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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.

Author(s): Okunishi, M; Ito, Y; Sharma, V; Aktar, S; Ueda, K; Lucchese, RR; Dnestryan, AI; Tolstikhin, OI; Inoue, S; Matsui, H; Morishita, T | Abstract: Photoelectron momentum distributions (PEMDs) generated in strong-field ionization of randomly oriented CO2 molecules by intense infrared laser pulses at 1300-, 1450-, and 1650-nm wavelengths are measured experimentally and analyzed theoretically. The experimental PEMDs extracted along the outermost backward rescattering caustic are well reproduced by theoretical calculations based on the recently derived factorization formula with an analytical returning photoelectron wave packet (RWP). The sensitivity of the theoretical results to the target structure models used in the calculations is investigated. It is shown that RWPs obtained in the single-active-electron (SAE) approximation and by the Hartree-Fock method have only minor differences. On the other hand, differential cross sections (DCSs) for elastic scattering of a photoelectron on the parent molecular ion calculated by ab initio, SAE, and independent-atom model methods are considerably different. This difference almost disappears after averaging over molecular orientations, so the present experiment does not enable us to discriminate between the different target structure models. However, we show that such a discrimination should become possible by measuring PEMD with aligned molecules. This will provide an access to the rich target structure information contained in the DCS.

We investigated the fragmentation dynamics of highly excited states of molecular oxygen using femtosecond transient photoelectron spectroscopy. An extreme ultraviolet (XUV) pulse populates the autoionizing Rydberg series converging to ${\mathrm{O}}_{2}^{+}\phantom{\rule{4pt}{0ex}}c\phantom{\rule{0.16em}{0ex}}^{4}\mathrm{\ensuremath{\Sigma}}_{u}^{\ensuremath{-}}$, and a femtosecond near-infrared (IR) pulse was used to photoionize these states as they dissociate. Monitoring the differential photoelectron spectra as a function of time delay allowed us to obtain the relaxation lifetimes of these Rydberg states. We observed a photoelectron signal corresponding to the formation of a $4p$ excited atomic oxygen fragment, which is not an expected dissociation product of the $({\mathrm{O}}_{2}^{+}\phantom{\rule{4pt}{0ex}}c\phantom{\rule{0.16em}{0ex}}^{4}\mathrm{\ensuremath{\Sigma}}_{u}^{\ensuremath{-}})nl{\ensuremath{\sigma}}_{g}$ Rydberg series. Analysis of the time-dependent photoelectron spectra and photoionization calculations indicate that this fragment results from a previously unexplored $({\mathrm{O}}_{2}^{+}\phantom{\rule{4pt}{0ex}}^{4}\mathrm{\ensuremath{\Pi}}_{g})4p$ repulsive state and that, contrary to expectations, this multielectron excitation pathway presents a substantial cross section. Our study demonstrates that two-color time-resolved differential photoelectron spectroscopy is an excellent tool to study the fragmentation dynamics of such multielectron excited states, which are not easily probed by other means.

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 complex Kohn variational method for electron--polyatomic-molecule scattering is formulated using an overset-grid representation of the scattering wave function. The overset grid consists of a central grid and multiple dense atom-centered subgrids that allow the simultaneous spherical expansions of the wave function about multiple centers. Scattering boundary conditions are enforced by using a basis formed by the repeated application of the free-particle Green's function and potential ${\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{G}}_{0}^{+}\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{V}$ on the overset grid in a Born-Arnoldi solution of the working equations. The theory is shown to be equivalent to a specific Pad\'e approximant to the $T$ matrix and has rapid convergence properties, in both the number of numerical basis functions employed and the number of partial waves employed in the spherical expansions. The method is demonstrated in calculations on methane and ${\mathrm{CF}}_{4}$ in the static-exchange approximation and compared in detail with calculations performed with the numerical Schwinger variational approach based on single-center expansions. An efficient procedure for operating with the free-particle Green's function and exchange operators (to which no approximation is made) is also described.

Author(s): Ito, Y; Carranza, R; Okunishi, M; Lucchese, RR; Ueda, K | Abstract: We measured angle-resolved high-energy electron spectra emitted from C2H4 in an intense laser field, extracted field-free electron-ion elastic scattering differential cross sections (DCSs) according to quantitative rescattering theory, and obtained molecular contrast factors (MCFs) subtracting the incoherent sum of DCSs of all the atoms in the molecule. Comparing the results with ab initio scattering calculations and employing least-squares fitting, we have extracted the C-C and C-H bond lengths of the molecule with ∼5% uncertainty. This approach opens the way to retrieve the structure of hydrocarbon molecules, potentially at high temporal resolution, employing low collision energies where electron scattering is sensitive to the hydrogen atoms; and where the independent atom model calculations may fail to reproduce the experimentally extracted MCF.

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

We report a joint theoretical-experimental investigation on elastic electron scattering by dimethyl disulfide in the low- and intermediate-energy regions. Experimental angular distributions of the elastically scattered electrons were measured in the 10--800 eV and ${5}^{\ensuremath{\circ}}$--${130}^{\ensuremath{\circ}}$ ranges using a crossed electron beam--molecular beam geometry. The absolute values of the differential cross sections were obtained using the relative-flow technique. Also, integral and momentum-transfer cross sections were derived from the experimental differential cross sections via a numerical integration procedure. Theoretically, differential, integral, momentum-transfer, grand-total, and total absorption cross sections are reported in the 1--500 eV range. In our calculations, a complex optical potential was used to represent the collision dynamics and a single-center expansion method combined with the Pad\'e approximation was used to solve the scattering equations. Our experimental data are in good agreement with the present calculated data. Comparisons with other theoretical results are also made.

A scheme for probing molecular vibrations is proposed by analyzing inner-shell-photoionization signals that use a weak infrared laser pump pulse to excite the vibrational mode, followed by an ultrashort soft-x-ray probe. It is shown, for SF${}_{6}$ molecules, that the sensitivity of the photoionization cross section to the molecular structure can be used to extract nuclear dynamical information in real time.

We investigate the resonance-like enhancement (RLE) in high-order above-threshold ionization (ATI) spectra of the polyatomic molecules ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$ and ${\mathrm{C}}_{2}{\mathrm{H}}_{6}$. In the spectrum-intensity maps, strong and weak RLE areas emerge alternatively for both ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$ and ${\mathrm{C}}_{2}{\mathrm{H}}_{6}$ but in different sequences. Theoretical calculations using the strong-field approximation reproduce the experimental observation and analysis shows that the different characteristics of the two molecules can be attributed to interference effects of molecular orbitals with different symmetries. For ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$, the RLE structures are attributed to C--C centers of the highest occupied molecular orbital (HOMO) orbital. For ${\mathrm{C}}_{2}{\mathrm{H}}_{6}$, in contrast, the C--C centers of the HOMO and HOMO-1 orbitals do not contribute to the RLE due to destructive interference but the hydrogen centers of the bonding HOMO-1 orbital give rise to the multiple RLE regions. In addition, clear experimental evidence of the existence of two types of the RLE and their dependence on the parity of ground state is shown. Our result, which strongly supports the channel-closing mechanism of the RLE, for the first time reveals the important role of low-lying orbitals and the differing roles of different atomic centers in the high-order ATI spectrum of molecules.

A theoretical and experimental investigation on elastic electron scattering by two small esters, namely, methyl formate and ethyl acetate, is reported. Experimental differential, integral, and momentum-transfer cross sections are given in the 30--1000 eV and ${10}^{\ensuremath{\circ}}--{120}^{\ensuremath{\circ}}$ ranges. The relative-flow technique was used to determine such quantities. Particularly for methyl formate, a theoretical study was also carried out in the 1--500 eV range. A complex optical potential derived from a Hartree-Fock molecular wave function was used to represent the collision dynamics, whereas the Pad\'e approximation was used to solve the scattering equations. In addition, calculations based on the framework of the independent-atom model (IAM) were also performed for both targets. In general, there is good agreement between our experimental data and the present theoretical results calculated using the Pad\'e approximation. The theoretical results using the IAM also agree well with the experimental data at 200 eV and above. Moreover, for methyl formate, our calculations reveal a ${}^{2}{A}^{\ensuremath{'}\ensuremath{'}}$ $({\ensuremath{\pi}}^{*})$ resonance at about 3.0 eV and a ${\ensuremath{\sigma}}^{*}$-type resonance centered at about 8.0 eV in the ${}^{2}{A}^{\ensuremath{'}}$ scattering channel. The ${\ensuremath{\pi}}^{*}$ resonance is also seen in other targets containing a carbonyl group.

We report an investigation on electron collisions with two nitrogen-containing compounds, namely ammonia (${\mathrm{NH}}_{3}$) and formamide (${\mathrm{NH}}_{2}\mathrm{CHO}$). For ammonia, both theoretical and experimental differential, integral, and momentum-transfer cross sections, as well as calculated grand-total and total absorption cross sections, are reported in the 50--500 eV incident energy range. Calculated results of various cross sections are also reported for energies below 50 eV. Experimentally, angular distributions of the scattered electrons were measured using a crossed electron beam-molecular beam geometry and then converted to absolute differential cross sections using the relative flow technique. Absolute integral and momentum-transfer cross sections for elastic ${e}^{\ensuremath{-}}$-ammonia scattering were also derived from the measured differential cross sections. For formamide, only theoretical cross sections are presented in the 1--500 eV incident energy range. A single-center-expansion technique combined with the method of Pad\'e was used in our calculations. For both targets, our calculated cross sections are compared with the present measured data and with the theoretical and experimental data available in the literature and show generally good agreement. Moreover, for formamide, two shape resonances located at 3.5 eV and 15 eV which correspond to the continuum ${}^{2}{A}^{\ensuremath{'}\ensuremath{'}}$ and ${}^{2}{A}^{\ensuremath{'}}$ scattering symmetries, respectively, are identified. The former can be associated to the ${}^{2}{B}_{1}$ shape resonance in formaldehyde located at around 2.5 eV, whereas the latter can be related to the ${}^{2}E$ resonance in ammonia at about 10 eV. Such correspondence is very interesting and so supports the investigation on electron interaction with small building blocks, instead of with larger biomolecules.

We demonstrate that dissociative ionization of O(2) can be controlled by the relative delay between an attosecond pulse train (APT) and a copropagating infrared (IR) field. Our experiments reveal a dependence of both the branching ratios between a range of electronic states and the fragment angular distributions on the extreme ultraviolet (XUV) to IR time delay. The observations go beyond adiabatic propagation of dissociative wave packets on IR-induced quasistatic potential energy curves and are understood in terms of an IR-induced coupling between electronic states in the molecular ion.

We propose a method, molecular polarimetry, applicable to an extended VUV--x-ray range, allowing us to determine the complete state of elliptically polarized light, including the challenging disentanglement of the circular and unpolarized components. It relies on the determination of the molecular frame photoelectron angular distributions derived from electron-ion velocity vector correlations in dissociative photoionization of simple molecules. The high accuracy of the full set of Stokes parameters determination has been established by comparison with data obtained in parallel with a VUV optical polarimeter used as a benchmark.

$K$-shell photodetachment of size-selected ${{\mathrm{B}}_{2}}^{\ensuremath{-}}$ and ${{\mathrm{B}}_{3}}^{\ensuremath{-}}$ cluster anions has been measured and calculated. The experimental absolute photodetachment cross sections exhibit bound resonances below threshold and two shape resonances above the $K$-shell threshold. Similar results were obtained for all of the cationic products observed, ${\mathrm{B}}^{+}$ and ${{\mathrm{B}}_{2}}^{+}$ from ${{\mathrm{B}}_{2}}^{\ensuremath{-}}$, as well as ${\mathrm{B}}^{+}$, ${{\mathrm{B}}_{2}}^{+}$, and ${{\mathrm{B}}_{3}}^{+}$ from ${{\mathrm{B}}_{3}}^{\ensuremath{-}}$. The overall agreement between measured and calculated photodetachment cross sections is very good. However, the theoretical study yielded additional bound resonances not observed in the experimental data.

We performed both single- and coupled-channel scattering calculations of the photoionization of HCN at a photon energy range of 40-1000 eV. Based upon the present results, we predict that HCN is a system for which the inner-valence photoionization is characterized by a complete breakdown of the quasiparticle picture of photoionization. We find that there is no inner valence main line and that the intensity from photoionization from the inner valence 3{sigma} orbital is split into two prominent satellites and two weaker satellites. These four satellites occur at binding energies of approximately 27.0, 29.5, 31.0, and 33.0 eV, and are almost solely of inner valence origin. We identify these satellites as being of inner-valence origin as a consequence of the theoretically predicted high-energy convergence of the photoionization cross-section rations to the ratio of corresponding spectroscopic intensity factors. Additionally, we find that there is one mixed provenance satellite that occurs on the shoulder of the highest binding energy (4{sigma}{sup -1}) line at 22.5 eV, i.e., lower than the expected energy range for satellites. The existence of this satellite is another result of the breakdown of the quasiparticle picture. On a phenomenological basis, all four of the inner-valence-derived satellites appear to be dynamicallymore » correlated shake-up states. (c) 1999 The American Physical Society.« less

The low-energy (0\char21{}6-eV) behavior of positrons scattering from gaseous ${\mathrm{C}}_{60}$ molecules is examined through a computational study which employs a nonempirical modeling of all the relevant interaction forces (static, correlation, and polarization potentials). There is uncertainty concerning the most appropriate correlation-polarization potential to use in the study of positron-molecule scattering. Here we have considered two such potentials which are representative of the potentials which have been proposed for this purpose. The coupled quantum scattering equations are solved in the body-fixed frame of reference, and the fixed-nuclei approximation is used to decouple the nuclear motion during the collision. The elastic integral cross sections without positronium formation exhibit strong one-particle resonances in the very-low-energy region up to $\ensuremath{\sim}4$ eV. The low-angular-momentum scattering components are shown to be the dominant contributions to trapped state wave functions. The existence of resonances in positron scattering from molecules is unusual. In the system studied here there are two types of resonances. First there are angular-momentum barrier resonances with the positron trapped outside of the ${\mathrm{C}}_{60}$ cage. Second, there are resonances of low angular momentum $(ll~2)$ with the positron trapped inside of the ${\mathrm{C}}_{60}$ cage by the repulsive interaction between the positron and the nuclei. The possibility of the experimental observation of these resonances is also discussed.

Calculations on the Auger decay of CO after the $\mathrm{C}1s\ensuremath{\rightarrow}2{\ensuremath{\pi}}^{*}$ excitation have been performed utilizing a multichannel Schwinger approach. The bound orbitals used to expand the multichannel configuration-interaction wave functions of the ion state were the natural orbitals of a single excitation and double excitation configuration-interaction (CI) calculation on the $\mathrm{C}{1s}^{\ensuremath{-}1}$ core-hole state of CO. Fixed-nuclei photoionization cross sections were then obtained with target states represented by complete active space CI wave functions. An analysis of the resulting cross sections using Fano's spectral line profile and the lifetime vibrational interference theory has lead to photoabsorption profiles in good agreement with experimental measurements. Additionally, we have found good agreement with experimental data for the resulting photoelectron and photoemission spectra.

We present theoretical calculations of high-order-harmonic generation (HHG) from stereoisomers of 1,2dichloroethylene (C2H2Cl2) and 2-butene (C4H8) based on the quantitative rescattering theory. Our results show that the HHG spectra from these cis and trans isomers with intense midinfrared laser pulses are clearly distinguishable, even when the molecules are randomly oriented, in good agreement with the recent experiments by Wong et al. [Phys. Rev. A 84, 051403(R) (2011)]. We found that the angle-averaged tunneling ionization yields and photoionization cross sections from the cis and trans isomers for both molecules are nearly identical. The origin of the differences in HHG spectra is traced as due to the interplay in the angular dependence of the photoionization (or photorecombination) cross section and ionization rate.

We report applications of the quantitative rescattering theory (QRS) for calculation of high-order-harmonic generation (HHG) from polyatomic molecules in ultrashort linearly polarized intense laser pulses, using the example of the CCl4 molecule. In particular, we present a detailed analysis and a treatment for the phase of the electron returning wave packet, which recollides with the parent molecular ion to emit high-energy photons. Our results show that Cooper-type minimum structures in the molecular photoionization cross section lead to quite distinguishable minima in the HHG spectra, even for unaligned polyatomic molecules.

We report a theoretical study on electron scattering by two strongly polar molecules, namely, formaldehyde (CH${}_{2}$O) and pyrimidine (C${}_{4}$H${}_{4}$N${}_{2}$), in the low- and intermediate-energy ranges. Calculated elastic differential, integral, and momentum-transfer cross sections, as well as total (elastic $+$ inelastic) and total absorption cross sections, are reported for impact energies ranging from 0.2 to 500 eV. A complex optical potential is used to represent the electron-molecule interaction dynamics, whereas a single-center-expansion method associated with the Pad\'e approximant technique is used to solve the scattering equations. Our calculated results are compared with experimental results and other theoretical data available in the literature. Generally good agreement is seen in these comparisons.

We investigate high-order-harmonic generation of isotropically distributed gas-phase ${\mathrm{H}}_{2}$O molecules exposed to an intense laser field. The induced dipole of each individual molecule by the laser field is first calculated using the recently developed quantitative rescattering theory. In a thin medium, harmonic spectra generated coherently from all the molecules are then calculated by solving Maxwell's equation of propagation. By using accurate transition dipoles of ${\mathrm{H}}_{2}$O, we show that the harmonics in the lower plateau region are quite different from models that employ the simpler strong-field approximation. We also examine the magnitude and phase of the harmonics and their dependence on laser focusing conditions.

Often the inclusion of electron-correlation effects in photoionization calculations is crucial; e.g., calculation of satellite cross sections and asymmetry parameters in core ionization of molecules. We have developed a computational approach to include the relaxation, multiconfiguration, and multichannel effects, in a multichannel configuration-interaction (MCCI) approximation, for the study of molecular photoionization. Results are presented of a detailed MCCI-Schwinger study of the C(1s) core-hole production in CO, including the formation of the lowest two π→π * shake-up satellites. It is shown that the multichannel effects are more important in describing the satellites than the core-hole channel