Anthony F. Starace PublicationsCopyright (c) 2016 University of Nebraska - Lincoln All rights reserved.
http://digitalcommons.unl.edu/physicsstarace
Recent documents in Anthony F. Starace Publicationsen-usWed, 28 Sep 2016 02:00:42 PDT3600Imaging population transfer in atoms with ultrafast electron pulses
http://digitalcommons.unl.edu/physicsstarace/219
http://digitalcommons.unl.edu/physicsstarace/219Mon, 26 Sep 2016 14:15:50 PDT
We propose the use of ultrafast electron diffraction (UED) to image a controllable, laser-driven coherent electron population transfer in lithium atoms with currently available femtosecond electron pulses. Our simulations demonstrate the ability of ultrafast electrons to image such an electronic population transfer, thus validating UED as a direct means of investigating electron dynamics. Provided the incident electron pulses have sufficient temporal resolution, the diffraction images are shown to resolve also the relative phases of the target electronic wave functions.
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Hua-Chieh Shao et al.Multistart spiral electron vortices in ionization by circularly polarized UV pulses
http://digitalcommons.unl.edu/physicsstarace/218
http://digitalcommons.unl.edu/physicsstarace/218Tue, 12 Jul 2016 14:37:07 PDTMultistart spiral vortex patterns are predicted for the electron momentum distributions in the polarization plane following ionization of the helium atom by two time-delayed circularly polarized ultrashort laser pulses. For two ultraviolet (UV) pulses having the same frequency (such that two photons are required for ionization), single-color two-photon interferometry with corotating or counter-rotating time-delayed pulses is found to lead respectively to zero-start or four-start spiral vortex patterns in the ionized electron momentum distributions in the polarization plane. In contrast, two-color one-photon plus two-photon interferometry with time-delayed corotating or counter-rotating UV pulses is found to lead respectively to one-start or three-start spiral vortex patterns. These predicted multistart electron vortex patterns are found to be sensitive to the carrier frequencies, handedness, time delay, and relative phase of the two pulses. Our numerical predictions are obtained by solving the six-dimensional two-electron time-dependent Schrödinger equation (TDSE). They are explained analytically using perturbation theory (PT). Comparison of our TDSE and PT results for single-color two-photon processes probes the role played by the time-delay-dependent ionization cross channels in which one photon is absorbed from each pulse. Control of these cross channels by means of the parameters of the fields and the ionized electron detection geometries is discussed.
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Jean Marcel Ngoko Djiokap et al.Atomic photoionization experiment by harmonic-generation spectroscopy
http://digitalcommons.unl.edu/physicsstarace/217
http://digitalcommons.unl.edu/physicsstarace/217Thu, 17 Mar 2016 14:26:39 PDT
Measurements of the high-order-harmonic generation yield of the argon (Ar) atom driven by a strong elliptically polarized laser field are shown to completely determine the field-free differential photoionization cross section of Ar, i.e., the energy dependence of both the angle-integrated photoionization cross section and the angular distribution asymmetry parameter.
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M. V. Frolov et al.Control of threshold enhancements in harmonic generation by atoms in a two-color laser field with orthogonal polarizations
http://digitalcommons.unl.edu/physicsstarace/216
http://digitalcommons.unl.edu/physicsstarace/216Tue, 01 Mar 2016 10:57:37 PST
Threshold phenomena (or channel-closing effects) are analyzed in high-order harmonic generation (HHG) by atoms in a two-color laser field with orthogonal linearly polarized components of a fundamental field and its second harmonic. We show that the threshold behavior of HHG rates for the case of a weak second harmonic component is sensitive to the parity of a closing multiphoton ionization channel and the spatial symmetry of the initial bound state of the target atom, while for the case of comparable intensities of both components, suppression of threshold phenomena is observed as the relative phase between the components of a two-color field varies. A quantum orbit analysis as well as phenomenological considerations in terms of Baz’ theory of threshold phenomena [Zh. Eksp. Teor. Fiz. 33, 923 (1957)] are presented in order to describe and explain the major features of threshold phenomena in HHG by a two-color field.
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M. V. Frolov et al.Favorable target positions for intense laser acceleration of electrons in hydrogen-like, highly-charged ions
http://digitalcommons.unl.edu/physicsstarace/215
http://digitalcommons.unl.edu/physicsstarace/215Mon, 14 Sep 2015 14:14:27 PDT
Classical relativistic Monte Carlo simulations of petawatt laser acceleration of electrons bound initially in hydrogen-like, highly-charged ions show that both the angles and energies of the laser-accelerated electrons depend on the initial ion positions with respect to the laser focus. Electrons bound in ions located after the laser focus generally acquire higher (≈GeV) energies and are ejected at smaller angles with respect to the laser beam. Our simulations assume a tightly-focused linearly-polarized laser pulse with intensity approaching 10^{22}W/cm^{2}. Up to fifth order corrections to the paraxial approximation of the laser field in the focal region are taken into account. In addition to the laser intensity, the Rayleigh length in the focal region is shown to play a significant role in maximizing the final energy of the accelerated electrons. Results are presented for both Ne^{9+} and Ar^{17+} target ions.
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Liang-Wen Pi et al.Electron Vortices in Photoionization by Circularly Polarized Attosecond Pulses
http://digitalcommons.unl.edu/physicsstarace/214
http://digitalcommons.unl.edu/physicsstarace/214Thu, 10 Sep 2015 11:43:36 PDT
Single ionization of He by two oppositely circularly polarized, time-delayed attosecond pulses is shown to produce photoelectron momentum distributions in the polarization plane having helical vortex structures sensitive to the time delay between the pulses, their relative phase, and their handedness. Results are obtained by both ab initio numerical solution of the two-electron time-dependent Schrödinger equation and by a lowest-order perturbation theory analysis. The energy, bandwidth, and temporal duration of attosecond pulses are ideal for observing these vortex patterns.
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Jean Marcel Ngoko Djiokap et al.Scaling laws for high-order-harmonic generation with midinfrared laser pulses
http://digitalcommons.unl.edu/physicsstarace/213
http://digitalcommons.unl.edu/physicsstarace/213Mon, 10 Aug 2015 10:04:45 PDT
We derive an analytic expression for thewavelength scaling of the high-order-harmonic generation (HHG) yield induced by midinfrared driving laser fields. It is based on a quasiclassical description of the returning electron wave packet, which is shown to be largely independent of atomic properties. The accuracy of this analytic expression is confirmed by comparison with results of numerical solutions of the time-dependent Schr¨odinger equation for wavelengths in the range of 1.4 μm ≤ λ ≤ 4 μm. We verify the wavelength scaling of the HHG yield found numerically for midinfrared laser fields in a recent paper by Le et al. [
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M. V. Frolov et al.Photodetachment of a model molecular system by an elliptically polarized field
http://digitalcommons.unl.edu/physicsstarace/212
http://digitalcommons.unl.edu/physicsstarace/212Sun, 26 Jul 2015 17:08:25 PDT
The differential cross section for one-photon molecular detachment by an elliptically polarized field is analyzed for a one-electron molecular model comprised of an electron in the field of two (generally nonequivalent) attractive zero-range potentials (ZRPs) separated by the distance R. A phenomenological parametrization of the photodetachment cross section for a fixed-in-space molecular system in terms of two scalar dynamical parameters is presented and circular dichroism effects are discussed. Analytic results for the dynamical molecular parameters within the ZRP molecular model are used to analyze interference phenomena (including two-center interference) and dichroic effects in the detached electron angular distributions and their dependence on the interatomic distance R and on the orientation of the molecular axis with respect to the polarization plane. Numerical ZRP results for angular distributions are presented for both symmetric and asymmetric molecules in an elliptically polarized field.
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M. V. Frolov et al.Nonlinear Dichroism in Back-to-Back Double Ionization of He by an Intense Elliptically Polarized Few-Cycle Extreme Ultraviolet Pulse
http://digitalcommons.unl.edu/physicsstarace/211
http://digitalcommons.unl.edu/physicsstarace/211Thu, 02 Apr 2015 13:54:35 PDT
Control of double ionization of He by means of the polarization and carrier-envelope phase (CEP) of an intense, few-cycle extreme ultraviolet (XUV) pulse is demonstrated numerically by solving the six-dimensional two-electron, time-dependent Schrödinger equation for He interacting with an elliptically polarized XUV pulse. Guided by perturbation theory (PT), we predict the existence of a nonlinear dichroic effect (∝ I^{3/2}) that is sensitive to the CEP, ellipticity, peak intensity I, and temporal duration of the pulse. This dichroic effect (i.e., the difference of the two-electron angular distributions for opposite helicities of the ionizing XUV pulse) originates from interference of first- and second-order PT amplitudes, allowing one to probe and control S- and D-wave channels of the two-electron continuum.We show that the back-to-back in-plane geometry with unequal energy sharing is an ideal one for observing this dichroic effect that occurs only for an elliptically polarized, few-cycle attosecond pulse.
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Jean Marcel Ngoko Djiokap et al.Rescattering effects in laser-assisted electron-atom
bremsstrahlung
http://digitalcommons.unl.edu/physicsstarace/210
http://digitalcommons.unl.edu/physicsstarace/210Tue, 17 Mar 2015 14:39:54 PDT
Rescattering effects in non-resonant spontaneous laser-assisted electron–atom bremsstrahlung (LABrS) are analyzed within the framework of time-dependent effective-range (TDER) theory. It is shown that high energy LABrS spectra exhibit rescattering plateau structures that are similar to those that are well-known in strong field laser-induced processes as well as those that have been predicted theoretically in laser-assisted collision processes. In the limit of a low-frequency laser field, an analytic description of LABrS is obtained from a rigorous quantum analysis of the exact TDER results for the LABrS amplitude. This amplitude is represented as a sum of factorized terms involving three factors, each having a clear physical meaning. The first two factors are the exact field-free amplitudes for electron–atom bremsstrahlung and for electron– atom scattering, and the third factor describes free electron motion in the laser field along a closed trajectory between the first (scattering) and second (rescattering) collision events. Finally, an extension of these TDER results to the case of LABrS in a Coulomb field is discussed.
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A. N. Zheltukhin et al.Comment on “Universality of Returning Electron Wave
Packet in High-Order Harmonic Generation with
Midinfrared Laser Pulses”
http://digitalcommons.unl.edu/physicsstarace/209
http://digitalcommons.unl.edu/physicsstarace/209Thu, 12 Feb 2015 08:58:48 PST
In conclusion, we have shown that when the same definition for the HHG yield is used [cf. Eq. (1)], the results of Ref. [1] give the same scaling law found earlier in Refs. [2–5] for wavelengths λ ≤ 2 μm. We note that this latter scaling law can be obtained analytically by using results of the model developed in Ref. [6] for the description of short-pulse HHG spectra. These analytic results as well as new numerical TDSE results for longer wavelengths, λ ≤ 4 μm, will be published elsewhere.
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M. V. Frolov et al.Imaging electronic motions in atoms by energy-resolved ultrafast electron diffraction
http://digitalcommons.unl.edu/physicsstarace/208
http://digitalcommons.unl.edu/physicsstarace/208Thu, 11 Sep 2014 21:55:34 PDT
We propose energy-resolved ultrafast electron diffraction as a means of directly imaging target electronic motions whose space, time, and energy information can be simultaneously retrieved from time-resolved diffraction measurements. The energy-resolved diffraction images are simulated for breathing, wiggling, and hybrid modes of electronic motion in the H atom. The simulations demonstrate the capabilities of ultrafast electron diffraction to image and distinguish different kinds of electronic motion. The theoretical analysis of the scattering process identifies the requirements for time- and state-resolved imaging of electronic motion and provides interpretations of the results.
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Hua-Chieh Shao et al.Potential-barrier effects in three-photon-ionization processes
http://digitalcommons.unl.edu/physicsstarace/207
http://digitalcommons.unl.edu/physicsstarace/207Mon, 04 Aug 2014 10:46:37 PDT
Resonance-like enhancements of generalized three-photon cross sections for extreme ultraviolet ionization of Ar, Kr, and Xe are demonstrated and analyzed within a single-active-electron, central-potential model. The resonant-like behavior is shown to originate fromthe potential barriers experienced by intermediate- and final-state photoelectron wave packets corresponding to absorption of one, two, or three photons. The resonance-like profiles in the generalized three-photon-ionization cross sections are shown to be similar to those found in the generalized two-photon-ionization cross sections [Phys. Rev. A 82, 053414 (2010)]. The complexity of Cooper minima in multiphoton-ionization processes is also discussed. Owing to the similar resonance-like profiles found in both two- and three-photon generalized cross sections, we expect such potential-barrier effects to be general features of multiphoton-ionization processes in most atoms with occupied p and d subshells.
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Liang-Wen Pi et al.Analytic model for the description of above-threshold ionization by an intense short laser pulse
http://digitalcommons.unl.edu/physicsstarace/206
http://digitalcommons.unl.edu/physicsstarace/206Tue, 08 Jul 2014 10:12:55 PDT
We present an analytic model for the description of above-threshold ionization (ATI) of an atom by an intense, linearly polarized short laser pulse. Our treatment is based upon a description of ATI by an infinitely long train of short laser pulses whereupon we take the limit that the time interval between pulses becomes infinite. In the quasiclassical approximation, we provide detailed quantum-mechanical derivations, within the time-dependent effective range (TDER) model, of the closed-form formulas for the differential probability P(p) of ATI by an intense, short laser pulse that were presented briefly by Frolov et al. [Phys. Rev. Lett. 108, 213002 (2012)] and that were used to describe key features of the high-energy part of ATI spectra for H and He atoms in an intense, few-cycle laser pulse, using a phenomenological generalization of the physically transparent TDER results to the case of real atoms. Moreover, we extend these results here to the case of an electron bound initially in a p state; we also take into account multiple-return electron trajectories. The ATI amplitude in our approach is given by a coherent sum of partial amplitudes describing ionization by neighboring optical cycles near the peak of the intensity envelope of a short laser pulse. These results provide an analytical explanation of key features in short-pulse ATI spectra, such as the left-right asymmetry in the ionized electron angular distribution, the multiplateau structures, and both large-scale and fine-scale oscillation patterns resulting from quantum interferences of electron trajectories. Our results show that the shape of the ATI spectrum in the middle part of the ATI plateau is sensitive to the spatial symmetry of the initial bound state of the active electron. This sensitivity originates from the contributions of multiple-return electron trajectories. Our analytic results are shown to be in good agreement with results of numerical solutions of the time-dependent Schrödinger equation for He and Ar atoms. Comparison of our results with those of quantitative rescattering theory is also discussed.
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M. V. Frolov et al.Resonant electron-atom bremsstrahlung in an intense laser field
http://digitalcommons.unl.edu/physicsstarace/205
http://digitalcommons.unl.edu/physicsstarace/205Sun, 09 Feb 2014 10:35:45 PST
We analyze a resonant mechanism for spontaneous laser-assisted electron bremsstrahlung (BrS) involving the resonant transition (via either laser-assisted electron-ion recombination or electron-atom attachment) into a laser-dressed intermediate quasibound state (corresponding, respectively, to either a field-free neutral atom or a negative-ion bound state) accompanied by ionization or detachment of this state by the laser field. This mechanism leads to resonant enhancement (by orders of magnitude) of the BrS spectral density for emitted photon energies corresponding to those for laser-assisted recombination or attachment. We present an accurate parametrization of the resonant BrS amplitude in terms of the amplitudes for nonresonant BrS, for recombination or attachment to the intermediate state, and for ionization or detachment of this state. The high accuracy of our general analytic parametrization of the resonant BrS cross section is shown by comparison with exact numerical results for laser-assisted BrS spectra obtained within time-dependent effective range theory. Numerical estimates of resonant BrS in electron scattering from a Coulomb potential are also presented.
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A. N. Zheltukhin et al.Imaging coherent electronic motion in atoms by ultrafast electron diffraction
http://digitalcommons.unl.edu/physicsstarace/204
http://digitalcommons.unl.edu/physicsstarace/204Sun, 29 Dec 2013 11:18:37 PST
Ultrafast electron diffraction from time-varying coherent electronic states of the H atom is analyzed theoretically. This theoretical analysis identifies the conditions necessary to obtain time-resolved measurements. Electron diffraction from coherent electronic states exhibiting breathing and wiggling modes of electronic motion are simulated numerically in order to demonstrate the capability of attosecond electron pulses to image electron dynamics. The scattering patterns and their temporal behaviors are shown to differentiate the two kinds of target electronic motion. Moreover, our simulations show that inelastic processes contribute significantly to the diffraction patterns. Thus, although the diffraction patterns clearly distinguish different modes of target electronic motion, they cannot be easily related to the time-dependent target charge density.
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Hua-Chieh Shao et al.Harmonic generation spectroscopy with a
two-colour laser field having orthogonal
linear polarizations
http://digitalcommons.unl.edu/physicsstarace/203
http://digitalcommons.unl.edu/physicsstarace/203Fri, 22 Nov 2013 13:03:05 PST
The interpretation of many high-order harmonic generation (HHG) experiments is based on the assumption that the HHG yield of an atom can be factorized into (i) a laser-dependent ‘electron wave packet’ with rather simple properties, including a nearly universal shape, and (ii) an atomic photorecombination cross section. We show that this factorization is restricted to linearly polarized laser fields and fails in two-colour laser fields with orthogonal polarizations. At the same time, we show how two-colour HHG spectroscopy using orthogonally polarized intense fundamental and relatively weak second harmonic fields makes a complete experiment possible that enables the retrieval of the angle-resolved photorecombination cross sections for atomic p states.
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T. S. Sarantseva et al.Resonant enhancement of the harmonic-generation spectrum of beryllium
http://digitalcommons.unl.edu/physicsstarace/202
http://digitalcommons.unl.edu/physicsstarace/202Mon, 18 Nov 2013 06:27:33 PST
The high-order harmonic-generation (HHG) spectrum of Be is investigated in the multiphoton regime by solving the full-dimensional, two-active-electron, time-dependent Schr¨odinger equation in an intense (≈10^{13} W/cm^{2}), 30-cycle laser field. As the laser frequency ω_{L}varies from 1.7 to 1.8 eV (which is in the tunable range of a Ti:sapphire laser), the seventh harmonic becomes resonant sequentially with the transition between the ground state and two doubly excited autoionizing states, 2p4s(^{1}P) (at ω_{L}= 1.734 eV) and 2p5s(^{1}P) (at ω_{L}= 1.785 eV), while the third harmonic becomes resonant with the 2s2p(^{1}P) singly excited state (at ω_{L}= 1.766 eV). At each of these resonant frequencies, the HHG power spectrum is found to increase by an order of magnitude over a range of harmonics that form a plateau, extending from the resonant harmonic up to a cutoff at the 25th harmonic. In contrast to the well-known rescattering plateau cutoff law appropriate in the tunneling regime (which predicts a cutoff at the fifth or seventh harmonic), the multiphoton regime plateau we find for Be originates from atomic resonance effects. Off resonance, the Be HHG spectrum decreases monotonically with harmonic order. By taking the ratio of the integrated harmonic power of the seventh harmonic to that of the fifth harmonic, one can isolate the resonant effects of the two doubly excited states in the HHG spectrum from those of singly excited resonance states. These ratios exhibit resonance profiles for driving laser-pulse durations much longer than the lifetimes of these autoionizing states. The energy widths of these resonance features are comparable to the widths of the laser pulse and are much smaller than the autoionizing state widths. These results demonstrate an important role for electron correlations in enhancing harmonic-generation rates in the multiphoton regime.
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Jean Marcel Ngoko Djiokap et al.Carrier-envelope-phase-induced asymmetries in double ionization of helium by an intense few-cycle XUV pulse
http://digitalcommons.unl.edu/physicsstarace/201
http://digitalcommons.unl.edu/physicsstarace/201Wed, 13 Nov 2013 14:57:24 PST
The carrier-envelope-phase (CEP) dependence of electron angular distributions in double ionization of He by an arbitrarily polarized, few-cycle, intense XUV pulse is formulated using perturbation theory (PT) in the pulse amplitude. Owing to the broad pulse bandwidth, interference of first- and second-order PT amplitudes produces asymmetric angular distributions sensitive to the CEP. The PT parametrization is shown to be valid by comparing with results of solutions of the full-dimensional, two-electron time-dependent Schrödinger equation for the case of linear polarization.
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Jean Marcel Ngoko Djiokap et al.Use of Attosecond Electron Pulses to Image Electronic
Motion in Atoms and Molecules
http://digitalcommons.unl.edu/physicsstarace/200
http://digitalcommons.unl.edu/physicsstarace/200Fri, 11 Oct 2013 08:45:46 PDT
We investigate theoretically the direct imaging of coherent electronic motion in atoms and molecules using attosecond electron pulses. The theories of time-resolved ultrafast electron diffraction and (e, 2e) momentum spectroscopy as well as the requisite conditions for carrying out time-resolved measurements to obtain timedependent images are discussed. Results of simulations showing images of the motions of coherent superposition states in both the hydrogen atom and the hydrogen molecular ion are shown, thus indicating the capability of ultrafast electron pulses to investigate time-dependent electron dynamics.
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Hua-Chieh Shao et al.