Energy Distribution Of Ejected Electrons Research Articles

The probe of electron correlation along the He isoelectronic sequence by intense and short laser pulses

The two-photon double ionization process along the He isoelectronic sequence (He − Ne8+) is investigated by solving the full-dimensional time-dependent Schrödinger equation in an intense laser field. The ionization is triggered from the ground state of each system with photons of energy higher than its second ionization potential, in which case the sequential two-electron ejection is allowed. The results of the total two-photon double ionization probability and the energy distribution of ejected electrons demonstrate the important effect of the nuclear charge Z on the dynamics of the interaction between the two ejected electrons.

Branching ratios of autoionization from Eu 476p1/26d [J] autoionizing states**Project supported by the National Natural Science Foundation of China (Grant No. 11174218).

The autoionization branching ratios from Eu 4f76p1/26d [J] autoionizing states to its 4f76s+ (9So), 4f76s+ (7So), and 4f75d+ (9Do) final ionic states are investigated with the combination of the three-step laser excitation and the velocity-map imaging technique. These different autoionizing states are excited via 4f76s6d 8DJ [J = 5/2, 7/2, and 9/2] intermediate states, respectively. The experimental photoelectron images are obtained, from which energy distributions of ejected electrons are achieved with the mathematical transformation. Furthermore, the energy dependence of the branching ratio is investigated within the autoionization resonance, by which population inversion is observed as an important characteristic. The J-dependence is also studied systematically. The validity of the well-known isolated core excitation technique used for obtaining the autoionization spectrum is also studied.

Stimulated Compton Scattering in Two-Color Ionization of Hydrogen with keV Electromagnetic Fields

We present a theoretical study of two-color ionization of hydrogen with keV photons at intensities ranging from 1016 to 1018 W/cm2. We consider the atom in interaction with a superposition of two electromagnetic pulses centered around two frequencies that differ by a few atomic units and we present in detail the case of the frequencies 55 and 50a.u. We present the electron energy spectra, angular distributions, and ionization rates based on nonperturbative and perturbative calculations. Although the ejected electron energy distribution is dominated by one-photon ionization from each pulse, we are able to identify the contribution of stimulated Compton scattering, a process in which one photon is absorbed while the other is emitted, the photon energy difference being transferred to the electron. This leads to low-energy electrons, and we show in particular that it is of crucial importance to consider the retardation effects on the ionization rates and the electron angular distributions. The relative propagation direction of the two fields also plays an important role; in the case of counterpropagating fields, the ionization by stimulated Compton scattering is dominated by A2 and competes with one-photon ionization at high intensities.

Measurement of Double Differential Cross-Section for Electron Impact Ionization of Argon at 348 eV

Renewed interest in measuring cross-sections for scattering electrons or positrons by noble gas atoms at intermediate energies has recently led to test various theoretical approximations. Ionization of the noble gases such as He, Ne, Ar, Kr, and Xe by electron impact is an important process in collision physics. Argon has been studied from the discharge point of view for more than 100 years and interest in this gas is due to several important fundamental and technical considerations. Cross-sections for electron-impact ionization have been measured and calculated since the early days of collision physics because of their importance to the kinetics and dynamics of collisions in relevance to many practical applications. Previous experimental measurements of double di erential cross-sections (DDCS) by electron impact for targets rather than helium are very few. The most itemized study is that of Opal et al. [1]. They presented DDCS data for several gases at di erent incident energies of a wide angular range. The literature contains the results from several experimental measurements on the double di erential cross-section studies of Ar. The elastic scattering of electrons by noble gases has been extensively studied and a large number of either theoretically or experimentally obtained relative and absolute di erential cross-sections are available at various incident electron energies and scattering angles. The critical minima appearing in Ar DCS spectra are the most sensitive test for comparison of experimental and theoretical results and, hence, give good possibilities for testing di erent theoretical models. DuBois and Rudd [2] measured DDCS for 100 500 eV electron impact of Ar including Ne, N2, and H2 data. Hippler et al. [3] and Chaudhry et al. [4] reported energy distributions of ejected electrons in double or (multiple) ionization of rare gases 300 eV to 10 keV electron impact. Another work is done by Santos et al. [5] at 500 750 and 1000 eV electron impact single or multiple ionization DDCSs of Ar. Yates and Khakoo [6] have presented DDCSs of Ar for near threshold electron impact single ionization of Ar at 17, 18, 20, and 30 eV. In this work, we used Ar target as a typical that is reasonable to expect to understand ionization mechanism of atomic systems. We have measured DDCS for asymmetric geometry for di erent kinematical conditions.

The VMI study on branching ratio decay from Eu 6p1/28s autoionizing state

A velocity-map-imaging (VMI) method is employed to investigate systematically the dynamical process of ejected electrons from autoionizing states of the Eu atom for the first time as far as we know. An atom is excited stepwise from the 4f76s6s8 S7/2 ground state to the 4f76s8s8 P7/2 Rydberg state via the 4f76s6p6 P5/2 intermediate state, then further excited to the 4f76p1/2(J=3)8s and 4f76p1/2(J=4)8s autoionizing states using the three-step isolated-core excitation method. According to the excitation pathways and selection rules, the value of total angular momentum of the autoionizing state can be calculated. The energy conservation and angular momentum parity conservation would enable us to determine the final states during the autoionizing process. The ejected electron, which decays from the autoionizing process, can be focused and imaged by the electron lens and the kinetic energy of it is resolved by the position sensitive detector. By combining velocity-map-imaging method with the mathematical transformation, the ejected electron energy distribution can be obtained, also the branching ratio is confirmed. Simultaneously, by tuning the wavelength of the third laser, the characteristic of the branching ratio following the variation of the photon energy, and the possibility of the population inversion have been discussed.

Ionization of the hydrogen atom by short half-cycle pulses: dependence on the pulse duration

A theoretical study of the ionization of hydrogen atoms by short external half-cycle pulses (HCPs) as a function of the pulse duration, using different quantum and classical approaches, is presented. Total ionization probability and energy distributions of ejected electrons are calculated in the framework of the singly-distorted Coulomb-Volkov (SDCV) and the doubly-distorted Coulomb-Volkov (DDCV) approximations. We also performed quasiclassical calculations based on a classical trajectory Monte Carlo method which includes the possibility of tunneling (CTMC-T). Quantum and classical results are compared to the numerical solution of the time-dependent Schrodinger equation (TDSE). We find that for high momentum transfers the DDCV shows an improvement compared to the SDCV, especially in the low-energy region of the electron emission spectra, where SDCV fails. In addition, DDCV reproduces successfully the TDSE electron energy distributions at weak momentum transfers. CTMC-T results reveal the importance of tunneling in the ionization process for relative long pulses and strong momentum transfers but fails to overcome the well-known classical suppression observed for weak electric fields.

Atomic ionization by ultrashort half-cycle pulses

A theoretical study of the ionization of hydrogen atoms as a result of the interaction with ultrashort external half-cycle pulses is presented. Total ionization probability and energy distributions of ejected electrons are calculated in the framework of the single distorted Coulomb-Volkov (CVA) and the doubly-distorted impulsive Coulomb-Volkov (ICV) approximations. Quantum and classical results are compared to the exact solution of the time dependent Schrödinger equation. We show that the ICV bears a considerable improvement to the CVA energy distribution in the low-energy region, where CVA fails.

Correlated electron detachment inH−−Hecollisions

Experimental and theoretical studies of the energy spectra of electrons ejected in ${\mathrm{H}}^{\ensuremath{-}}\text{\ensuremath{-}}\mathrm{He}$ collisions have been performed. Comparison of calculations using the Sturmian and advanced adiabatic theories with experimental data reveals the existence of a correlated electron detachment mechanism, which yields the main contribution to formation of the high-energy part of the ejected electron energy distribution. This mechanism is associated with dynamical energy transfer to the loosely bound $1{s}^{\ensuremath{'}}$ electron of ${\mathrm{H}}^{\ensuremath{-}}$ in the course of superpromotion of the inner $1s$ electron.

Ejected electron energy distribution in the ionization of atomic hydrogen by C6+ impact

We report doubly differential cross section (DDCS) for C6+ impact ionization of atomic hydrogen at the incident energy of 2.5MeV/amu. The calculation is based on the eikonal approximation (EA) method. A comparison is made of the present DDCS with the results of other theoretical methods and experiment. It is found that the multiple scattering effect has a significant influence on the energy distributions of the ejected electrons. The cross sections predicted by the present EA also show reasonably good agreement with experiment.

Electron Emission in H-–He, H2 Collisions

Experimental and theoretical study of energy spectra of electrons ejected in H-–He, H2 collisions has been performed. Comparison of the calculations using the Sturmian and advanced adiabatic theories with experimental data reveals existence of a new, correlated S-detachment mechanism responsible for production of high-energy part of ejected electron energy distribution. This mechanism is associated with dynamical energy transfer to the loosely bound 1s-electron of H- in the course of superpromotion of the inner 1s-electron.

Development of a new experimental setup for studying collisions of keV-electrons with thick and thin targets

Development of a new lectron-recoil ion/photon coincidence setup for investigating some of the electron induced collision processes, such as electron bremsstrahlung, electron backscattering, innershell excitation and multiple ionization of target atoms/molecules in bombardment of electrons having energies from 2.0 keV to 30.0 keV with solid and gaseous targets is described. The new features include the use of a compact multipurpose scattering chamber, a time-of-flight spectrometer for detection of multiply charged target ions, a 45°-parallel plate electrostatic analyzer for measuring energy and angle of the ejected electrons, a room temperature high resolution Si-PIN photo diode X-ray detector for counting the collisionally induced photons, a coincidence data acquisition system consisting of a 200 MHz Pentium based 8K-multichannel analyzer and a standard network of a fast/slow coincidence electronics. In particular, the details of design, fabrication and assembly of indigenous components employed in the setup are presented. Selected experiments planned with the setup are mentioned and briefly discussed. A report on performance, optimization, efficiency, time resolution etc. of the time-of-flight (TOF) spectrometer and that of the 45°-parallel plate electrostatic analyzer (PPEA) is presented. Test spectra of electron-recoil ion coincidences, energy distribution of ejected electrons and characteristic plus non-characteristic X-ray spectrum are illustrated to exhibit the satisfactory performance of the developed setup.

Coulomb-Volkov approach of atom ionization by intense and ultrashort laser pulses

We present a nonperturbative theoretical approach, based on Coulomb-Volkov-type states, which is able to predict both angular and energy distributions of ejected electrons when atoms interact with a very short and intense laser pulse. In a previous paper [Eur. Phys. J. D 11, 191 (2000)], it was shown that, for atomic hydrogen targets, this theory makes accurate predictions as long as the interaction time does not allow more than two optical cycles. Recently, multigigawatt laser pulses with a full width at half maximum of less than two optical cycles have been generated by Nisoli et al. [Opt. Lett. 22, 522 (1997)] at $\ensuremath{\lambda}=800\mathrm{nm}.$ In the present paper, it is shown that predictions of the Coulomb-Volkov approach for the ionization of a hydrogen atom by laser pulses similar to the ones generated by Nisoli et al. are in very good agreement with the predictions of an ``exact'' numerical treatment. Further, the domain where the Coulomb-Volkov theory applies is marked out by means of a consistent accuracy parameter and by comparison with an ``exact'' numerical treatment. It is shown that, subject to the above-mentioned condition, good predictions may always be issued as long as the interaction time does not exceed half the initial orbital period of the electron. For a given laser pulse duration, predictions are all the better that the laser field amplitude is high and the initial quantum number is large.

A simple non-perturbative approach of atom ionisation by intense and ultra-short laser pulses

A simple theoretical approach based on Coulomb-Volkov states is introduced to predict ionisation of atoms by intense laser pulses in cases where the effective interaction time does not exceed one or two optical cycles [M. Nisoli et al., Opt. Lett. 22, 522 (1997)]. Under these conditions, the energy distributions of ejected electrons predicted by this non-perturbative approach are in very good agreement with “exact" results obtained by a full numerical treatment. The agreement is all the better that the principal quantum number of the initial state is high. For very strong fields, most electrons are ejected at an energy \(\) which is close to the classical kinetic energy that would be transferred to free electrons by the electromagnetic field during the pulse. The power of the present approach appears when \(\) keV. In this region, full numerical treatments become very lengthy and finally do not converge. However, the present Coulomb-Volkov theory still makes reliable predictions in very short computer times.

Exact double-continuum solution of a correlated atomic model

The dynamics of double ionization is of considerable interest as being a striking manifestation of the electron correlation. This is an essentially nonseparable problem for which it is highly desirable to have an analytically soluble model for reference. In this paper, the correlated double-continuum solution of a two-particle atomic model is obtained in closed form. Although particles are one-dimensional and potentials are of -type, this model provides for double photoionization of helium, energy distributions of ejected electrons and total cross sections in qualitative agreement with those of more realistic calculations.

Transfer and excitation in high-energy He+-He collisions: V.Electroniccontinuum influence on ejected electron distributions and TE crosssections

Compared to experimental data based on 0° electron spectroscopy, previous theoretical predictions related to simultaneous transfer and excitation (TE) processes occurring in collisions between fast He+ projectiles and helium targets are not fully satisfactory. In fact, peaks in energy distributions of forward-emitted electrons have always been considered as the result of the production of (bound) doubly excited states followed by post-collisional Auger decay. However, two indistinguishable collision channels, hereafter referred to as TI, can also produce forward-ejected electrons: (a) direct target ionization and (b) simultaneous electron transfer and projectile ionization. They are ignored in these first studies, although experimental electron spectra show asymmetrical profiles. Here the TE process is re-examined within the framework of the continuum distorted wave with four-body (CDW-4B) formalism. Our approach sticks to Fano's description of a resonance, i.e. the adjacent electronic continuum of a doubly excited state produced during the collision is included. Very good agreement is found between the present predictions and the experimental data both for energy distributions of ejected electrons and for so-called total TE cross sections. Further, it is shown that TE cross sections, which are extracted directly from lineshapes, always contain non-separable contributions from the two above-mentioned TI channels.

Energy distribution of relativistic electrons in the tunneling ionization of atoms by super-intense laser radiation

We studied theoretically the influence of relativistic effects on the energy distribution of electrons in the tunneling ionization of atoms by a field of linearly polarized super-intense laser radiation. It was shown that the energy distribution of ejected electrons is determined by relativistic law though the electron kinetic energy can be less than its rest energy. The relativistic probability of ionization along the field strength decreases exponentially with the electron kinetic energy, but more quickly than in the non-relativistic case.

Angular and energy distributions of ejected electrons in the direct and resonance He(e, 2e)He+ reactions

Angular and energy distributions of ejected electrons in the direct and resonance He(e, 2e)He+ reactions are studied. It is shown that the first Born approximation (FBA) can be used to described ejected electron characteristics at incident energies E0>or=500 eV and ejection energies Eb<or=40 eV. It is also shown that the anisotropy of ejected electrons and the predominance of higher-order multipole terms are due to the peculiarities of the angular distributions of double-differential cross sections under different kinematical conditions. Under the conditions of FBA validity the profiles of the overlapping 1P and 1D autoionization resonances (AIR) in the ejection spectra of the He(e, 2e)He+ reaction are investigated by using the resonance structure of the angular coefficients. The resonance structure of the angular coefficients might provide more detailed information on the profiles and interaction mechanism of the 1P and 1D AIR at different multipoles L. Calculations carried out in the concrete structure model are compared with the available experimental data and theoretical estimates.

Deexcitation of helium 2 (3)S, 2 (1)S, and 2 (3)P atoms at Ar and Xe films.

Measurements of ejected electron energy distributions are used in conjunction with electron spin labeling techniques to probe the mechanisms by which He(2 $^{3}$S), He(2 $^{1}$S), and He(2 $^{3}$P) atoms are deexcited at Ar and Xe films adsorbed on a cooled Cu(100) substrate. The data for both surfaces contain features similar to those observed in gas-phase Penning ionization, indicating that ejection results, in part, from Auger deexcitation, i.e., surface Penning ionization. For Xe, however, additional features are observed that can be attributed to resonance ionization of an incident excited atom followed by neutralization of the resulting ${\mathrm{He}}^{+}$ ion through an interaction that involves neighboring Xe atoms in the film. Indeed, the Xe data provide an exceptional example of a surface at which Auger deexcitation and resonance ionization occur in parallel with one another, with a branching ratio that changes significantly as the internal energy of the incident atoms increases. The ejected electron yield from both Ar and Xe films is substantially higher than for clean Cu(100), indicating that such films might form the basis of an efficient thermal-energy helium metastable-atom detector.

Theory of spin-polarized metastable-He deexcitation on a metal surface with an overlayer

In a recent experiment by Hart et al. the spin-polarization and energy distribution of electrons emitted in the interaction between metastable He(2 3S) and Cu with a K overlayer was measured as a function of their kinetic energy. The ejected electron energy distribution is found to be sharply peaked at the highest energies with a spin-polarization around 75%. The purpose of this work is to show the crucial role played by Auger deexcitation and secondary electrons to explain both the energy distribution and the spin-polarization. Using a simple model for Cu with an overlayer our calculation shows a good agreement with the experimental results.

Statistics of ion-induced electron emission from a clean metal surface

With a novel combination of experimental approaches, ion-induced electron emission from clean polycrystalline gold bombarded with (1–5 keV/amu) H+, H+2, and H+3 has been studied by measuring total electron yields, ejected-electron energy distributions, and, in particular, electron emission statistics (ES). Evaluation of ES included corrections for backscattering of electrons from the surface of the applied solid-state electron detector. By comparing ES for atomic and molecular ions of equal impact velocities, a negative molecular effect could be clearly demonstrated. This effect was further investigated by comparing measured ES for molecular ion impact with ES synthesized from measured ES for H+ and H+2 ion impact, respectively. The quality of approximating measured ES by Poisson and Polya statistical distributions was investigated in detail and found to be generally unsatisfactory.