Shake-off Mechanism Research Articles

Dehydrogenation in Electron-Induced Dissociative Ionization of Pyridine Molecule

"The electron-impact dissociative ionization of pyridine has been investigated using mass spectrometry. Thirty-two well-resolved mass peaks have been identified in the cation mass spectra and assigned to the most likely ionic molecular fragments. The new sixteen ionic fragments' appearance energies have been determined, and sixteen others remeasured. The total cross-section for electron-impact ionization of pyridine has been measured at 100 eV. Thorough analysis indicates that at least three peaks' groups in the mass spectra occur via the shake-off dehydrogenation mechanism, i.e., by sequential hydrogen atoms elimination from the pyridine parent cation or its fragmentation products. The nature of this process is deciphered. "

Multiple Auger decay following ionization-excitation of Ne i

The photoionization-excitation spectrum of Ne i, in which an electron in the $1s$ shell is ionized and the $2p$ orbital electron simultaneously excited to the $np$ orbital, is investigated in detail for complex satellite states ${\mathrm{Ne}}^{+}$ $1{s}^{\text{--}1}2{p}^{\text{--}1}{(}^{1,3}P)np$ $(n=3,4,5,6,7)$. Then, the multiple Auger decay process is studied using the multistep method, including cascade, knock-out, and shake-off mechanisms, based on perturbation theory for the satellite states ${\mathrm{Ne}}^{+}$ $1{s}^{\text{--}1}2{p}^{\text{--}1}({}^{1,3}\mathrm{P})np$ $(n=3,4)$ with two holes. The major single Auger decay can be described as a spectator process that valence electrons remain in the original orbitals. For double Auger decay, direct and cascade processes are considered, in which the direct mechanism is dominated by the spectator process, while the cascade mechanism is mainly from the participator process. For triple Auger decay, the important contribution is from the process consisting of direct double Auger and the subsequent single Auger electron emissions. The calculated ion yields of ${\mathrm{Ne}}^{2+}$, ${\mathrm{Ne}}^{3+}$, and ${\mathrm{Ne}}^{4+}$ for multiple Auger process of satellite states ${\mathrm{Ne}}^{+}$ $1{s}^{\text{--}1}2{p}^{\text{--}1}{(}^{1,3}P)np$ $(n=3,4)$ are consistent with experimental values [Hikosaka et al., Phys. Rev. A 97, 023405 (2018)].

Quasifree Photoionization under the Reaction Microscope

We experimentally investigated the quasifree mechanism (QFM) in one-photon double ionization of He and H2 at 800 eV photon energy and circular polarization with a COLTRIMS reaction microscope. Our work provides new insight into this elusive photoionization mechanism that was predicted by Miron Amusia more than four decades ago. We found the distinct four-fold symmetry in the angular emission pattern of QFM electrons from H2 double ionization that has previously only been observed for He. Furthermore, we provide experimental evidence that the photon momentum is not imparted onto the center of mass in quasifree photoionization, which is in contrast to the situation in single ionization and in double ionization mediated by the shake-off and knock-out mechanisms. This finding is substantiated by numerical results obtained by solving the system’s full-dimensional time-dependent Schrödinger equation beyond the dipole approximation.

Competition between the shake-off and knockout mechanisms in the double and triple photoionization of the halothane molecule (C2HBrClF3).

The ratios of single, double, and triple ionizations to the total photoionization of the halothane (C2HBrClF3) molecule have been investigated by a single-photon ionization in the energy range from 21.21 eV to 320 eV. In the valence region, the multiple ionization results can be described by a sum of contributions generated from the shake-off and the two-step one models. At low photon energies (from the threshold of triple ionization up to 100 eV), the triple photoionization dynamics of halothane can be reasonably well described by a model involving a classical electron impact double ionization of the singly ionized parent ion.

Dominance of double processes in complete Auger decay of Rb+(3d−1)

The complete Auger decay of ${\mathrm{Rb}}^{+}(3{d}^{\ensuremath{-}1})$ including single and double processes is investigated using the distorted wave approximation. The direct double Auger decay was calculated by separating the knock-out and shake-off mechanisms. For single Auger decay of the $3{d}^{\ensuremath{-}1}$ hole state, it is generally believed that the dominant channels should be levels belonging to the configurations of $[1{s}^{2}2{s}^{2}2{p}^{6}3{s}^{2}3{p}^{6}3{d}^{10}]4{s}^{2}4{p}^{4}5s$, $4s4{p}^{5}5s$, and $4{s}^{0}4{p}^{6}5s$. Here we predict that the strongest channels originate from $4{s}^{2}4{p}^{3}4d5s$, which accounts for 36.3% of the single Auger decay rate. The levels belonging to this configuration can further decay to ${\mathrm{Rb}}^{3+}$, resulting in a large fraction of cascade double Auger decay. Moreover, the probability of direct double Auger decay in Rb is also high because of its $5s$ electron. The branching ratios of cascade and direct double Auger decay are predicted to be 50.60% and 22.13%, respectively, resulting in the dominance of double Auger decay with a fraction of 72.73% in the complete Auger decay of ${\mathrm{Rb}}^{+}(3{d}^{\ensuremath{-}1})$.

Highly efficient double ionization of mixed alkali dimers by intermolecular Coulombic decay

As opposed to purely molecular systems where electron dynamics proceed only through intramolecular processes, weakly bound complexes such as He droplets offer an environment where local excitations can interact with neighbouring embedded molecules leading to new intermolecular relaxation mechanisms. Here, we report on a new decay mechanism leading to the double ionization of alkali dimers attached to He droplets by intermolecular energy transfer. From the electron spectra, the process is similar to the well-known shake-off mechanism observed in double Auger decay and single-photon double ionization1,2, however, in this case, the process is dominant, occurring with efficiencies equal to, or greater than, single ionization by energy transfer. Although an alkali dimer attached to a He droplet is a model case, the decay mechanism is relevant for any system where the excitation energy of one constituent exceeds the double ionization potential of another neighbouring molecule. The process is, in particular, relevant for biological systems, where radicals and slow electrons are known to cause radiation damage3. Using alkali metal dimers attached to helium droplets, a new decay mechanism for intermolecular Coulombic decay is demonstrated. The process leads to previously unresolved double ionization for excitation energies exceeding double ionization energies.

Single and double Auger decay of 4f-ionized mercury including cascade and direct processes**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0402300), the National Natural Science Foundation of China (Grant Nos. 11774344 and 11474033), and the Joint Foundation of the National Natural Science Foundation and the China Academy of Engineering Physics (Grant No. U1330117).

Single (SA) and double (DA) Auger decay including cascade and direct processes are investigated for Hg 4f−1 with multiconfiguration Dirac–Fock method and two-step approaches, i.e., knockout and shakeoff mechanisms. Due to the computational effort, only the major transitions are considered to describe the SA and DA decays for the Hg+ ions with complex electronic configurations. In order to estimate the Auger transition energies and amplitudes, the reference configuration sets producing the configuration state functions are carefully chosen for balancing electron correlations among the successive singly, doubly and triply ionized mercury. The Auger rates and electron spectra, DA probabilities as well as the populations of the final Hg3+ states are obtained. Our results well explain the recent experimental data about the 4f hole states of Hg [Palaudoux J et al., Phys. Rev. A 91 012513 (2015)], and could provide guidance for further studies on complex atoms. Particularly for the DA decay, the contributions of the direct processes, which are neglected in their calculations, are found to be important, accounting for as high as about 38% and 34% of the total DA decays for the and , respectively.

Auger decay including direct double processes of K-shell hollow states of Ne+ and the related hypersatellite radiative transitions

In this study, we theoretically investigated single and double Auger decay processes from the K−2V (V = 3s, 3p, 3d, 4s, 4p, 4d) hollow states of Ne+ in the framework of the first- and second-order perturbation theory implemented by the distorted wave approximation. The direct double Auger decay rates were calculated based on the separation of knock-out and shake-off mechanisms. Atomic data, including the transition energy, single and double Auger decay rates and natural lifetime width, are obtained and compared with the experimental and theoretical results available in the literature. The natural lifetime widths of the K−2V hollow states, including the contributions from single and direct double Auger decay, are in excellent agreement with a recent experiment. Using our theoretical results, we diagnosed the relative population fractions of the initial quantum states prepared in the experiment and interpreted the measured Auger spectra. There is an excellent agreement between the theoretical and experimental results. Finally, the hypersatellite Kα radiative transitions relevant to these hollow states were investigated using the natural lifetime widths determined by single and direct double Auger rates.

A practical theoretical formalism for atomic multielectron processes: direct multiple ionization by a single auger decay or by impact of a single electron or photon

Multiple electron processes occur widely in atoms, molecules, clusters, and condensed matters when they are interacting with energetic particles or intense laser fields. Direct multielectron processes (DMEP) are the most complicated among the general multiple electron processes and are the most difficult to describe theoretically. In this work, a unified and accurate theoretical formalism is proposed on the DMEP of atoms including the multiple auger decay and multiple ionization by an impact of a single electron or a single photon based on the atomic collision theory described by a correlated many-body Green's function. Such a practical treatment is made possible by taking consideration of the different coherence features of the atoms (matter waves) in the initial and final states. We first explain how the coherence characteristics of the ejected continuum electrons is largely destructed, by taking the electron impact direct double ionization process as an example. The direct double ionization process is completely different from the single ionization where the complete interference can be maintained. The detailed expressions are obtained for the energy correlations among the continuum electrons and energy resolved differential and integral cross sections according to the separation of knock-out (KO) and shake-off (SO) mechanisms for the electron impact direct double ionization, direct double and triple auger decay, and double and triple photoionization (TPI) processes. Extension to higher order DMEP than triple ionization is straight forward by adding contributions of the following KO and SO processes. The approach is applied to investigate the electron impact double ionization processes of C+, N+, and O+, the direct double and triple auger decay of the K-shell excited states of C+ and , and the double and TPI of lithium. Comparisons with the experimental and other theoretical investigations wherever available in the literature show that our theoretical formalism is accurate and effective in treating the atomic multielectron processes.

Multiple Auger decay probabilities of neon from the 1s -core-hole state

The multiple Auger decays of the $\mathrm{Ne}\phantom{\rule{0.28em}{0ex}}1{s}^{\ensuremath{-}1}$ state including double and triple Auger processes are investigated within the framework of perturbation theory. The contributions of the cascade and direct processes are determined for the double Auger decay. In the cascade processes, the choice of the orbital sets and configuration interaction can strongly affect the partial probabilities for the specific configurations of $\mathrm{N}{\mathrm{e}}^{3+}$. The multistep approaches, i.e., the knockout and shakeoff mechanisms, are implemented to deal with the direct double Auger processes for which the total and partial probabilities corresponding to specific configurations of $\mathrm{N}{\mathrm{e}}^{3+}$ are calculated and reveal that the knockout is dominant. Finally, the probabilities of the triple Auger decays that are decomposed into a double Auger process and a subsequent emission of a single electron are obtained using the cascade and knockout mechanisms. The calculated probabilities agree reasonably well with the available experimental data.

Relativistic effects in double ionization of helium via Compton scattering

In this article we present the relativistic calculations, based on the QED theory, for double ionization of helium by the Compton scattering. In particular, we calculate the contribution of the spin–flip amplitude to the total cross section. Due to this amplitude the final triplet spin state of the ejected electrons is possible. In the calculations based on the non–relativistic A2 term of the electron–photon interaction only the singlet spin state for the final electrons is allowed. We further assume the shake-off mechanism for process of double ionization. For the ground state of helium we use both the non–correlated and highly correlated wave function. We also discuss a degree of the scattered photon polarization in correlation with the formation of spin triplet state. Our calculations cover the photon impact energy range from 150 to 1000keV.

Single, double, and triple Auger decay probabilities ofC+(1s2s22p22D,2P)resonances

Single, double, and triple Auger decay rates of ${\mathrm{C}}^{+}(1s2{s}^{2}2{p}^{2}{\phantom{\rule{0.16em}{0ex}}}^{2}D,{\phantom{\rule{0.16em}{0ex}}}^{2}P)$ resonances were calculated in the framework of perturbation theory. The direct double Auger decay probabilities were calculated by using the approximate formulas according to the knockout and shakeoff mechanisms, in which the knockout mechanism was found to be dominant. Then the knockout mechanism was employed to investigate the complex triple Auger decay process, and the calculated rates have good agreement with the available experimental values.

Single-photon double and triple ionization of acetaldehyde (ethanal) studied by multi-electron coincidence spectroscopy

Single-photon multiple ionization processes of acetaldehyde (ethanal) have been experimentally investigated by utilizing a multi-particle coincidence technique based on the time-of-flight magnetic bottle principle, in combination with either a synchrotron radiation source or a pulsed helium discharge lamp. The processes investigated include double and triple ionization in the valence region as well as single and double Auger decay of core-ionized acetaldehyde. The latter are studied site-selectively for chemically different carbon core vacancies, scrutinizing early theoretical predictions specifically made for the case of acetaldehyde. Moreover, Auger processes in shake-up and core-valence ionized states are investigated. In the cases where the processes involve simultaneous emission of two electrons, the distributions of the energy sharing are presented, emphasizing either the knock-out or shake-off mechanism.

2D momentum distribution of electron in transfer ionization of helium atom by fast proton

2D distribution of momentum components of the ejected electron in the reaction H+ +He → H+He2++e at 630 keV proton is studied both theoretically and experimentally. This allows to unambiguously identify contributions from the shake-off and binary encounter mechanisms of transfer ionization. It is shown that the results are highly sensitive to the quality of the initial-state wave function.

Multielectron processes in photoionization of endohedral atoms at high photon energies

We calculate the probability of inelastic processes in the fullerene shell of an endohedral atom which follow the ionization of the caged atom by a high-energy photon. We demonstrate that the probability is close to unity in a large interval of the photoelectron energies. The inelastic processes are determined mainly by the final-state interactions, which have to be included beyond the perturbative approach. At photoelectron energies $E$ of the order of several keV the probability drops as $1/E$. It is determined by the final-state interactions, which can be treated perturbatively. At still larger energies the probability is determined by the shake-off mechanism and does not depend on energy $E$. A model for calculating the shake-off contribution is also suggested. The actual calculations are carried out for the endohedral atoms $A@{\text{C}}_{60}$ and $A@{\text{C}}_{20}$.

Level-to-level and total probability for Auger decay including direct double processes of Ar2p−1hole states

Auger decay including direct double processes of Ar $2{p}^{\ensuremath{-}1}$ hole levels is investigated in the framework of perturbation theory implemented by distorted wave approximation with balanced large-scale configuration interaction among the successive ions being taken into account. The complex transition amplitude obtained from the second perturbation theory for the direct double Auger decay (DAD) is decomposed into approximate formulas according to two generally agreed mechanisms of shake-off and knock-out. Practical computations showed that the knock-out condition is fulfilled and thus justified such a decomposition treatment of knock-out mechanism. The contribution to the DAD probability from knock-out mechanism is larger than shake-off by an order of magnitude and therefore the former is dominant. The interference effect between the knock-out and shake-off mechanisms should be trivial and neglecting it we obtained a branching ratio of 12.0$%$ for the direct DAD into triply charged states of Ar${}^{3+}$. By including cascade double decay, a total branching ratio of 14.9$%$ is obtained. Our result explained recent experimental results on the branching ratio into triply charged ion, and should be useful for further detailed experimental investigations.

Transfer ionization and its sensitivity to the ground-state wave function

We present kinematically complete theoretical calculations and experiments for transfer ionization in H${}^{+}+\phantom{\rule{0.16em}{0ex}}$He collisions at 630 keV/u. Experiment and theory are compared on the most detailed level of fully differential cross sections in the momentum space. This allows us to unambiguously identify contributions from the shake-off and binary encounter mechanisms of the reaction. It is shown that the simultaneous electron transfer and ionization is highly sensitive to the quality of a trial initial-state wave function.

Double and transfer ioinization in collisions of He with bare ions

Double and transfer ionization processes are theoretically investigated for collisions of He atoms with bare energetic projectile ions. Two-electron transition amplitudes are described by linear combinations of products of one-electron transition amplitudes, which enables one to incorporate the effects of electron correlation in the asymptotic channels. A collision mechanism in which the projectile interacts separately with both electrons gives a realistic account of the processes only at medium impact energies, while the final state correlation and the shake-off mechanism together with correlations in the initial state are found to be important for the lower and higher collision energies, respectively.

Probing the mechanism of simultaneous two-electron emission on core-hole decay

Simultaneous two-electron emission upon $2p$ core-electron excitation in Ar has been studied using a state-of-the-art multielectron coincidence method. Simultaneous two-electron emission effectively populates Rydberg-excited ${\text{Ar}}^{2+}$ states, in which the excited electron behaves as a spectator of the direct double Auger decay of the core hole. This observation constitutes experimental evidence that the shake-off mechanism is not sufficient to model direct double Auger decay of a $2p$ hole in Ar and suggests an important contribution from the knock-out mechanism.

Perturbative calculation of the cross section in double ionization by high-energy Compton scattering

In this paper we investigate double ionization in high-energy Compton scattering from the He-atom including both the shake-off mechanism and a perturbative correction to that mechanism. The correction is calculated in second-order perturbation theory and includes Coulomb electron-electron interaction in addition to the correlation in the ground state of the He-atom. Our calculations for the ratio of double to single cross section cover the range from 30 to 300 keV of impact photon energy and explain the slow convergence of the ratio towards the asymptotic value.