Search

Author(s): Streeter, ZL; Yip, FL; Lucchese, RR; Gervais, B; Rescigno, TN; McCurdy, CW | Abstract: The measurement of the triple differential cross section in the body frame for double photoionization of a molecule can be made in principle by detecting the ionic fragments and the two photoelectrons in coincidence - but only if the dynamics and geometry of dissociation of the doubly charged molecular ion are known. A classical trajectory study of the nine lowest states of the water dication is presented using high quality ab initio potential-energy surfaces. Sampling from a semiclassical initial distribution of positions and momenta is used to approximate ionization from the Frank-Condon region of the ground vibrational state of neutral H2O. Excellent agreement in comparison with preliminary experimental momentum imaging measurements of double photoionization of water show that eight dication states can be unambiguously identified in the experiment with the aid of theory. The theoretical trajectory results allow body frame measurements of double photoionization to yield all eight states even though the usual assumption of direct dissociation, the "axial recoil" approximation, breaks down for three of the dication electronic states seen in the experiment.

The electronic and nuclear dynamics in methanol, following 156~nm photoexcitation, are investigated by combining a detailed analysis of time-resolved photoelectron spectroscopy experiments with electronic structure calculations. The photoexcitation pump pulse is followed by a delayed 260~nm photoionization probe pulse, to produce photoelectrons that are analyzed by velocity map imaging. The yield of mass-resolved ions, measured with similar experimental conditions, are found to exhibit the same time-dependence as specific photoelectron spectral features. Energy-resolved signal onset and decay times are extracted from the measured photoelectron spectra to achieve high temporal resolution, beyond the 20~fs pump and probe pulse durations. When combined with {\it ab initio} calculations of selected cuts through the excited state potential energy surfaces, this information allows the dynamics of the transient excited molecule, which exhibits multiple nuclear and electronic degrees of freedom, to be tracked on its intrinsic few-femtosecond timescale. Within 15~fs of photoexcitation, we observe nuclear motion on the initially bound photoexcited 2$^{1}$A$''$ (S$_2$) electronic state, through a conical intersection with the 1$^{1}$A$'$ (S$_3$) state, which reveals paths to photodissociation following C--O stretch and C--O--H angle opening., 8 pages, 5 figures + Supporting Info

We investigate the photodouble ionization of H2 molecules with 400 eV photons. We find that the emitted electrons do not show any sign of two-center interference fringes in their angular emission distributions if considered separately. In contrast, the quasiparticle consisting of both electrons (i.e., the "dielectron") does. The work highlights the fact that nonlocal effects are embedded everywhere in nature where many-particle processes are involved, This work was funded by the Deutsche Forschungsgemeinschaft (DFG), the BMBF, the European COST Action Grant No. XLIC CM1204, the European Research Council Advanced XCHEM Grant No. 290853, and the MINECO Project Grant No. FIS2013-42002-R. J. L. thanks the DFG for support. We are grateful to the staff of PETRA III for excellent support during the beam time. Raw data are archived at the Goethe-University Frankfurt am Main and are available on request

Double photoionization of the core 1s electrons in atomic beryllium is theoretically studied using a hybrid approach that combines orbital and grid-based representations of the Hamiltonian. The 1S ground state and 1P final state contain a double occupancy of the 2s valence shell in all configurations used to represent the correlated wave function. Triply differential cross sections are evaluated, with particular attention focused on a comparison of the effects of scattering the ejected electrons through the spherically symmetric valence shell with similar cross sections for helium, representing a purely two-electron target with an analogous initial-state configuration, Work partially supported by the MICINN projects FIS2010-15127, ACI2008-0777, and CSD 2007-00010; the ERA-Chemistry project PIM2010EEC- 00751; European grants MC-ITN CORINF and MC-RG ATTOTREND; and the European COST Action CM0702. F.L.Y. acknowledges support from Extesp-MICINN Modalidad B. Work at the Lawrence Berkeley National Laboratory was performed under the auspices of the US DOE under Contract No. DE-AC02-05CH11231 and was supported by the US DOE Office of Basic Energy Sciences, Division of Chemical Sciences

We present the results of ab initio calculations for elastic electron scattering by tetrahydrofuran (THF) using the complex Kohn variational method. We carried out fixed-nuclei calculations at the equilibrium geometry of the target molecule for incident electron energies up to 20 eV. The calculated momentum transfer cross sections clearly reveal the presence of broad shape resonance behavior in the 8-10 eV energy range, in agreement with recent experiments. The calculated differential cross sections at 20 eV, which include the effects of the long-range electron-dipole interaction, are alsofound to be in agreement with the most recent experimental findings.

We describe a simple model for electron-molecule collisions that has one nuclear and one electronic degree of freedom and that can be solved to arbitrarily high precision, without making the Born-Oppenheimer approximation, by employing a combination of the exterior complex scaling method and a finite-element implementation of the discrete variable representation. We compare exact cross sections for vibrational excitation and dissociative attachment with results obtained using the local complex potential approximation as commonly applied in the "boomerang" model, and suggest how this two-dimensional model can be used to test the underpinnings of contemporary nonlocal approximations to resonant collisions.

A method is described for computing scattering amplitudes for a cylindrically symmetric potential using square-integrable basis functions. The addition of variational corrections using the Kato formula is also outlined. Results for a two-center Gaussian potential are presented.

We show that the minimization of the variance integral provides a method for the determination of scattering wave functions which uses discrete basis functions exclusively. By using a separable representation of the scattering potential only one new class of matrix elements appears in the evaluation of the variance integral which is not already required in the diagonalization of the Hamiltonian. The choice of Gaussian basis functions for the expansion of the scattering wave function should make the method particularly applicable to electron-molecule scattering. Some advantages and limitations of the method are discussed.

Theoretical investigations of photoexcitation and ionization cross sections in molecular nitrogen are reported employing the recently devised Stieltjes–Tchebycheff moment-theory technique in the static-exchange approximation. The coupled-channel equations for photoabsorption are separated approximately by identifying the important physically distinct excitation processes associated with formation of the three lowest electronic states of the parent molecular ion. Approximate Rydberg series and pseudospectra of transition frequencies and oscillator strengths are constructed for the seven individual channel components identified using Hartree–Fock ionic core functions and normalizable Gaussian orbitals to describe the photoexcited and ejected electrons. Detailed comparisons of the theoretically determined discrete excitation series with available spectral data indicate general accord between the calculated and observed excitation frequencies and oscillator strengths, although there are some discrepancies and certain Rydberg series have apparently not yet been identified in the measured spectra. The total Stieltjes–Tchebycheff vertical photoionization cross section obtained from the discrete pseudospectra is in excellent agreement with recent electron–ion coincidence measurement of the cross section for parent–ion production from threshold to 50 eV excitation energy. Similarly, the calculated vertical partial cross sections for the production of the three lowest electronic states in the parent molecular ion are in excellent accord with the results of recent electron–electron coincidence and synchrotron–radiation branching ratio measurements. The origins of particularly intense resonancelike features in the discrete and continuum portions of the photoabsorption cross sections are discussed in terms of excitations into valencelike molecular orbitals. Small discrepancies between theory and experiment are attributed to specific autoionization processes and channel couplings not included in the calculations. In contrast to previously reported model or local potential studies, the present results employ the full nonlocal and nonspherical molecular Fock potential in ab initio photoabsorption calculations. The excellent agreement obtained between theory and experiment in molecular nitrogen suggests that highly reliable photoabsorption cross sections for diatomic molecules can be obtained from Hilbert space calculations and the Stieltjes–Tchebycheff method in the static-exchange approximation under appropriate conditions.