No abstract

We present an implementation of the time-dependent configuration-interaction singles (TDCIS) method for treating atomic strong-field processes. In order to absorb the photoelectron wave packet when it reaches the end of the spatial grid, we add to the exact nonrelativistic many-electron Hamiltonian a radial complex absorbing potential (CAP). We determine the orbitals for the TDCIS calculation by diagonalizing the sum of the Fock operator and the CAP using a flexible pseudospectral grid for the radial degree of freedom and spherical harmonics for the angular degrees of freedom. The CAP is chosen such that the occupied orbitals in the Hartree-Fock ground state remain unaffected. Within TDCIS, the many-electron wave packet is expanded in terms of the Hartree-Fock ground state and its single excitations. The virtual orbitals satisfy nonstandard orthogonality relations, which must be taken into consideration in the calculation of the dipole and Coulomb matrix elements required for the TDCIS equations of motion. We employ a stable propagation scheme derived by second-order finite differencing of the TDCIS equations of motion in the interaction picture and subsequent transformation to the Schrödinger picture. Using the TDCIS wave packet, we calculate the expectation value of the dipole acceleration and the reduced density matrix of the residual ion. The technique implemented will allow one to study electronic channel-coupling effects in strong-field processes.

Elementary processes associated with ionization of liquid water provide a framework for understanding radiation-matter interactions in chemistry and biology. Although numerous studies have been conducted on the dynamics of the hydrated electron, its partner arising from ionization of liquid water, H2O+, remains elusive. We used tunable femtosecond soft x-ray pulses from an x-ray free electron laser to reveal the dynamics of the valence hole created by strong-field ionization and to track the primary proton transfer reaction giving rise to the formation of OH. The isolated resonance associated with the valence hole (H2O+/OH) enabled straightforward detection. Molecular dynamics simulations revealed that the x-ray spectra are sensitive to structural dynamics at the ionization site. We found signatures of hydrated-electron dynamics in the x-ray spectrum.

We introduce a unified and simplified theory of atomic double ionization. Our results show that at high laser intensities (I ≥ 10 14 watts/cm 2 ) purely classical correlation is strong enough to account for all of the main features observed in experiments to date.Short-pulse lasers with high peak intensities (10 14 ≤ I ≤ 10 16 , in watts/cm 2 ) now produce multiphoton generation of double ionization, the two-electron photoelectric effect, with surprising results. To summarize briefly, the experimental data show that two atomic (or molecular) outer-shell electrons are highly correlated when photoejected, with a double ionization rate that can be 1-million times higher than uncorrelated sequential theory [1] allows, so the process is called non-sequential double ionization (NSDI). The first laboratory results were reported in 1992 and 1993 [2,3], showing an anomalously high double ionization yield, the principal experimental signature of NSDI. Additional data is being reported from momentum spectroscopy experiments [4,5,6,7,8,9,10,11]. The momentum distribution data, along with the ion-yield data, serve as the benchmarks for various theoretical models.The mechanism that makes NSDI correlation so effective is far from settled, and theoretical exploration has been extensive [12,13,14,15,16,17]. Almost all existing calculations refer to, or are closely guided by, a single few-step rescattering model [18,19], which is based on an imagined picture in which one electron escapes the atom by quantum tunneling through a fieldlowered barrier and is then phase-coherently and classically forced by the laser away from and then back to the core where a quantum collision liberates both electrons at once (consistent with the term non-sequential). However, the patchwork of ad hoc elements typically employed [20] has not been claimed to make a complete, i.e., self-contained, theory. It is the purpose of this note to show that a self-contained theory exists that is compatible with essentially all prominent features of NSDI.Our theory is dynamically classical, and discards all aspects of quantum mechanics including tunneling. It is built on the need for strong electron correlation to explain NSDI, and so must be intrinsically a two-electron theory. We do not advocate such a theory for an electron that does not have the advantage of a strongly correlated partner. It turns out that entirely classical interactions are adequate to generate very strong two-electron correlation, as observed in NSDI, and quantum theory is not needed. Of course atoms are quantum objects but in such strong fields as are used for NSDI it is mainly electron physics rather than atomic physics that determines the experimental outcome. In this sense the early remark of Corkum [19] advocating the adoption of a plasma per- spective was quite appropriate.We note that a new form of energy analysis is very helpful. The graph in Fig. 1 shows the sum of kinetic energy, electron-nucleus binding energy, e-e correlation energy and laser field interaction energy for each of...

We show that high fluence, high-intensity x-ray pulses from the world's first hard x-ray free-electron laser produce nonlinear phenomena that differ dramatically from the linear x-ray-matter interaction processes that are encountered at synchrotron x-ray sources. We use intense x-ray pulses of sub-10-fs duration to first reveal and subsequently drive the 1s↔2p resonance in singly ionized neon. This photon-driven cycling of an inner-shell electron modifies the Auger decay process, as evidenced by line shape modification. Our work demonstrates the propensity of high-fluence, femtosecond x-ray pulses to alter the target within a single pulse, i.e., to unveil hidden resonances, by cracking open inner shells energetically inaccessible via single-photon absorption, and to consequently trigger damaging electron cascades at unexpectedly low photon energies.