Excitation Of Autoionizing States Research Articles

Obrazovanie i raspad avtoionizatsionnykh sostoyaniy - osnovnoy mekhanizm neuprugikh poter' pri stolknoveniyakh atomov keV-energiy

It has been shown that the formation of autoionization states makes the dominant contribution to inelastic energy loss and to ionization in keV atomic collisions, i.e., at velocities lower than the velocities of atomic electrons. Scaling laws have been proposed to calculate the cross sections for the formation of vacancies in inner K and L electron shells of colliding atoms. A model has been proposed to relate ionization processes and the observed inelastic energy losses. Auger transitions in a short-lived quasimolecule formed by two atoms approaching each other have been studied. The nature of the continuous component in electron spectra emitted in collisions has been determined. It has been shown that the excitation of autoionization states determines the stopping cross sections for keV atoms in matter.

Line Identification and Excitation of Autoionizing States in a Late-type, Low-mass Wolf–Rayet Star

Abstract Identifications of a large fraction of previously unidentified lines in the complex spectrum of the low mass, late-type LMC [WC11] star J060819.93−715737.4 have been made utilizing electronic databases. There are an exceptionally large number of C ii emission lines originating from autoionizing (AI) levels. Resonance fluorescence between the C ii ground state and excited AI levels is shown to be an important photoabsorption process that is competitive with dielectronic recombination in exciting AI emission lines in stellar winds, and has broad application to many types of emission-line stars. In addition, numerous C ii quartet multiplets appear in emission that are not excited directly by recombination or resonance fluorescence, signifying high wind densities in the emission region that enhance collisional transfer between doublet and quartet states.

Contribution of Autoionization Processes to the Electronic Stopping Power

It is shown that the excitation of autoionization states during the collisions of keV-energy ions with a solid plays a determining role in the formation of inelastic energy losses and, accordingly, the electronic-stopping cross section Se. It is proposed to estimate the Se values using the relationship between the cross section for autoionization-state excitation and the ionization cross section. For cases where the ionization cross sections are unknown, scaling is used to calculate the ionization cross sections when the L and M shells are excited. The threshold dependence of Se versus the energy of bombarding ions is predicted.

Energy levels, oscillator strengths, radiative transition probabilities, level lifetimes and electron-impact excitation rate coefficients for Ne-like Mo XXXIII

The energy levels, oscillator strengths, radiative decay rates, lifetimes, collision strengths, direct and resonance electron-impact excitation rate coefficients have been computed for the 257 fine-structure levels arising from 1s22s22p5nl and 1s22s2p6n′l′ configurations belonging to the Mo32+ ion with n≤7, l≤4 and n′≤5, l′≤4. The model-potential approach is used for the target ion structure calculations. Additionally, we employ the multiconfiguration Dirac–Hartree–Fock method to further assess the energy levels and transition probabilities. The collision strengths for the electron-impact direct excitation are computed within the relativistic distorted-wave approximation at 34, 135, 680, 1700, 5436, and 12,740 eV scattered electron energy values. We also perform collision calculations at 85, 175, and 450 keV electron energies using the plane-wave approximation and interpolate the reduced cross section within Fano plots, thus accounting for relativistic effects at asymptotic energies. This assures the convergence of Maxwellian integration for effective collision strengths calculation at electron temperatures up to 80 keV. The resonance contribution to the excitation rates is accounted for within the independent process isolated resonance approximation by including Na-like Mo31+ doubly excited autoionization states arising from the 1s227nln′l′ configurations with n≤7,l≤4,n′≤20,l′≤8. Contributions from high n′≥21 Rydberg states to the excitation rate coefficients are included by employing the n′−3 extrapolation law for radiative and autoionization decay rates. Radiative decay of resonances to lower autoionization states followed by autoionization cascade as well as radiative damping via core transitions are included in our model. The highest rate coefficients corresponding to ground state excitations are the 2p–3d allowed and 2p–3p electric monopole transitions, respectively. Intercombination and generally higher-order electric multipole transitions amount to lower excitation rate coefficients but are not to be neglected. Magnetic transitions are only relevant for low electron temperatures since both their direct and resonance contributions to excitation significantly decrease with increasing electron energy. Present results compare well with existing data from literature. The resonance contributions play an important role in the accuracy of rate coefficients, especially for weak forbidden transitions at low electron temperatures. These results may be useful in fusion related plasmas, astrophysics and fundamental physics.

KLL Dielectronic Satellite Spectra from the Photorecombination of He-like Fe and Ni Ions

Abstract The radiative decay of a doubly excited autoionizing state is characterized by the dielectronic recombination in the photorecombination (PR) process. We calculate PR cross sections for the ground states of the Fe xxv and Ni xxvii ions by employing the Dirac R-matrix technique. The autoionizing states are treated in the framework of Davies and Seaton’s theory to account for the radiative decay. The strengths of the KLL resonances that equate directly to the dielectronic satellite line strengths are evaluated. Besides, the Multiconfiguration Dirac Fock formalism is utilized to obtain the dielectronic satellite line strengths under the isolated resonance approximation. A very good agreement is found for the wavelengths and transition parameters belonging to the KLL autoionizing states with the previous theoretical and experimental results. The dominant KLL dielectronic satellite line strengths from both theoretical models are consistent with each other and are in good agreement with the established theoretical models and measurements at the electron beam ion trap.

Theoretical study of 4p5nln'l'LSJstates of Sr ion excited by electron impact on Sr atom

Calculations of energy levels and electron-impact excitation cross sections of the Auger autoionizing states 4p5nln'l'(nl= 4d, 5s, 5p; n'l'= 4d, 5s, 5p, 5d, 6s, 6p, 6d, 7s, 7p) of Sr ion were performed. A large-scale configuration interaction method on the basis of the solutions of Dirac–Fock–Slater equations was used. The cross sections of electron-impact simultaneous ionization and excitation, and Auger decay of the electron-impact excited states of Sr atom to the 4p-core excited autoionizing states of Sr ion were calculated for the first time and used to estimate the intensity of ejected electron lines. Tentative identification of the Auger electron lines of Sr ion registered in a number of experiments is presented.

Excitation of Autoionization States and Electronic Stopping Powers at Collisions of Slow Ions with a Solid

It has been shown that the excitation of autoionization states at collisions of keV ions with a solid is decisive for inelastic energy loss and, correspondingly, the electronic stopping power dE/dx. It has been proposed to estimate the electronic stopping power dE/dx using the relation of cross sections for the excitation of autoionization states to ionization cross sections. When ionization cross sections are unknown, scaling is used to calculate ionization cross sections at the excitation of the L and M shells. A threshold dependence of the electronic stopping power dE/dx on the energy of bombarding ions has been predicted.

Low-energy photodetachment ofGa−and elastic electron scattering from neutral Ga

We present a comprehensive study of the photodetachment of the negative gallium ion and elastic electron scattering from neutral Ga for photon and electron energies ranging from threshold to 12 eV. The calculations are carried out with the $B$-spline $R$-matrix method. A multiconfiguration Hartree-Fock method with nonorthogonal term-dependent orbitals is employed to generate accurate initial- and final-state wave functions. The close-coupling expansions include the $4s{\phantom{\rule{0.16em}{0ex}}}^{2}4pnl(kl)$ bound and continuum states of Ga and the $4s$-excited autoionizing states $4s4{p}^{2}$. The calculated photodetachment and elastic cross sections exhibit prominent resonance features. In order to clarify the origin of these resonances, the contributions of the major ionization channels to the partial cross sections are analyzed in detail.

Two-photon double ionization of metastable1,3S1s2shelium

We investigate the process of two-photon double ionization (TPDI) of the metastable helium ${}^{1,3}S$ $1s2s$ states. The process has been simulated within a fully $ab$ $initio$ numerical framework, solving the time-dependent Schr\"odinger equation in full dimensionality for the two interacting electrons, in a $B$-spline-based methodology. The presence of doubly excited (autoionizing) states in the direct TPDI regime causes resonance-enhanced multiphoton ionization, and we demonstrate this effect by accessing the ${}^{1,3}P$ $2s2p$ doubly excited states. Fully converged theoretical calculations are presented, and a well-defined cross section is extracted for the direct TPDI process and compared in the context of its analogous process in the helium ground state. In addition, the resonance-enhanced two-photon double-ionization mechanism is explored, and we discuss how this process differs from both direct and sequential ionization processes.

Strong asymmetry of the angular distribution of Auger electrons from electron-impact excited autoionizing states of Rb

The asymmetry parameters of the angular distribution of Auger electrons for a number of electron-impact excited states were calculated in single configuration intermediate coupling approximation. Strong dependence on the energy of exciting electrons was obtained for the angular distribution of the emitted Auger electrons from the states with J > 1/2.

Laser-induced breakup of helium3S1s2swith intermediate doubly excited states

Solving the time-dependent Schr\odinger equation in full dimensionality for two electrons, it is found that in the XUV regime the two-photon double ionization dynamics of He($1s2s$) is predominantly dictated by the process of resonance enhanced multiphoton ionization via doubly excited states (DESs). We have studied a pump-probe scenario where the full laser-driven breakup of the ${}^{3}S$ $1s2s$ metastable state is dominated by intermediate quasiresonant excitation to doubly excited (autoionizing) states in the ${}^{3}{P}^{o}$ series. Clear evidence of multipath interference effects is revealed in the resulting angular distributions of the ejected electrons in cases where more than one intermediate DES is populated in the process.

Enhanced asymmetry in few-cycle attosecond pulse ionization of He in the vicinity of autoionizing resonances

By solving the two-active-electron, time-dependent Schrödinger equation in its full dimensionality, we investigate the carrier-envelope phase (CEP) dependence of single ionization of He to the He+(1s) state triggered by an intense few-cycle attosecond pulse with carrier frequency ω corresponding to the energy ℏω = 36 eV. Effects of electron correlations are probed by comparing projections of the final state of the two-electron wave packet onto field-free highly correlated Jacobi matrix wave functions with projections onto uncorrelated Coulomb wave functions. Significant differences are found in the vicinity of autoionizing resonances. Owing to the broad bandwidths of our 115 and 230 as pulses and their high intensities (1–2 PW cm−2), asymmetries are found in the differential probability for ionization of electrons parallel and antiparallel to the linear polarization axis of the laser pulse. These asymmetries stem from interference of the one- and two-photon ionization amplitudes for producing electrons with the same momentum along the linear polarization axis. Whereas these asymmetries generally decrease with increasing ionized electron kinetic energy, we find a large enhancement of the asymmetry in the vicinity of two-electron doubly excited (autoionizing) states on an energy scale comparable to the widths of the autoionizing states. The CEP dependence of the energy-integrated asymmetry agrees very well with the predictions of time-dependent perturbation theory (Pronin et al 2009 Phys. Rev. A 80 063403).

The decay of autoionizing states in electric field formed during collisions of multiply charged ions with metal surface

When a multicharged ion approaches the metal surface, a strong electric field occurs due to interaction between the ion and the charge induced in the metal. During ion-surface scattering, double excited autoionizing states can be formed, and their decay occurs in the presence of strong electric field. The Auger transition rates for such states are higher than those in the absence of the electric field; this is connected with the increase in the wave functions overlap and also with mixing of states with different orbital momentums. This effect changes our view on the population scheme of states with the lowest principal quantum numbers for the collisions considered.

Transitions from the 3p63d ground state of potassium-like vanadium

The three lowest ions in the K-like sequence, Ca II, Sc III and Ti IV have excited states 3p53d2 and 3p53d 4s which lie above the 3p6 ionization threshold. These excited (resonance or autoionizing) states increase greatly the cross sections for electron and photon ionization of the ground state of the ion. As the charge of the ion increases the number of resonance states of the form 3p53d2 or 3p53d 4s which lie above the 3p6 ionization threshold decreases. At present there is no experimental evidence for the existence of these resonances in V V, the next ion in the K-like sequence. Theoretical calculations (Tiwary et al 1983 J. Phys. B: At. Mol. Phys. 16 2457) suggest that there is a [3d 1P4s] 2P resonance in V V at an excitation energy of 2.81 au above the 3p63d ground state. This paper presents an ab initio calculation of the transition energies and A-values between individual J levels of the ground 3p63d state and the J levels of the 3p6np, 3p6nf and the 3p53d2 and 3p53d 4s excited states of V V. These calculations indicate that there is a resonance multiplet [3d 1P 4s] 2P at an excitation energy of 2.73 au which is 0.33 au (9.0 eV) above the 3p 6 ionization threshold.

Resonances at collisions of slow electrons with cadmium atoms and ions

Using the crossed-beam method and a hypocycloidal electron spectrometer, the energy dependence of the ionization cross section for the cadmium atom has been studied in the near-threshold region and the elastic scattering of slow electrons at an angle close to 180° by Cd+ ions was studied for the first time. Within the region under study (0–7 eV above the first atomic ionization potential), a resonance structure determined by the contribution of atomic autoionization states is revealed in both the ionization curve and differential cross section of elastic scattering. The structure in the measured curves has been analyzed with the use of data on the ejected-electron spectra obtained under the excitation of autoionization states of Cd atoms, as well as the data on the optical excitation functions for the atomic spectral lines at λ=430.7 nm (51 P 1-81 S 0), 515.5 nm (51 P 1-71 S 0), 298.0 nm (53 P 2-63 D j ), and 361.0 nm (53 P 2-53 D j ).

One- and two-electron numerical models of multiphoton ionization of helium

We discuss the calculation of ionization rates of helium using time-dependent solutions of the full-dimensional Schrödinger equation in conjunction with time-dependent solutions of a single active electron (SAE) model of helium. The SAE model is a one-electron atom with a non-Coulombic effective potential that can be tuned in certain limits to give near quantitative agreement with the results of the full-dimensional integration. We show how the tuned SAE model can be used to improve the accuracy and reliability of the calculations of ionization cross sections. In addition we consider a case in which failure of the SAE model can be attributed to strong correlation effects in laser-driven helium, specifically interaction with an intermediate doubly excited (autoionizing) state of helium.

Electron-impact excitation cross sections of K*(3p54s2 2P3/2, 2P1/2) autoionizing states: strong fine-structure dependence near threshold

The electron-impact cross sections of the inner-shell excited autoionizing states K*(3p54s2 2P3/2) and K*(3p54s2 2P1/2) were measured for the incident energy ranges from threshold up to 1 keV and up to 37.2 eV, respectively. A strong negative-ion resonance structure was found in the excitation cross section of the 2P3/2 state close to the threshold, whereas the excitation function of the 2P1/2 state is rather smooth. For impact energies below 40 eV the cross section ratio R of the fine-structure components shows a substantial energy-dependent deviation from the statistical value R = 2. The present experimental values are compared with recent theoretical results obtained in the distorted-wave Born approximation.

The effect of electronic autoionization on vibrational branching ratio in (1 + 1')-photon resonance-enhanced multiphoton ionization of molecules: dependence on laser intensity

We have studied the laser intensity effect on the vibrational branching ratio (VBR) in -photon resonance-enhanced two-photon ionization of molecules, considering the electronic autoionization through the lowest autoionizing (AI) state of symmetry. In this ab initio calculation we have studied resonant two-photon transitions from the ground state to the doubly excited autoionizing state of symmetry as well as to the electronic continuum (:) of molecules via the resonant intermediate levels. Transitions to two different continuum energies (-0.4500 au and -0.5853 au) have been considered; the first one is 1.36 eV above the dissociation threshold (-0.50 au) of the ion and the second one is 0.048 eV above the vibrational level of the ion in the ground state . Parallel transitions through other intermediate near-resonant rovibrational levels v = 0-3 and 5-20 with the allowed values of total angular momentum j of the state have been included in the calculation. Besides the effect of interference of direct ionization with autoionization channels, we have considered here (i) the effects of mutual interference among parallel autoionization channels and that among parallel ionization channels via the near-resonant and resonant rovibrational levels of the state and (ii) the effect of the two-photon coupling between intermediate rovibrational levels via the autoionizing state and the continuum in order to study the laser intensity dependence of vibrational distribution of ions in the ground state. It has been shown that these two effects (channel interference effect and two-photon coupling effect) lead to non-Franck-Condon type VBR and the values differ from those obtained by considering the direct ionization channels alone. The laser intensity dependence of VBR is also different from that obtained by neglecting the autoionization channels. For transitions to two different continuum energies these effects become prominent at two different laser intensities. Furthermore, it has been found that the transitions through different intermediate resonant rotational levels of the state give rise to different vibrational branching ratios.

On amplification of radiation emitted from the excited autoionizing state of the H2 molecule in the presence of incoherent pumping to the lowest autoionizing state

We have shown that the amplification without population inversion (AWOPI) from the first excited autoionizing (AI) state of the1Σg symmetry of the H2 molecules can be obtained when the lowest AI state of the1Σg symmetry is pumped incoherently. We have considered a resonant two-photon transition scheme from the groundX1Σg(v = 0;j = 1) state to the ionization continuum embracing the two AI states (the lowest and the first excited autoionizing states) of the1Σg symmetry via the resonant intermediateB1Σu(v = 4;j = 2) level. The effect of two-photon near-resonant autoionizing channels via the near-resonant rovibrational levels of theB1Σu state has also been considered. We have found that the gain can be obtained in different spectral regions corresponding to the resonances with the lowest and first excited AI states from the intermediate levels of theB1Σu state. The inherent coherence between two AI states due to the configuration interaction coupling via the common continuum has been found to have a significant effect on the amplification process, which occurs around all the resonance frequencies connecting this AI state with the intermediate levels. This gain can be significantly modified by increasing the coupling strength of this excited AI state with the continuum. Moreover, the gain from the excited AI state remains unaffected by the damping caused by incoherent pumping to the lowest AI state. It has also been shown that the persistence of inversionless gain from the excited AI state is shorter than that from the lowest AI state. The presence of close-lying near-resonant rovibrational levels leads to the increase in gain around the resonances with two AI states from theB1Σu (v = 4;j = 2) level. The advantages of this type of scheme are: i) AWOPI can be obtained from an excited AI state although it has not been pumped and ii) the amplification obtained from the excited AI state is almost unaffected by the distortion caused by incoherent pumping to the lower AI state.

Resonance yields and excitation cross sections for atomic ionization

A physically correct introduction of a cross section for the excitation of autoionization states into the theory of resonant ionization is considered and the relation of this quantity to measurable values is discussed. Excitation cross sections have been shown to differ from resonance yields even for isolated resonances without any post-collision interactions, so that this difference might be used to study the interplay of various atomic interactions in ionization processes. Numerical results for helium ionization by charged-particle impact in a broad range of velocities are presented in support of the conclusions made. It has been found that the resonance yield can provide a lower estimate for an excitation cross section for asymptotic collision velocities.