rather than using the calculated cross sections for H as in a previous report. We believe these experimental data provide stronger support for the evidence of the interference effect. We show that it is not only a feature of very high energy collisions, but also a feature to be observed in relatively lower energy collisions.

We have investigated K-shell vacancy production due to ionization and electron transfer processes, in collisions of highly charged oxygen ions with various solid targets such as Cl, K, Ti, Fe, and Cu at energies between 1.5 and 6.0 MeV/u. The K-shell ionization cross sections were derived from the measured K x-ray cross sections. An ab initio theoretical model based on the local plasma approximation (LPA), which is an extension of the dielectric formalism to consider core electrons, provides an explanation of the measured data only qualitatively. In case of asymmetric collisions ${(Z}_{p}{/Z}_{t}l0.35,$ ${Z}_{p},$ ${Z}_{t}$ being the atomic numbers of the projectile and target, respectively) and at higher energies, the LPA model explains the data to some extent but deviates for more symmetric collision systems. On the other hand, a perturbed-stationary-state (PSS) calculation (ECPSSR), including the corrective terms due to energy (E) loss, Coulomb (C) deflection, and relativistic (R) wave functions designed for ion-atom collisions agree quite well with the data for different combinations of target and projectile elements. In addition, we have also measured the $K(\mathrm{target})\ensuremath{-}K(\mathrm{projectile}) \mathrm{electron}$ transfer cross sections and compared them with a model based on perturbed-stationary-state approximation.

M-subshell x-ray production cross sections are measured for Au, induced by F ions in the energy range of 20 to 102 MeV. For a comparative study the measurements are also carried out for protons, He, Li, C ions at a few energies. We have derived the absolute cross sections for $M\ensuremath{\alpha}\ensuremath{\beta}$ and $M\ensuremath{\gamma}$ x rays as well as the total M x-ray cross sections. The intensity ratios, $I(\ensuremath{\gamma})/I(\ensuremath{\alpha}\ensuremath{\beta}),$ of the $M(\ensuremath{\gamma})$ and $M(\ensuremath{\alpha}\ensuremath{\beta}),$ as well as the energy shifts of these lines signifying multiple vacancies in outer shells have also been studied as a function of projectile atomic number. The measured cross sections are compared with available theoretical calculations, namely, the ECPSSR based on the perturbed stationary state (PSS) approximation including the effects due to the increased binding energy, Coulomb deflection (C), energy loss (E), and relativistic (R) wave function. In case of the $M\ensuremath{\gamma}$ the measured cross sections are much higher compared to the ECPSSR prediction which is attributed to a dramatic enhancement in the ${M}_{3}$-subshell fluorescence yield owing to multiple vacancies in N subshells. The enhancement strongly depends on the projectile atomic number.

The hard x-ray emission in the energy range of 30-300 keV from copper plasmas produced by 100 fs, 806 nm laser pulses at intensities in the range of 10$^{15}-10^{16}$ W cm$^{-2}$ is investigated. We demonstrate that surface roughness of the targets overrides the role of polarization state in the coupling of light to the plasma. We further show that surface roughness has a significant role in enhancing the x-ray emission in the above mentioned energy range., Comment: 5 pages, 4 figures, to appear in Phys. Rev. A

We have investigated ionization mechanisms in fast ion-atom collisions by measuring the low-energy electron emission cross sections in a pure three-body collision involving bare carbon ions $(v=6.35\mathrm{a}.\mathrm{u}.)$ colliding with atomic hydrogen targets. The measurements have also been extended to molecular hydrogen and helium targets. In this paper we provide the energy and angular distributions of double differential cross sections of low-energy electron emission for atomic hydrogen targets. The Slevin rf source with a high degree of dissociation was used to produce the atomic H target. It is found that the two-center effect has a major influence on the observed large forward-backward angular asymmetry. A detailed comparison is presented with calculations based on the continuum distorted-wave (CDW) and CDW-EIS (eikonal initial-state) approximations. Both the continuum distorted-wave calculations provide a very good understanding of the data, whereas the first Born calculation predicts almost symmetric forward-backward distributions that do not agree with the data. The two-center effect is slightly better represented by the CDW calculations compared to the CDW-EIS calculation. The total cross sections are, however, in good agreement with the theories used. The results for molecular hydrogen and helium will be discussed in the following paper.