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We present an experimental and theoretical energy- and angle-resolved study on the photoionization dynamics of nonresonant one-color two-photon single-valence ionization of neutral ${\mathrm{N}}_{2}$ molecules. Using 9.3-eV photons produced via high-order harmonic generation and a three-dimensional momentum imaging spectrometer, we detect the photoelectrons and ions produced from one-color two-photon ionization in coincidence. Photoionization of ${\mathrm{N}}_{2}$ populates the $X$ $^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{+}, A$ $^{2}\mathrm{\ensuremath{\Pi}}_{u}$, and $B$ $^{2}\mathrm{\ensuremath{\Sigma}}_{u}^{+}$ ionic states of ${\mathrm{N}}_{2}{}^{+}$, where the photoelectron angular distributions associated with the $X$ $^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{+}$ and $A$ $^{2}\mathrm{\ensuremath{\Pi}}_{u}$ states both vary with changes in photoelectron kinetic energy of only a few hundred meV. We attribute the rapid evolution in the photoelectron angular distributions to the excitation and decay of dipole-forbidden autoionizing resonances that belong to series of different symmetries, all of which are members of the Hopfield series, and compete with the direct two-photon single ionization.

We investigate the two-photon double ionization of atomic magnesium induced by ultrashort pulses. Though the initial and final state symmetries are comparable to the same process in helium, in stark contrast the range of photon energies for which nonsequential ionization is the only open pathway is narrow (less than 1 eV) in magnesium. Thus several sequential ionization pathways feature heavily in these processes. Nonetheless, it is found that for pulse durations between 0.25 and 2.0 fs, the joint angular dependence of the ejected electrons can depend sensitively on pulse length, varying between the strictly back-to-back ejection characteristic of nonsequential ionization to other distributions. The significance of excited-state correlating configurations in representing the initial state of magnesium is discussed in the light of their consequences for the resulting angular distributions at photon energies where sequential ionization can access intermediate states that lie nearby in energy, particularly for longer pulses.

Author(s): Marante, CA; Greenman, L; Trevisan, CS; Rescigno, TN; McCurdy, CW; Lucchese, RR | Abstract: The simple single-channel static-exchange approximation completely ignores correlation between the continuum and molecular ion electrons. In molecular systems with symmetry equivalent atoms, the single-channel approximation can seriously fail in core ionization when using delocalized orbitals to represent the core hole states. We present cross sections and molecular frame photoelectron angular distributions with both localized and delocalized core orbitals in CF4 F (1s) ionization. We show that only a full coupled-channel calculation can recover an accurate description of the physics of inner-shell photoionization when using delocalized orbitals, whereas nearly the same result can be obtained from independent single-channel static-exchange calculations when localized core orbitals are used. A grid-based variational method described here makes such single-channel calculations possible on larger systems without local-exchange approximations. Illustrative calculations on the core ionization of SF6 are presented to illustrate the power of the grid-based method.

A hybrid method that combines Gaussian basis functions typically used in bound-state molecular electronic structure calculations with a grid-based discrete-variable representation with finite elements suitable for a general electronic continuum representation is used to fully describe the double ionization of molecular ${\mathrm{H}}_{2}$ by a single photoabsorption. This work expands the hybrid method, previously applied to single-ionization events, to double photoionization. Constructing the full two-electron operator encoding the electron correlation necessary to doubly ionize the target via the action of a single photon requires all classes of mixed integrals between combinations of the different basis-function types. Comparison of the present results with benchmark theoretical calculations and experimental results shows excellent agreement for both molecular ${\mathrm{H}}_{2}$ and its united-atom limit, atomic helium; the triply differential cross sections that relate the angular distribution and energy sharing of all of the particles in the frame of the molecule are compared. The two-electron results computed using this hybrid basis hint at application of this general descriptive scheme beyond this simplest molecular target towards describing double ionization in more complicated and experimentally relevant molecules.

We present a theoretical study of $K$-shell photodetachment of ${\mathrm{C}}_{2}{}^{\ensuremath{-}}$ and ${\mathrm{C}}_{3}{}^{\ensuremath{-}}$ ions. The calculations were carried out using the complex Kohn variational method. Our study addresses the recent observation that what appears as a single, narrow shape resonance close to threshold in the $K$-shell photodetachment of ${\mathrm{C}}^{\ensuremath{-}}$ splits into several peaks in the longer ${\mathrm{C}}_{n}{}^{\ensuremath{-}}$ carbon anion chains, with the number of peaks increasing with increasing $n$ values. We argue that the number of expected peaks is related to the number of symmetry-equivalent carbon atoms.

Author(s): Bello, RY; Yip, FL; Rescigno, TN; Lucchese, RR; McCurdy, CW | Abstract: By decomposing the initial-state wave function into its unique natural orbital expansion, as defined in the 1950s by Lowdin and used in modern studies of entanglement, we analyze the role of electron correlation in the initial state of an atom or molecule in determining the angular distribution of one-photon double ionization. Final-state correlation of the two ejected electrons is treated completely in numerically accurate calculations as the initial states of He, H-, and H2 are built up from correlating configurations in strict order of decreasing natural orbital occupations. In the two-electron atoms it is found that the initial-state correlation plays a sometimes modest but generally measurable role. In striking contrast, for H2 a large number of correlating configurations in the ground state is often necessary to produce angular distributions even approximately resembling the correct ones. One-photon double photoionization of oriented H2 is found to be particularly sensitive to left-right correlation along the bond.

The dynamics of autoionizing Rydberg states of oxygen are studied using attosecond transient absorption technique, where extreme ultraviolet (XUV) initiates molecular polarization and near infrared (NIR) pulse perturbs its evolution. Transient absorption spectra show positive optical density (OD) change in the case of $ns\sigma_g$ and $nd\pi_g$ autoionizing states of oxygen and negative OD change for $nd\sigma_g$ states. Multiconfiguration time-dependent Hartree-Fock (MCTDHF) calculation are used to simulate the transient absorption spectra and their results agree with experimental observations. The time evolution of superexcited states is probed in electronically and vibrationally resolved fashion and we observe the dependence of decay lifetimes on effective quantum number of the Rydberg series. We model the effect of near-infrared (NIR) perturbation on molecular polarization and find that the laser induced phase shift model agrees with the experimental and MCTDHF results, while the laser induced attenuation model does not. We relate the electron state symmetry dependent sign of the OD change to the Fano parameters of the static absorption lineshapes., Comment: 15 pages, 8 figures

Citation: McCurdy, C. W., Rescigno, T. N., Trevisan, C. S., Lucchese, R. R., Gaire, B., Menssen, A., . . . Weber, T. (2017). Unambiguous observation of F-atom core-hole localization in CF4 through body-frame photoelectron angular distributions. Physical Review A, 95(1). doi:10.1103/PhysRevA.95.011401

The valence-shell double ionization of atomic magnesium is calculated using a grid-based representation of the $3{s}^{2}$ electron configuration in the presence of a fully occupied frozen-core configuration of the remaining ten electrons. Atomic orbitals are constructed from an underlying finite-element discrete variable representation that facilitates accurate representation of the interaction between the inner-shell electrons with those entering the continuum. Length and velocity gauge results are compared with recent theoretical calculations and experimental measurements for the total double-, single-, and triple-differential cross sections, particularly at the photon energy of 55.49 eV for the last one. Comparison between the similar processes of double ionization of the $n{s}^{2}$ atoms helium, beryllium, and magnesium further illuminates the role of valence-shell electron correlation in atomic targets with heliumlike electronic configurations and symmetry.

© 2016 American Physical Society. Ab initio theoretical studies and momentum-imaging experiments are combined to provide a consistent picture of the dynamics of dissociative electron attachment to ammonia through its 5.5- and 10.5-eV resonance channels. The present study clarifies the character and symmetry of the anion states involved and the dynamics that leads to the observed fragment-ion channels, their branching ratios, and angular distributions.

We consider the effect of substituting atomic ligands $X$ with different electronic properties in the carbon-$K$-shell photoionization of the linear $X\mathrm{CC}X$ molecules. We study the cases of lithium, hydrogen, and fluorine as ligands bonded to the carbon atoms. The molecular frame photoelectron angular distributions are computed using the variational complex Kohn technique. The electronic properties of the ligands have direct observable effects on the angular distribution of the emitted carbon core-hole photoelectron. These effects have already been observed experimentally using the cold target recoil ion momentum spectroscopy technique. We propose a simple classical explanation based on the intramolecular electrostatic potential to qualitatively explain the preferred directions of electron emission.

A combined basis of analytic Gaussian functions and grid-based finite element-discrete variable representation spherical harmonic expansion is specialized for the description of continuum electron dynamics in the presence of electrons held fixed in core molecular orbitals. The applicability of this hybrid representation designed for general problems involving accurate determination of molecular continua wave functions is illustrated for photoionization of second-row diatomic molecules. Accurate descriptions of such electron continuum dynamics are a necessary step towards analyzing correlated double continua photoejections. Examination of this hybrid method in comparison to a more computationally expensive pure grid-based single-center expansion reveals several key advantages that by design make it attractive for describing processes involving one or more electrons moved to the continuum.

We present experimental results for dissociative electron attachment to acetylene near the 3 eV ${}^{2}{\ensuremath{\Pi}}_{g}$ resonance. In particular, we use an ion-momentum imaging technique to investigate the dissociation channel leading to ${\mathrm{C}}_{2}{\mathrm{H}}^{\ensuremath{-}}$ fragments. From our measured ion-momentum results we extract fragment kinetic energy and angular distributions. We directly observe a significant dissociation bending dynamic associated with the formation of the transitory negative ion. In modeling this bending dynamic with ab initio electronic structure and fixed-nuclei scattering calculations we obtain good agreement with the experiment.

Triply differential cross sections for electron-impact ionization of the $1{t}_{2}$ and $2{a}_{1}$ orbitals of methane are calculated using the complex Kohn variational method. The calculations are carried out for scattered electron energies of 500 eV and coplanar asymmetric kinematics far from the Bethe ridge, where previous experiment and theory are available for comparison. The present results are in reasonably good agreement with experiment for $1{t}_{2}$ ionization and in excellent agreement for $2{a}_{1}$ ionization, in contrast with previous theory.

The complex Kohn variational method, which has been extensively applied to low-energy molecule scattering, is extended to treat molecular ionization by fast electrons under the assumption that the incident and scattered electrons can be described by plane waves. The formulation reduces to the computation of the continuum generalized oscillation strength, which amounts to a generalization of the molecular photoionization problem to which the Kohn method has been successfully applied. To illustrate the approach, we present fully differential cross sections for the case of water, where good experimental data is available for comparison.

We analyze the consequences of discretizing one of the two continua in three-body breakup to reduce it to a two-body close-coupling problem. We identify the origin of oscillations in the singly differential cross section in those {open_quotes}convergent close-coupling{close_quotes} calculations as lying only in the way the cross section is calculated from the wave function and not in the wave function itself. The anomalous {open_quotes}step-function{close_quotes} behavior of those calculations is derived from a stationary-phase argument. Calculations are presented on the Temkin-Poet model for electron-impact ionization of hydrogen, a breakup problem with exponential potentials, and an analytically solvable model. The anomalies associated with two-body close-coupling calculations are demonstrated using wave functions from complex exterior scaling calculations that otherwise give converged results without any anomalies. {copyright} {ital 1999} {ital The American Physical Society}

We report the results of a theoretical study of electron scattering by CO[sub 2] at incident electron energies ranging from 0.25 to 10 eV using the complex Kohn variational method. These are the first fully [ital ab initio] calculations to accurately reproduce the two dominant features observed in experiment, namely, the dramatic rise in the integral cross sections below 2.0 eV and the resonance enhancement near 3.8 eV. Both of these effects are sensitive to the inclusion of electronic correlation effects involving long-range target polarization and short-range distortion. We have also carried out a preliminary study of effects of target vibrational motion in [sup 2][Pi][sub u] symmetry with an adiabatic nuclei treatment of the symmetric stretch mode. We find that this has a substantial effect on the width of the 3.8 eV resonance feature and gives results for both the integral elastic and total cross sections in excellent agreement with experiment. Our calculated differential elastic cross sections are also in good accord with recent experimental results. [copyright] [ital 1999] [ital The American Physical Society]

Exterior complex scaling enables one to compute the outgoing wave portion of the wave function for three charged particles without explicitly imposing the asymptotic boundary condition for three-body breakup. This technique is used in connection with a high-order finite difference scheme to provide numerically accurate single-differential ionization cross sections for the Temkin-Poet (s-wave) model of $e\ensuremath{-}\mathrm{H}$ scattering. These benchmark values are compared with results obtained from several recent close-coupling approaches that employ pseudostates to discretize the ionization continuum, but use a strictly two-body scattering formalism.

Despite the importance of BCl{sub 3} in plasmas used for commercial etching processes, no calculations or measurements of elastic low-energy electron scattering from BCl{sub 3} have previously appeared in the literature. We therefore present calculations based on the complex Kohn method for elastic electron-BCl{sub 3} scattering at incident electron energies below 8 eV. We find a near-zero-energy virtual state and a sharp temporary negative-ion resonance at 0.25 eV, which likely contributes to the large electron attachment cross sections observed in swarm studies. {copyright} {ital 1998} {ital The American Physical Society}

We describe complex Kohn calculations for elastic e{sup {minus}}-CF{sub 4} scattering at incident electron energies of 0.04{endash}20 eV. An accurate representation of the electronic correlation effects involving target polarization is critical in the explicit demonstration of a Ramsauer-Townsend minimum. We use a set of polarized virtual orbitals to form a compact representation of closed channels in the trial scattering function to include these effects. These are the first {ital ab initio} calculations to verify the Ramsauer-Townsend minimum in CF{sub 4}. We find excellent comparison between the complex Kohn results and measured differential and integral cross sections. {copyright} {ital 1998} {ital The American Physical Society}

An extension of the Kohn variational method for computing scattering amplitudes is demonstrated that requires only matrix elements of the resolvent of the Hamiltonian between energy-independent test functions. Scattering boundary conditions are imposed by expanding the resolvent in a basis of square-integrable functions and outgoing-wave continuum functions. By employing several continuum basis functions with overlaps defined by suitable analytic continuation, the scattering amplitude can be expressed efficiently over a continuous range of energies. The method described here differs from previous approaches using time-independent wave packets in that the wave packets which generate the initial and final states in this approach can lie within the interaction region. {copyright} {ital 1998} {ital The American Physical Society}

We describe the calculation of singly differential (energy-sharing) cross sections for electron-impact ionization. First, using exterior complex scaling, we calculate the outgoing portion of the scattering wave function without explicit use of asymptotic boundary conditions. Once that wave function is known for a finite region of space, the outgoing flux can be calculated and extrapolated to large distances according to behavior that is specific to the ionization problem. The differential cross section is proportional to the outgoing flux in specific directions of a hyperspherical angle in the coordinates. Calculations on the singlet s-wave radial limit (Temkin-Poet and collinear) models of electron-hydrogen atom ionization are presented. {copyright} {ital 1997} {ital The American Physical Society}

The complex Kohn variational method is used to study low-energy electron collision processes involving CH{sub 3}Cl. The elastic differential and momentum transfer cross sections are studied from low electron energy (0.5 eV) through the resonance region (3.5 eV) to 10 eV. The effects of target correlation and polarization are studied. A comparison is made between the results of this study and experimental results available for this system. {copyright} {ital 1997} {ital The American Physical Society}

We propose a general approach to the electron-impact ionization problem that involves two distinct steps. First, using exterior complex scaling, we calculate the outgoing portion of the scattering wave function without explicit use of boundary conditions for ionization or any other channel. Once that wave function is known for a finite region of space, we make use of a general formula for extracting the integral cross section for the breakup process from any wave function. That formula is a projected form of the optical theorem and has no relation to complex coordinates or other methods of computing the scattering wave function. A preliminary calculation on the s-wave radial limit form of electron{endash}hydrogen-atom ionization is presented in excellent agreement with calculations by other approaches. {copyright} {ital 1997} {ital The American Physical Society}

We examine finite basis set implementations of complex scaling procedures for computing scattering amplitudes and cross sections. While ordinary complex scaling, i.e., the technique of multiplying all interparticle distances in the Hamiltonian by a complex phase factor, is known to provide convergent cross-section expressions only for exponentially bounded potentials, we propose a generalization, based on Simon's exterior complex scaling technique, that works for long-range potentials as well. We establish an equivalence between a class of complex scaling transformations carried out on the time-independent Schr\"odinger equation and a procedure commonly referred to as the method of complex basis functions. The procedure is illustrated with a numerical example.

We describe a variational method for calculating photoionization cross sections from a matrix element of the resolvent operator. Scattering boundary conditions are enforced by expanding the resolvent in a basis of square-integrable (L{sup 2}) and outgoing wave continuum basis functions. By employing several continuum basis functions, with overlaps defined by a suitable analytic continuation, we can, in a single calculation, express the cross section over a continuous range of energies without explicitly resolving the variational equations at each desired energy. The method is illustrated by calculation of the photoionization cross section of atomic Be in the autoionizing region between the 1s{sup 2}2s and 1s{sup 2}2p states of Be{sup +}. {copyright} {ital 1997} {ital The American Physical Society}

Ion Momentum Imaging of Dissociative Electron Attachment Dynamics in Methanol D.S. Slaughter, 1, ∗ D.J. Haxton, 1 H. Adaniya, 1 T. Weber, 1 T.N. Rescigno, 1 C.W. McCurdy, 1 and A. Belkacem 1 Lawrence Berkeley National Laboratory, Chemical Sciences, Berkeley, California 94720,USA A combined experimental and theoretical investigation of the dissociative electron attachment (DEA) dynamics in methanol are presented for the Feshbach resonance at 6.5 eV incident electron energy. Highly-differential laboratory-frame momentum distributions have been measured for each fragmentation channel using a DEA reaction microscope. These measurements are combined with calculations of the molecular frame electron attachment probability in order to investigate the dy- namics of the dissociating methanol transient negative anion. In contrast to previous comparisons between water and methanol [1, 2] we find subtle differences in the dissociation dynamics of the two fragment channels that are direct evidence of planar symmetry-breaking of warm methanol in its electronic ground state. We also find that the DEA fragmentation does not strictly follow the axial recoil approximation and we describe the dynamics that enable an accurate prediction of the fragment angular distributions. PACS numbers: 34.80.Ht I. INTRODUCTION Low-energy free electrons are widely considered to play an important role in the radiation-induced chemistry of bio-molecules [3] and organic chemistry in the interstellar medium [4]. Dissociative electron attachment (DEA) is one of the primary fundamental interactions that drives free electron chemistry, and attracts considerable inter- est, not only for the need to understand electron-induced molecular breakup and negative ion production in nature, but also in systems of technological interest [5]. Both gaseous methanol and its ice are ubiquitous in interstellar clouds and comets, but the mechanisms of methanol synthesis in these systems are not well under- stood, although low-energy electrons are likely to play an important role [6, 7]. Free-electron driven chemistry also provides many reaction pathways leading to more complex molecules of which methanol may be an inter- mediate [4]. On Earth, methanol has widely been pro- posed as a potential large-scale alternative to fossil fuels, as it can be produced both agriculturally and syntheti- cally. Today, industrial production of methanol is typi- cally achieved by reaction of synthesis gas (mixtures of H 2 , CO 2 and CO) in the presence of a Cu/Zn/Al 2 O 3 cat- alyst at high temperatures and pressures [8]. A similar scheme has been proposed for storage of CO 2 and elec- trical energy [9] and it is possible that DEA or the time- reverse process, associative detachment, play important roles in these industrial applications. Several experimental studies have been conducted on DEA to methanol in the past, including measurements of the translational kinetic energy release (KER) [1, 10] and velocity slice imaging [11] of the dissociation prod- ucts. K¨ uhn et al. [10] found evidence of hydrogen scram- bling in the production of OH − in the 10.5 eV resonance in their measurements of DEA relative cross sections and DSSlaughter@lbl.gov KER spectra in methanol over electron energies spanning 0 eV to 17 eV. Curtis and Walker [1] extended the study to the H − ion channel, making comparisons between the three Feshbach resonances in methanol and the Fesh- bach resonances in the relatively well-understood water molecule,[1, 11–20] in addition to comparing the DEA spectra to the corresponding parent Rydberg states in both VUV photo-absorption spectra and near-threshold electron energy loss spectra. They identified each of the first two Feshbach resonances to be due to promotion of an electron from either of the highest lying occupied or- bitals, (7a ) 2 or (2a ) 2 , of methanol in its ground state, having planar (C s ) symmetry, and simultaneous elec- tron attachment to the 3s Rydberg orbital; the 6.5 eV and 8 eV resonances being 2 A [(2a ) 1 (3s) 2 ] and 2 A [(7a ) 1 (3s) 2 ], respectively. More recently, Ib˘anescu et al. [21] provided further insight on these resonances by comparing their high- precision measurements of DEA ion yields, photoelectron spectra and vibrational excitation cross-sections from several alcohols, characterizing a previously unknown shape resonance at around 3 eV. Ib˘anescu and Allan [22] followed this with a time-dependent density functional theory description of the dynamics of the first two Ry- dberg states in methanol, predicting that the dynamics of dissociation of the 1 2 A Feshbach resonance in the methanol anion would follow the dynamics of the parent Rydberg excited state [23] of the molecule. Prediction and measurement of the dynamics of the methanol anion dissociation processes are formidable challenges from both theoretical and experimental per- spectives. In general, detailed measurement and analysis of the dynamics of DEA to polyatomic molecules have, until recently, been limited to small molecules containing only a few atoms. Our understanding of multidimen- sional dynamics resulting from DEA are hindered by the few elements of symmetry and many degrees of freedom that exist in larger polyatomics. Methanol is one of the simplest systems that possesses relatively weak planar symmetry and many vibrational modes, presenting an ac

We report the results of ab initio calculations on valence photoionization of the acetylene monocation in its ground ($X$) and electronically excited ($A$) states using the complex Kohn variational method. In contrast to the $K$-shell molecular frame photoelectron angular distributions (MFPADs), which are only sensitive to the molecular geometry, these results show that the valence MFPADs are also sensitive to the electronic state of the target. Hence, the isomerization path from acetylene to vinylidene, which is mediated by a conical intersection responsible for a change in electronic state of the monocation, could, in principle, be traced through valence photoionization.

We report the results of close-coupling complex Kohn calculations of cross sections for electron-impact excitation of the water molecule into electronically dissociative states in the energy range from 8 to 30 eV. We have coupled five channels including the [sup 1][ital A][sub 1] ground state and excited singlet and triplet states formed by promoting the occupied 3[ital a][sub 1] and 1[ital b][sub 1] electrons to the lowest unoccupied 4[ital a][sub 1] orbital. These states have theoretical vertical energies in the range from 8 to 12 eV and are known to be dissociative. Our calculations have confirmed the existence of Feshbach resonances related to the [ital A][sub 1] and [ital B][sub 1] excited states which are responsible for dissociative attachment in water at rather high energies. We found that these resonances indeed correspond to configurations 3[ital a][sub 1]4[ital a][sub 1][sup 2] and 1[ital b][sub 1]4[ital a][sub 1][sup 2]. Our results include both total and differential cross sections. We also make comparison between theory and available experimental data.

Following prior work on the lower-energy resonances, we apply techniques of momentum imaging and ab initio scattering calculations to the process of dissociative electron attachment to water via the highest-energy ${}^{2}{B}_{2}$ resonance. We focus on the H${}^{\ensuremath{-}}$ anion fragment, which is produced via dynamics passing through and avoiding the conical intersection with the lower ${A}_{1}$ state, leading to OH (${}^{2}\ensuremath{\Pi}$) and OH (${}^{2}\ensuremath{\Sigma}$), respectively. The momentum imaging technique, when combined with theoretical calculations on the attachment amplitude and dissociation dynamics, demonstrates that the angular distributions provide a signature of the location of the conical intersection in the space of nuclear configurations.