Photoionization Of He Research Articles

He i 10830 Å Dimming during Solar Flares. I. The Crucial Role of Nonthermal Collisional Ionizations

Abstract While solar flares are predominantly characterized by an intense broadband enhancement to the solar radiative output, certain spectral lines and continua will, in theory, exhibit flare-induced dimmings. Observations of transitions of orthohelium He i λ λ 10830 Å and the He i D3 lines have shown evidence of such dimming, usually followed by enhanced emission. It has been suggested that nonthermal collisional ionization of helium by an electron beam, followed by recombinations to orthohelium, is responsible for overpopulating those levels, leading to stronger absorption. However, it has not been possible observationally to preclude the possibility of overpopulating orthohelium via enhanced photoionization of He i by EUV irradiance from the flaring corona followed by recombinations. Here we present radiation hydrodynamics simulations of nonthermal electron-beam-driven flares where (1) both nonthermal collisional ionization of helium and coronal irradiance are included, and (2) only coronal irradiance is included. A grid of simulations covering a range of total energies deposited by the electron beam and a range of nonthermal electron-beam low-energy cutoff values were simulated. In order to obtain flare-induced dimming of the He i 10830 Å line, it was necessary for nonthermal collisional ionization to be present. The effect was more prominent in flares with larger low-energy cutoff values and longer lived in weaker flares and flares with a more gradual energy deposition timescale. These results demonstrate the usefulness of orthohelium line emission as a diagnostic of flare energy transport.

Studies of H2 photoionization by a strong ultrashort x-ray pulse on base of a trajectory method

Photoionization of the hydrogen molecule was studied using our previously developed semiclassical trajectory method. The focus was on the parameters of the field for which there are no other results, namely a short photo-pulse with photon energies from soft to hard x-rays and with intensities including superatomic values. We calculated photoionization probabilities for the single and double ionization of H2. In the superatomic field we observed the transient from single-photon to multiphoton double photoionization, the inversion of single and double ionization probabilities, and distinct stabilization effect for the single photoionization. We also observed a specific behavior in the single photoionization probability dynamics with a dip in the middle of the pulse, which becomes extremely deep for a field approaching relativistic intensity. The agreement of our previous results with independent data on the helium atom in superatomic field is an important argument in favor of the efficiency of the used method. This allows us to hope that the probabilities of photoionization of a hydrogen molecule in an atomic and superatomic x-ray field obtained in this work are not only the first, but also quite reasonable results on the indicated process.

Photoionization of H2 using the molecular R-matrix with time approach

SynopsisWe present results of the first calculations using the variational ab initio molecular R-matrix with time approach. We have calculated two and four-photon ionization cross sections for H2 and studied the effects of electron correlation and choice of the Gaussian atomic basis sets. Our results are compared with earlier calculations.

H2 photoionization induced by XUV pulses and X-ray free electron lasers

SynopsisWe present a theoretical study on H2 photionization taking into account coupled electron-nuclear motion and non-dipole effects. In the first part we study the effects of harmonic filtering in a pump-probe scheme using attosecond train pulses (ATP) in dipole approximation. In the second part we investigate nondipole effects in XUV and soft X-ray regimes.

Parametrization of energy sharing distributions in direct double photoionization of He

We present experimental results on the characteristic sharing of available excess energy, ranging from 11–221 eV, between two electrons in single-photon direct double ionization of He. An effective parametrization of the sharing distributions is presented along with an empirical model that describes the complete shape of the distribution based on a single experimentally determinable parameter. The measured total energy sharing distributions are separated into two distributions representing the shake-off and knock-out parts by simulating the sharing distribution curves expected from a pure wave collapse after a sudden removal of the primary electron. In this way, empirical knock-out distributions are extracted and both the shake-off and knock-out distributions are parametrized. These results suggest a simple method that can be applied to other atomic and molecular systems to experimentally study important aspects of the direct double ionization process.

Doubly resonant photoionization of He below the N = 2 ionization threshold

We present an extended theoretical study of two-colour doubly resonant photoionization of helium involving two doubly excited states below the N = 2 ionization threshold. We model the effects of a strong laser coupling between the lowest 1Po and the selected 1Se, 1Pe and 1De doubly excited states on the photoelectron (photoion) yield by taking into account the spatio-temporal structure of the pulses and by considering the effects of the neighbouring resonance states. We show that the laser coupling results in a measurable modification of the line profiles and in a reduction of the photoelectron yield under resonant or near-resonant conditions.

Dynamic interference in the photoionization of He by coherent intense high-frequency laser pulses: Direct propagation of the two-electron wave packets on large spatial grids

The direct ionization of the helium atom by intense coherent high-frequency short laser pulses is investigated theoretically from first principles. To this end, we solve numerically the time-dependent Schr\odinger equation for the two-electron wave packet and its interaction with the linearly polarized pulse by the efficient time-dependent restricted-active-space configuration-interaction method (TD-RASCI). In particular, we consider photon energies which are nearly resonant for the $1s\ensuremath{\rightarrow}2p$ excitation in the ${\mathrm{He}}^{+}$ ion. Thereby, we investigate the dynamic interference of the photoelectrons of the same kinetic energy emitted at different times along the pulse in the two-electron system. In order to enable observation of the dynamic interference in the computed spectrum, the electron wave packets were propagated on large spatial grids over long times. The computed photoionization spectra of He exhibit pronounced interference patterns the complexity of which increases with the decrease of the photon energy detuning and with the increase of the pulse intensity. Our numerical results pave the way for experimental verification of the dynamic interference effect at presently available high-frequency laser pulse sources.

Photoelectron sidebands induced by a chirped laser field for shot-by-shot temporal characterization of FEL pulses

We theoretically investigate the laser-assisted photoionization of He by an extreme ultra violet (XUV) pulse in the presence of a linearly chirped intense laser pulse by solving the time-dependent Schrödinger equation within the single-active-electron approximation. Analysis based on the time-dependent perturbation theory is also carried out to provide more physical insights. A new scheme is shown to be capable of extracting the arrival time of an XUV free-electron laser (FEL) pulse relative to an external laser pulse as well as the XUV pulse duration from the photoelectron sidebands resulting from XUV ionization in the presence of a chirped laser pulse. This scheme is independent of the energy fluctuation and the timing jittering of the FEL pulse. Therefore it can be implemented in a non-invasive way to characterize FEL pulses on a shot-by-shot basis in time-resolved spectroscopy.

Application of R-Matrix with Time-dependence Theory to Double Ionisation Using a 2-electron Outer Region

SynopsisR-Matrix with Time-dependence (RMT) theory has been extended to cover double-ionisation processes. An application to photoionisation of He is demonstrated, with an emphasis on double-ionisation cross sections.

Reply to comment on ‘Time-resolved resonant photoionization of He using a time-dependent Feshbach method with ultrashort laser pulses’

In this reply we amend the effect of misusing an analytical formula derived in 2007 Phys. Rev. A 75 013407, but containing a misprint subsequently corrected in a recently published erratum, 2013 Phys. Rev. A 87 069905(E). Two new corrected figures are included in this reply with the gradual formation of the resonance Fano profile for the lowest and third-lowest He 1Po resonant states. The comparison between our ab initio time-dependent Feshbach method and the analytical perturbative formula is rather good in the temporal domain for the time sequence shown during and after the laser pulse up to 50 fs, although still far from reaching the final asymptotic form. Other comments are also replied to. Finally, we emphasize that in the LabTalk published on 4 June 2013, we do not mention the inadequacy of any analytical formula whatsoever and the figure caption clearly refers to the asymptotic form of the Fano profile.

Comment on ‘Time-resolved resonant photoionization of He using a time-dependent Feshbach method with ultrashort laser pulses’

We compare the contents of the recent paper by Granados-Castro and Sanz-Vicario (2013 J.Phys. B 46 055601) to our previously published work on the same problem (Mercouris, Komninos and Nicolaides 2007 Phys. Rev. A 75 013407; 2013 Phys. Rev. A 87 069905(E)).

Time-resolved resonant photoionization of He using a time-dependent Feshbach method with ultrashort laser pulses

We study the resonant photoionization of the Helium atom subject to ultrashort laser pulses by using a Feshbach formalism in the time domain. We solve the projected time-dependent Schrodinger equation in terms of a configuration interaction spectral method, with a total wave function expanded with configurations defined within bound-like () and scattering-like () halfspaces. The method allows for accurate descriptions of both the atomic structure (energy positions and widths) as well as for the resonant photodynamics using ultrashort laser pulses. Special attention is given to the temporal formation of Fano profiles in the one- and two-photon ionization cross sections.

Time-dependent Feshbach method to study resonant photoionization of He with ultrashort laser pulses

A time-dependent Feshbach formalism is proposed to study the resonant photoionization of the helium atom using ultrashort laser pulses. This spectral method consist in solving the time-dependent Schrodinger equation by expanding the time-dependent wavepacket in terms of eigenfunctions defined in two orthogonal halfspaces: a bound-like resonant and a non-resonant scattering-like . The latter eigenfunctions for the projected Hamiltonians H and H are not indeed eigenfunctions of the total Hamiltonian, so that the electrostatic coupling H acts as a leaking operator → responsible for the temporal decay of resonances into the underlying continuum, keeping the physical insight of the Fano-Feshbach time independent formalisms. This method allows not only for accurate descriptions of the resonance parameters (energies, widths and Fano shape parameters) but also for the temporal evolution of the photodynamics involved in the resonant photoionization when using short laser pulses. We illustrate the performance of the method by analyzing the temporal formation of i) the one-photon ionization cross section below the second ionization threshold and the buildup of Fano profiles and ii) the up-down asymmetry of photoelectron angular distributions resulting from interferences of S-, P- and D-waves after simultaneous photoexcitation and decay of the lowest 1Se, 1P° and 1De resonant states, by using two sequential laser pulses with XUV harmonic frequencies separated by a time delay.

Autoionization of Molecular Hydrogen: Where do the Fano Lineshapes Go?

Atomic autoionization following photoabsorption is a typical example of quantum interferences governed by electron-electron correlation. Coherence between direct photoionization and autoionization paths results in "Fano profiles", widely explored in atoms in the last 60 years. The advent of femto- and attosecond laser technology made time-resolved images of the delayed electron ejection in autoionization accessible, leading to the reemergence of such studies in atomic systems. The counterpart molecular phenomena show the richness, as well as the complexity, added by nuclear motion, which may proceed on similar time scales. However, Fano profiles are usually absent in measured molecular photoionization cross sections and an unequivocal parametrization of molecular autoionization signatures, similar to that introduced by Fano in atoms [U. Fano, Phys. Rev. 1961, 124, 1866] has not yet been achieved. In this work we introduce a simple semiclassical model that accounts for all the features observed in H2 photoionization and demonstrate that the interference structures observed in dissociative ionization spectra are almost exclusively due to the phase accumulated in the nuclear motion. Furthermore, we show that the temporal build-up of these structures in the energy-differential cross sections is also determined by nuclear motion. We validate our models by comparing with full-dimensional ab initio calculations solving the time-dependent Schrödinger equation.

Time-resolved resonant photoionization of He using a time-dependent Feshbach method with ultrashort laser pulses

We study the photoionization and autoionization of the helium atom subject to ultrashort laser pulses by using a Feshbach formalism in the time domain. We solve the time-dependent Schrödinger equation in terms of a configuration interaction (CI) spectral method, in which the total wavefunction is expanded with configurations defined within bound-like () and scattering-like () halfspaces. The method allows one to provide accurate descriptions of both the atomic structure (energy positions and widths) and the photodynamics. We illustrate our approach by (i) calculating the time-resolved one-photon ionization below the He+ (n = 2) ionization threshold, from 11Se and 21Po initial states, then reaching the lowest autoionizing states of 1Se, 1Po and 1De final symmetries, (ii) studying the temporal formation of the Fano profile of 1Po resonances and (iii) showing its performance in obtaining the perturbative long-time limit of one- and two-photon ionization cross sections using ultrashort laser pulses following a recently developed procedure in Palacios et al (2008 Phys. Rev. A 77 032716).

Sensitivity of cross sections to the diffuseness of the confining potential in time-dependent close-coupling calculations of the double photoionization of He@C60

Calculations of the photoionization of endofullerenes often use a square-well potential to represent the confining fullerene. The sensitivity of the photoionization cross sections to the form of this model potential is explored here by the use of a diffuse potential. The cross sections for single photoionization with excitation and double photoionization of He@C${}_{60}$ are found to show damping of the confinement resonance structure as the degree of diffuseness is increased using a commonly adopted well depth of 8.22 eV. For a deeper well depth of 11.00 eV, the double-photoionization cross section is found to be insensitive to the diffuseness of the confining potential, as has also been reported recently for the single photoionization of H@C${}_{60}$ and Xe@C${}_{60}$ [Dolmatov, King, and Oglesby, J. Phys. B 45, 105102 (2012)].

Near-threshold photoelectron angular distributions from two-photon resonant photoionization of He

Two-photon resonant photoionization of helium is investigated both experimentally and theoretically. Ground state helium atoms are excited to the 1s4p, 1s5p and 1s6p 1P states by synchrotron radiation and ionized by a synchronized infrared pulsed picosecond laser. The photoelectron angular distributions of the emitted electrons are measured using a velocity map imaging (VMI) spectrometer. The measured asymmetry parameters of the angular distribution allow the phase differences and the ratios of the dipole matrix elements of the 1sϵs and 1sϵd channels to be determined. The experimental results agree with the calculated values obtained in a configuration–interaction calculation with a Coulomb–Sturmian basis set. The effects of the radiative decay of the intermediate state and the static electric field of the VMI spectrometer on the measurements are discussed.

Circular Dichrosim in Photoionization of H2 and D2

In this work, circular dichroism in H2 (D2) photoionization is studied in detail. We have selected several photon energies for a case study: 19 eV for which only direct ionization to the 1sσg ionization channel is present, 27 eV where autoionization of Q1 1Σ+u doubly excited states takes place, and 32.5 eV for which autoionization from Q1 and doubly excited states and direct ionization to 1sσg and 2pσu channels strongly interfere. The latter case shows clear evidence of different behavior of the photoionization against radiation helicity.

Experimental and theoretical asymmetry parameters for photoionization of H2 showing interference from the Q1 and Q2 doubly excited states

The photoelectron asymmetry parameter for single photon ionization of H2 between hν = 31 eV and 35 eV has been measured. At these photon energies the doubly excited Q1 and Q2 states are energetically accessible and lie within the Franck-Condon absorption range. For electron energies greater than ~6 eV, the β parameter oscillates strongly, mainly due to interferences in autoionization decay of the Q1 1Σu+ and Q2 1Πu doubly excited states. The experimental results are compared with close coupling calculations.

Two-center interference in photoionization of H2+ in circularly polarized laser fields

We theoretically investigate the strong-field ionization of H2+ in circularly polarized laser fields by calculating the photoelectron energy spectra and the logarithm of the differential ionization rate. In circular polarization laser fields, if the emission directions of a photoelectron are averaged, the photoelectron energy spectra usually have a bell shape and a single broad peak at the middle-energy region (Ren et al. [Phys. Rev. A78, 043411 (2008)]). In this paper, our theoretical results show that when the emission directions of a photoelectron are not averaged, the photoelectron energy spectrum has a complex double-peak or a multipeak structure with interference valleys. The locations of the interference valleys are independent of the laser intensity, but dependent on the emission direction and final kinetic energy of the photoelectron. In order to completely understand the interference effect, we also carry out the calculations for the logarithm of the differential ionization rate. We find that the interference light and dark stripes alternate with the emission directions and kinetic energies of the photoelectron: if Pf·R≃2kπ (k=0,1,2,3,⋯), the interference stripes are light; if Pf·R≃(2k+1)π (k=0,1,2,3,⋯), the interference stripes are dark. No matter what the kinetic energies of the photoelectron are, for the emission direction of φf=π/2 the stripes are always light.