Measurement of the transverse kinetic energies of argon recoil ions produced in 120 MeV -Ar collisions

Measurements were carried out to deduce the transverse kinetic energies of highly charged argon recoil ions produced in single collisions of 120 MeV ions with argon atoms in which the post collision charge states of the projectiles were not determined. A time of flight spectrometer was designed and fabricated to detect the charge states of recoils. Experimental procedures for optimizing the spectrometer for extraction, transmission and detection of recoils are described. A simple approach for determining the transverse kinetic energy of the recoil ions from FWHM of the peaks is reported. This method is shown to be independent of the choice of collision partners and requires only the knowledge of the physical values of “optimized parameters” of time-of-flight spectrometer used in the experiment. The transverse kinetic energy of the recoil ions determined from the present approach is found to vary from 0.03 eV for to 4.02 eV for Ar¹⁰⁺. These values are compared with the results reported by earlier workers and are shown to follow a q²-behaviour up to a charge state q =8+ of the recoil ions. Received: 5 February 1998 / Revised: 8 June 1998 / Accepted: 11 June 1998     

Note on electron impact excitation for transition of Be-like isoelectronic sequence

Relativistic distorted-wave method was used to calculate the electron impact excitation collision strengths for transitions 2 s2 ¹ S0-2 s 2 p ³ P1 of Be-like isoelectronic ions. The target states were described, respectively, by 10-level, 46-level and 133-level MCDF configuration-expansion. The relativistic continuum orbitals were calculated in the potential field of frozen target-ion charge distribution with semi-classical exchange potential. The influence of the target states on this collision process along the isoelectronic sequence was investigated in the above three MCDF configuration-expansion modes. It was found that the configurations in the n =3 and the n =4 complexes have great influence on both the high and the low Z ions but the influence is relatively small for intermediate Z ions. The latter phenomenon was attributed to competition between opposing correlation and relativistic effects on the collision strengths. Received: 25 February 1998 / Received in final form: 18 June 1998     

Ionization of Rydberg atoms in THz-laser fields at the transition from low to high scaled frequencies

We have studied the ionization of Rydberg-excited xenon atoms in THz-laser fields and by quantum dynamical calculations. The experimental threshold laser field strength for 10% ionization probability follows an n^*-1.68 (1.04 THz) dependence (n^* effective principal quantum number) with additional weak resonance structures and shows that ionization does not occur by a Landau-Zener mechanism. At scaled frequencies of to 5.6 the simulated threshold fields for ionization in oscillatory fields show a dependence on the principal quantum number n of n^-4.1 to n^-1.35. Received: 17 February 1998 / Revised: 20 April 1998 / Accepted: 21 April 1998     

Ionization of iridium ions in the Dresden EBIT studied by X-ray spectroscopy of direct excitation and radiative recombination processes

Ir^q+ ( 41≤q≤64) ions with open-shell configurations have been produced in the electron beam of the room-temperature Dresden Electron Beam Ion Trap (Dresden EBIT) at electron excitation energies from 2 keV to 13 keV. X-ray emission from direct excitation processes and radiative capture in krypton-like to aluminium-like iridium ions is measured with an energy dispersive Si(Li) detector. The detected X-ray lines are analyzed and compared with results from multiconfigurational Dirac-Fock (MCDF) atomic structure calculations. This allows to determine dominant produced ion charge states at different electron energies. The analysis shows that at the realized working gas pressure of 5×10^-9mbar for higher charged ions the maximum ion charge state is not preferently determined by the chosen electron beam energy needed for ionization of certain atomic substates, but by the balance between ionization and charge state reducing processes as charge exchange and radiative recombination. This behaviour is also discussed on the basis of model calculations for the resulting ion charge state distribution. Received 12 July 2001 and Received in final form 10 September 2001     

Intercombination transition rates in Al-like ions

N =3, intercombination transitions in Al-like ions of Au have been studied by time-resolved EUV spectroscopy of foil-excited ion beams. Wavelengths and lifetimes are compared to the available relativistic calculations. The theoretical data are found to be useful for guidance, but of clearly insufficient precision, in particular for the transition probabilities. Received: 23 February 1998 / Accepted: 5 March 1998     

Electron impact ionization of SiCl<Subscript>2</Subscript> and SiCl

We measured absolute partial cross sections for the formation of all singly charged positive ions produced by electron impact on SiCl₂ and SiCl from threshold to 200 eV using the fast-neutral-beam technique. Some of the cross section curves exhibit an unusual energy dependence with a pronounced low-energy maximum at an energy around 30 eV, which may be indicative of the presence of indirect ionization channels. Dissociative ionization channels are dominant for both species. The experimentally determined total single ionization cross sections for both species agree very well with calculated cross sections using the Deutsch-M?rk (DM) formalism. A brief summary of the ionization cross sections determined for all four SiCl_x (x=1–4) species is given highlighting similarities and differences.     

Vibrational Feshbach resonances in electron attachment to carbon dioxide clusters

Using a high resolution ( meV) laser photoelectron attachment method, we have studied the formation of (CO ₂) _q ⁻ ions (q = 4−22) in collisions of low energy electrons (1−180 meV) with (CO₂) _N () clusters. The previously reported “zero energy resonance”, observed at much larger electron bandwidths, actually consists of several narrow vibrational Feshbach resonances of the type [(CO ₂) _{N −1}CO which involve a vibrationally-excited molecular constituent ( denotes vibrational mode) and a diffuse electron weakly bound to the cluster by long range forces. The resonances occur at energies below those of the vibrational excitation energies of the neutral clusters [(CO ₂) _{N −1}CO ]; the redshift rises with increasing cluster ion size q by about 12 meV per unit; these findings are recovered by a simple model calculation for the size dependent binding energies. The size distribution in the cluster anion mass spectrum, resulting from attachment of very slow electrons, mainly reflects the amount of overlap of solvation-shifted vibrational resonances with zero energy; the cluster anion size q is identical with or close to that of the attaching neutral cluster. Received 11 January 2000 and Received in final form 10 April 2000     

Electron impact single and double ionization of magnesium

Electron impact single and double ionization cross-sections for magnesium have been calculated in the binary encounter model using accurate expression for (cross-section for energy transfer ΔE) as given by Vriens. Hartree-Fock velocity distributions for the target electrons have been used throughout the calculations. In case of double ionization contributions of inner shell ionization and Auger emission have been included in the present work. The results obtained in case of single ionization are excellent and at the same time the double ionization cross-sections show reasonably good agreement with the recent experimental observations. Substantiation of the viewpoint of Peach, and Boivin and Srivastava that a vacancy in the 2p shell of magnesium leads to double ionization is a remarkable feature of the present investigation. Received 9 October 2001 / Received in final form 8 January 2002 Published online 28 June 2002     

Regenerative soot as a source of broad band VUV light

Certain important and pertinent questions have been raised by E. Träbert in his comment on the paper [Eur. Phys. J. D 22, 189 (2003)] by one of us. We provide explanations as well as new and recent data using a calibrated monochromator on the existence of the main intercombination multiplet 233 nm which is the bone of contention. It is shown that there is no conflict with the established atomic physics in the regenerative sooting discharges; only the interpretations are at variance with those of the comment.Received: 30 July 2003, Published online: 30 September 2003PACS: 32.70.Fw Absolute and relative intensities - 34.50.Dy Interactions of atoms and molecules with surfaces; photon and electron emission; neutralization of ions - 34.80.Dp Atomic excitation and ionization by electron impact     

Multiple ionization of argon in coincidence with projectile ions in 60–120 MeV Si q+-Ar collisions

A projectile ion-recoil ion coincidence technique has been employed to study the multiple ionization and the charge transfer processes in collisions of 60–120 MeV Si ^q+ (q = 4−14) ions with neutral argon atoms. The relative contribution of different ionization channels, namely; direct ionization, electron capture and electron loss leading to the production of slow moving multiply charged argon recoil ions have been investigated. The data reported on the present collision system result from a direct measurement in the considered impact energy for the first time. The total ionization cross-sections for the recoil ions are shown to scale as q ^1.7/E _p ^0.5 , where E _p is the energy in MeV of the projectile and q its charge state. The recoil fractions for the cases of total- and direct ionizations are found to decrease with increasing recoil charge state j. The total ionization fractions of the recoils are seen to depend on q and to show the presence of a ‘shell-effect’ of the target. Further, the fractions are found to vary as 1/j ² upto j = 8+. The average recoil charge state 〈j〉 increases slowly with q and with the number of lost or captured electrons from or into the projectile respectively. The projectile charge changing cross-sections σ _qq′ are found to decrease with increasing q for loss ionization and to increase with q for direct-and capture ionization processes respectively. The physics behind various scaling rules that are found to follow our data for different ionization processes is reviewed and discussed.     

Measurement of K X-ray intensity ratios in Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn employing carbon and nitrogen projectiles – VTT method

A method is suggested to effect the self absorption correction in a different way to estimate the K X-ray intensity ratios particularly when heavy ions are used as projectiles. Employing this method, the K_β/K_α intensity ratios are measured in some 3d shell elements by using Carbon and Nitrogen ions as exciting agents. The K_β/K_α intensity ratios thus obtained in the present work are compared with the intensity ratios due to some previous authors and also with Scofield theoretical values. Received 14 August 2000 and Received in final form 3 January 2001     

Multiple ionization and fragmentation of the DNA base thymine by interaction with C q+ ions

Ionization and fragmentation of the DNA base thymine upon interaction with keV C^q+ ions (q = 1 - 6) has been studied. By means of time-of-flight spectrometry of two or more thymine fragments in coincidence with an ejected electron we could investigate particular dissociation channels by means of their associated kinetic-energy-release. The fragmentation dynamics are strongly influenced by the C^q+ charge state: for low q values mainly fragmentation due to direct collisions is observed. With increasing q, electron capture becomes more important. For larger q we could identify several Coulomb explosion channels, leading to very energetic fragments.     

Ionisation of phosphorus, arsenic, antimony, and bismuth by electron impact

Total ionization cross sections of neutral phosphorus, arsenic, antimony, and bismuth atoms by electron impact are reported and compared to the only available experimental results by Freund et al. [Phys. Rev. A 41, 3575 (1990)]. These calculations take into account the possibilities that some target atoms used in the experiments were in metastable states close to the ground state, the excitation-autoionization of nsnp⁴ excited states may be substantial, and the ions produced in experiments may be in excited, low-lying metastable states. The cross sections for direct ionization calculations are based on the BEB model by Kim and Rudd [Phys. Rev. A 50, 3954 (1994)]. Plane-wave Born cross sections scaled by the method developed by Kim [Phys. Rev. A 64, 3954 032713 (2001)] are used to determine the contributions from excitation-autoionization. The combination of the BEB model and the scaled Born cross sections is in agreement with the experimental data by Freund et al. These theoretical data are useful to experimentalists and can be used to complete data tables needed for plasma or astrophysical studies.     

Calculation of absolute electron-impact ionization cross-sections of dimers and trimers

We report calculations of the electron-impact ionization cross-sections of selected dimers (homonuclear diatomic molecules) and trimers (homonuclear triatomic molecules) using a method which relies only on macroscopic quantities in conjunction with a “defect concept”. The empirically determined defect describes the deviation of the cluster (dimer, trimer) cross-sections from a simple linear dependence on the cluster size. We compare the calculated cross-sections to experimental data for the dimers S₂ and F₂ and the trimer O₃ and we present predictions for the ionization cross-sections of Br₂, I₂, C₂ and C₃ for which no experimental data are available. Lastly, we extend the method to the calculation of ionization cross-sections for the fullerenes C₆₀ and C₇₀. Received 6 December 1999 and Received in final form 10 April 2000     

Calculation of the contributions from high-n dielectronic satellites to the resonance line in helium-like iron

A simplified relativistic configuration interaction method is used to study the dielectronic satellite transition processes. In this method, the infinite resonant doubly excited states can be calculated, and furthermore, the whole high-n dielectronic satellite transition processes can be treated conveniently by interpolation (rather than extrapolation) in the frame of Quantum Defect Theory. As an example, we calculate the contributions from high-n dielectronic satellites to the resonance line in helium-like iron, and the results are in good agreement with the experimental measurements. Received: 16 April 1998 / Revised: 8 September 1998 / Accepted: 14 September 1998     

Electron impact single detachment on the F - ions using the heavy ion storage ring CRYRING: Cross-section determination

Electron Impact Single Detachment (EISD) of F^- has been studied using the heavy ion storage ring CRYRING at the Manne Siegbahn Laboratory, Stockholm, Sweden. F^- ions stored in the ring were merged with an electron beam in one of the ring sections. Neutral F atoms produced in the EISD process were detected in the zero-degree direction using a surface barrier detector. The threshold for the detachment process was found to be around 7.6 eV, thus more than twice the binding energy of F^-. The cross-sections increased smoothly up to 55 eV where it reached a maximum of 1.9×10 ^-16 cm ² . At higher energies a slow decrease of the cross-section was observed, which follows the energy dependence predicted by the Bethe-Born approximation. The experiment showed that CRYRING can be used favourably for studies of anions, and several experiments are forthcoming. Received 14 June 2000 and Received in final form 11 September 2000     

Dynamics of ionization mechanisms in relativistic collisions involving heavy and highly-charged ions

The dynamics of mechanisms associated with the ionization of inner-shell electrons in relativistic collisions involving heavy and highly-charged ions is investigated within a nonperturbative approach formulated explicitly in the time domain. The theoretical treatment is based on the exact numerical solution of the time dependent Dirac equation for two Coulomb centers on a lattice in momentum space. We present results for ionization in encounters between 100 MeV/u Au⁷⁹⁺ projectile ions impinging on a hydrogen-like uranium target. By directly visualizing the collision dynamics we identify a new ionization mechanism in which electrons are emitted from the internuclear region preferentially in the transverse direction with respect to the projectile trajectory. A striking characteristic of this ionization mechanism is that the velocity of the electron is higher than the projectile velocity. Received 26 June 2001 and Received in final form 27 November 2001     

Theoretical total ionization cross-sections of CH x,CF x,SiH x,SiF x ( ) and CCl 4 targets by electron impact

Total ionization cross-sections of electron impact are calculated for the molecular targets CH_x, CF_x, SiH_x, SiF_x (x = 1-4) and CCl₄ at incident energies 20-3 000 eV. The calculation is based on Complex Scattering Potential approach, as developed by us recently. This leads to total inelastic cross-sections, from which the total ionization cross-sections are extracted by reasonable physical arguments. Extensive comparisons are made here with the previous theoretical and experimental data. The present results are satisfactory except for the CF_x and SiF_x (x = 1-3) radicals, for which the experimental data are lower than most of the theories by more than 50%. Received 23 May 2002 / Received in final form 24 October 2002 Published online 21 January 2003 RID="a" ID="a"e-mail: knjoshipura@yahoo.com     

Multiple ionization of argon accompanied by electron loss and capture of 0.22-6.35 MeV C^q＋ ions

In this paper a projectile ions recoil ions coincidence technique is employed to investigate the target ionization and projectile charge state changing processes in the collision of 0.22-6.35 MeV Cq^＋ （q = 1 - 4） ions with argon atoms. The partial cross section ratios of the double, triple, quadruplicate ionization to the single ionization （or the single capture） of argon associated with single electron loss （or single electron capture） by the projectile are measured and compared with the previous experimental results. In the present experiment, it is observed that the ratios of ionization cross sections R associated with single loss and single capture depend strongly on the projectile charge state and vary significantly with different reaction channels as impact energy increases. In addition, this paper gets empirical scaling laws for the ionization cross section ratios R corresponding to the projectile single loss and finds that the ratios of the double ionization to the single ionization associated with single electron capture remain constant in the present energy range.     

Ionization of sodium by the impact of alpha particle

A one channel distorted wave approximation has been used to calculate the ionization cross section for the 3s electron of Na atom by alpha particle impact. For a consistency check we have also run the standard CDW-EIS code of McSherry et al. [Comput. Phys. Commun. 155, 144 (2003)] and found reasonable agreement in the high energy region among the present results, the CDW-EIS results and the classical trajectory Monte Carlo (CTMC) calculations. It is also found that the present results tend to agree qualitatively with the low energy measurement of Knoop et al. [J. Phys. B 38, 1987 (2005)] at as low as 10 keV amu^-1, although the present theoretical approach is expected to be suitable in the high energy area. This qualitative agreement in the low energy region is attributed to the partially correlated ground state wave function used in the present calculation.