Contribution Of Ionization Research Articles

Selective control over fragmentation reactions in polyatomic molecules using impulsive laser alignment.

We investigate the possibility of using molecular alignment for controlling the relative probability of individual reaction pathways in polyatomic molecules initiated by electronic processes on the few-femtosecond time scale. Using acetylene as an example, it is shown that aligning the molecular axis with respect to the polarization direction of the ionizing laser pulse does not only allow us to enhance or suppress the overall fragmentation yield of a certain fragmentation channel but, more importantly, to determine the relative probability of individual reaction pathways starting from the same parent molecular ion. We show that the achieved control over dissociation or isomerization pathways along specific nuclear degrees of freedom is based on a controlled population of associated excited dissociative electronic states in the molecular ion due to relatively enhanced ionization contributions from inner valence orbitals.

Ionization of metastable nitrogen and oxygen atoms by electron impact: Relevance to auroral emissions

AbstractTotal cross sections are theoretically evaluated for electron impact ionization of initially metastable states of nitrogen as well as oxygen atoms, known to play roles in auroral emissions observed in the Earth's polar regions as also in the cometary coma and airglows. If the long‐lived N(2P), N(2D), O(1S), and O(1D) metastable species get ionized in their interactions with energetic electrons, then they would not contribute to auroral emission. Therefore, we investigate electron ionization of the said metastable species by calculating relevant total cross sections. Our quantum mechanical calculations are based on projected approximate ionization contribution in the total inelastic cross sections. In all the four targets, the resulting cross sections are fairly large, indicating the importance of the electron ionization process.

On the electron impact ionization of silicon and metal containing organic molecules

Calculation of electron impact total inelastic cross sections for three silicon containing organic molecules (Trimethylsilane, Tetraethoxysilane and Hexamethyldisiloxane) and three organometallic complexes (Cyclopentadienyltrimethyl-platinium, Bismethylcyclopentadienyl-ferrum and Bismethylcyclopentadienyl-ruthenium) were performed employing spherical complex optical potential formalism. The complex scattering potential ionization contribution method was then used to derive total ionization cross sections from inelastic cross sections for these targets. The results presented here are for the incident electron energy ranging from ionization threshold to 2000eV. The comparison with existing measurement shows promising results.

Ionization cross sections and rate coefficients for the ground state of AsI-like ions

Detailed level-by-level calculations of cross sections and rate coefficients for electron impact direct ionization and excitation-autoionization of AsI-like ions in the 3d 104s 24p 3 (4S3/2) ground state have been performed. The cross sections are presented in the energy range near the threshold for the five ions Mo9+, Xe21+, Pr26+, Dy33+ and W41+. The rate coefficients are given for all the ions from Sr5+ to U59+ in the AsI sequence at the seven electron temperatures (k T e = 0.1E I ,0.3E I ,0.5E I ,0.7E I ,E I ,2E I and 10E I , where E I is the first ionization energy). The calculations include the contribution of direct ionization (DI) calculated using the Lotz formula approximation and the contributions of excitation-autoionization (EA) computed in the framework of the Distorted Wave (DW) approximation for the 4s − nl, 3d − nl and 3p − nl resonant inner-shell excitations. The ionization enhancement due to the EA channels is presented as a function of Z along the AsI isoelectronic sequence. The present results show the great importance of the EA processes; an ionization enhancement factor of up to 6.5 is predicted for instance for Dy33+ (Z = 66) at electron temperature of coronal equilibrium maximum abundance.

Electron impact ionization cross sections for chlorinated and brominated methane and CnH2n+1Cl (where n = 2, 3, 4) molecules

Theoretical determination of electron impact total ionization cross sections for chloromethane (CH2Cl2, CHCl3), bromomethane (CH2Br2, CHBr3, and CBr4) and some mono chloroalkane (C2H5Cl, 1-C3H7Cl and 1-C4H9Cl) molecules in gaseous ground state are reported over the energy range from ionization threshold to 5keV. Here, the target molecule is represented as an aggregate of scattering centres, which is assumed to scatter the electron independently. Hence, an approximation called group additivity rule is adopted for the determination of cross sections. Spherical complex optical potential formalism is employed to calculate the inelastic cross section for the constituent scattering centres. Sum of the inelastic cross sections of the constituent centres gives the molecular total inelastic cross section. Further, the total ionization cross section of the target molecule is derived from the molecular inelastic cross section using the complex scattering potential ionization contribution method. The results obtained are compared with previous measurements and theoretical values wherever available and satisfactorily good agreement is observed. The cross section predicted presently for CBr4 molecule is a maiden attempt.

Electron impact total ionisation cross sections for simple bio-molecules: a theoretical approach

In this article, we have reported electron impact total ionisation cross sections for the bio-molecules pyridine, pyrimidine, n-propylamine, urea, formamide and N-methylformamide from ionisation threshold to 2000 eV. The present calculations are based on the spherical complex optical potential formalism and complex scattering potential ionisation contribution method. The results obtained for pyridine and pyrimidine are compared with available theoretical and experimental results and are found to be in excellent agreement with existing data. The ionisation cross sections for other molecules are reported for the first time. An interesting relation between the peak of inelastic and ionisation cross sections with target parameters is also reported. It was found that both the cross sections at their maximum depend linearly with these parameters, confirming the consistency of the values reported here.

Subcycle Dynamics of Coulomb Asymmetry in Strong Elliptical Laser Fields

We measure photoelectron angular distributions of noble gases in intense elliptically polarized laser fields, which indicate strong structure-dependent Coulomb asymmetry. Using a dedicated semiclassical model, we have disentangled the contribution of direct ionization and multiple forward scattering on Coulomb asymmetry in elliptical laser fields. Our theory quantifies the roles of the ionic potential and initial transverse momentum on Coulomb asymmetry, proving that the small lobes of asymmetry are induced by direct ionization and the strong asymmetry is induced by multiple forward scattering in the ionic potential. Both processes are distorted by the Coulomb force acting on the electrons after tunneling. Lowering the ionization potential, the relative contribution of direct ionization on Coulomb asymmetry substantially decreases and Coulomb focusing on multiple rescattering is more important. We do not observe evident initial longitudinal momentum spread at the tunnel exit according to our simulation.

Electron impact total cross section calculations for CH3SH (methanethiol) from threshold to 5 keV

We report calculated total elastic cross sections Qel, total ionisation cross sections, Qion, summed total excitation cross sections ∑Qexc and total cross sections QT for CH3SH upon electron impact for energies from ionisation threshold to 5 keV. We have employed Spherical Complex Optical Potential (SCOP) formalism to calculate total elastic cross section Qel, and total inelastic cross section Qinel and used Complex Scattering Potential – the ionisation contribution (CSP-ic) method to extract the ionisation cross sections, Qion, from the calculated Qinel. The calculated total cross sections are examined as functions of incident electron energy and are compared with available data wherever possible and overall good agreement is observed. In this work Qel, Qion, and ∑Qexc are reported for the first time for CH3SH in this energy range.

Ionization within the Local Cavity by Hot White Dwarfs

ABSTRACTWe present estimates of the contribution from 33 nearby (d < 100 pc) hot DA white dwarfs to the total ionization of the Local Cavity. The EUV spectra of these white dwarfs have been derived using non-LTE stellar atmosphere models (with the inclusion of metals where necessary) to calculate the incident flux at the Earth using up-to-date distances and interstellar column densities. Our model spectra give a very good approximation to the spectra obtained with observations from the NASA Extreme Ultraviolet Explorer satellite. The number of Lyman continuum photons per second emitted by each of the white dwarfs for wavelengths over the 30–912 Å range was then derived. Using these numbers, the respective sizes of the ionized Strömgren spheres of HII were then calculated for each star under the assumption of ambient interstellar electron density values of ne = 0.04 cm-3 and 0.10 cm-3. The location and sizes of these ionized regions have been plotted on a 3D map of the neutral absorption contours of the local ISM, as derived for both in-plane and out of plane directions. A similar procedure was also carried out for the determination of the sizes of the ionized HeII regions surrounding the same 33 white dwarfs. We conclude that only a few of the very hottest DA white dwarfs located within the Local Cavity can possess appreciably sized ionized Strömgren spheres of HII and HeII. The size of these ionized regions can potentially range between 20 pc and 40 pc in diameter, dependent on the ambient value of the electron density of the interstellar medium. However, the volumetric filling factor of these ionized spheres within the Local Cavity is small (∼6%), even at the lowest value of electron density considered in this study (i.e., ne = 0.04 cm-3). Thus, the contribution from DA white dwarfs to the ionization of both the local interstellar cloud complex and other clouds located within the Local Cavity is of secondary importance compared to the ionization contribution provided by the two early B-type stars of ϵ and β CMa.

Contribution of recollision ionization to the cross-shaped structure in nonsequential double ionization

With the three-dimensional classical ensemble model, we investigate the correlated electron emission in nonsequential double ionization (NSDI) of argon atoms by few-cycle laser pulses. Our calculations well reproduce the experimentally observed cross-shaped structure in the correlated two-electron momentum spectrum [ Nature Commun. 3, 813 (2012)]. By tracing these NSDI trajectories, we find that besides the process of recollision-induced excitation with subsequent ionization just before the next field maximum, the recollision ionization also significantly contributes to the cross-shaped structure.

Photoion-yield study of the 3p–3d giant resonance excitation region of isolated Cr, Mn and Fe atoms

Using the charge-separated photoion-yield method combined with monochromatized synchrotron radiation, single (M+) and double (M2+) photoionization spectra in the 3p excitation region were observed for isolated atoms M = Cr, Mn and Fe. The relaxation processes of excited states to double ionization, as well as to single ionization, were considered with the help of existing photoabsorption and photoelectron data and some calculations of highly-excited states of M+ using the atomic code GRASP92. In the 3p→3d giant resonance region before the 3p→n(s, d) Rydberg series starts, the spectral features in the M2+ channel are similar to those in the M+ channel. The ratios M2+/M+ in this region are rather constant accordingly against the photon energy with values 0.045 ± 0.005 for Cr, 0.17 ± 0.03 for Mn and 0.30 ± 0.08 for Fe at the giant resonance. The increase in the M2+/M+ ratio with increasing the atomic number Z can be explained by the increasing contribution of two-step double ionization via highly-excited M+ states. It is expected that, at the 3p→3d giant resonance, the one-step double ionization process occurs within a few per cent of the entire decay.

Electron impact total and ionization cross-sections for DNA based compounds

This paper reports computational results of the total (complete) and total ionization cross- sections, for electron impact on Uracil (C4H4N2O2) and PO3OH for impact energies from the ionization threshold to 2 keV. The total cross-section is evaluated using quantum mechanical approach using Spherical Complex Optical Potential (SCOP) presented as sum of the elastic and inelastic cross-sections. The ionization cross-sections are extracted from total inelastic cross-section using Complex Optical Potential–ionization contribution (CSP-ic) method. The present results are, in general, found to be in good agreement with previous theoretical results. In absence of any theoretical or experimental data, present results for PO3OH will serve to fill the void in the data base and may also inspire the experimentalists for some measurements as it is very important target.

Electron impact ionization of Si and SiClX (X=1-4) targets: Theoretical investigations

In present paper we have calculated ionization cross sections of Si and SiClX(X=1-4) targets using "Complex Scattering Potential - ionization contribution" (CSP-ic) method.

Binary encounter models for electron ionization of atoms: A one parameter expression for electron energy dependent (E < 200 eV) single ionization cross section

A one parameter expression for the single ionization cross-section of atoms by electrons is presented. This is done following analysis of current binary encounter based approaches and models. Using this expression, the agreement obtained with available experimental data for 45 elements (from ionization threshold up to 200eV ionizing electron energy) is shown to be comparable with (or better than) that achieved using other multi-parameter approaches. We associate the single parameter used here with the effective reduction in the number of equivalent electrons within a given shell accessible for electron ionization. We attribute this reduction effect to intra-shell shadowing of part of the shell electrons by other electrons of the same shell. For atoms with 2s2pi (i=3, 4, 5, 6) outer shell configurations we suggest also an intershell shadowing effect. Finally, we discuss the possibility of a meaningful contribution of deep inner shell ionization (50–100eV binding energy) to the single ionization cross-section due to various post-collision interactions. These interactions may be responsible for decreasing the formation probability of doubly charged ions.

Electron ionization of exotic molecular targets CN, C2N2, HCN, HNC and BF—Theoretical cross sections

Total ionization cross sections Qion of the electron impact on exotic but lesser known targets CN, C2N2, HCN, HNC and BF have been calculated at incident energies Ei from ionization threshold (IE) to 2keV, by employing the Complex potential ionization contribution (CSP-ic) method. Since most of the present results are new, comparable theoretical data are obtained by Additivity Rules and by generating the BEB (Binary Encounter Bethe) values through the Quantemol-N package. A good accord is found among the present and the other results, except in BF. Our ionization cross sections on HCN, HNC and BF are also compared with the available data on iso-electronic 14-electron systems N2, CO and C2H2. Moreover in e−–HCN scattering, the relative contributions of various allowed scattering channels are analyzed at the energy of the peak of ionization (70eV) and presented here as a bar chart. The present work is relevant due to importance in mass spectrometry and in planetary environments.

Ionization cross sections and contributions of continuum to optical characteristics of Rydberg states

Simple analytical properties of ionization cross sections allow high-precision evaluations of contributions from integrals over continuum to sum rules for oscillator-strength moments and to the blackbody-radiation-induced shifts of atomic levels. The contributions from continuum and from infinite sums over discrete states are presented in terms of the cross section for the threshold photoionization. General properties of the indicated contributions are determined for bound states with small angular momenta (l ⩽ 3) in hydrogen atoms. Asymptotic presentations for the contributions from continuum to the sums of oscillator-strength moments are proposed for highly excited Rydberg states. The estimate is given to the accuracy of determining the infinite sum over discrete states by a sum of a finite number of terms.

Analysis of plasma-mediated ablation in aqueous tissue

Plasma-mediated ablation using ultrafast lasers in transparent media such as aqueous tissues is studied. It is postulated that a critical seed free electron density exists due to the multiphoton ionization in order to trigger the avalanche ionization which causes ablation and during the avalanche ionization process the contribution of laser-induced photon ionization is negligible. Based on this assumption, the ablation process can be treated as two separate processes – the multiphoton and avalanche ionizations – at different time stages; so that an analytical solution to the evolution of plasma formation is obtained for the first time. The analysis is applied to plasma-mediated ablation in corneal epithelium and validated via comparison with experimental data available in the literature. The critical seed free-electron density and the time to initiate the avalanche ionization for sub-picosecond laser pulses are analyzed. It is found that the critical seed free-electron density decreases as the pulse width increases, obeying a tp−5.65 rule. This model is further extended to the estimation of crater size in the ablation of tissue-mimic polydimethylsiloxane (PDMS). The results match well with the available experimental measurements.

Electron-impact ionization of Al

Perturbative distorted-wave and non-perturbative close-coupling methods are used to calculate electron-impact ionization cross sections for the ground state of the neutral Al atom. Configuration-average distorted-wave calculations are made for both direct ionization and excitation-autoionization contributions. The total perturbative results are found to be almost a factor of 2 higher than experiment over a wide energy range. On the other hand, the R-matrix with pseudo-states results for total ionization are found to be in good agreement with experiment. Comparison of time-dependent close-coupling calculations for the direct ionization with the R-matrix with pseudo-state calculations for total ionization reveals that both the direct ionization and excitation-autoionization contributions are strongly affected by correlation effects.

Competition between two-photon-resonant three-photon ionization and four-wave mixing in Xe

Competitive inhibition of a resonance enhanced multiphoton ionization process by a resonant four-wave mixing has been observed in Xe atoms. When an intense IR (1064 nm) laser was applied to a sample of Xe which was also being irradiated by a UV laser tuned to the two-photon absorption line of Xe, the two-photon-resonant three-photon ionization signals decreased with increasing IR laser power. This phenomenon is dependent on the resonant states of Xe and the polarization of the two laser beams. Three 6$s$ excited states [5/2]${}_{2}$, [3/2]${}_{2}$, and [1/2]${}_{0}$ were examined. At the [1/2]${}_{0}$ resonant state, the ion signals were not decreased but slightly increased with the increase of the IR laser power. No suppression of the ion signal was observed at the [5/2]${}_{2}$ resonant state, when the polarization directions of the lasers were perpendicular to each other. The result of the polarization dependence reflects the selection rules of four-wave mixing. A simple rate equation analysis including the contribution of two-photon ionization from the [1/2]${}_{0}$ state by the IR laser well represents the IR laser-power dependence of the ion signal.

Calculations of total ionization cross sections for halogen compounds on electron impact from threshold to 2 keV

Calculation for electron impact total ionization cross sections on halogen compounds (BF, SiF, BCl, SiCl) are performed employing Spherical Complex Optical Potential and Complex Optical Potential — ionization contribution (CSP-ic) formalisms. In this article we are presenting data for energies ranging from above threshold to 2000 eV. Our results are compared with available experimental and theoretical data wherever available. It is found that the present result gives a better account of the ionization cross sections.