Ionization and transient absorption control with a resonant attosecond clock

Metastable states are important actors in the ionisation of atoms and molecules. Sub-femtosecond extreme ultraviolet pulses can coherently populate several transiently bound states at once, thus starting the attosecond clocks which are required to monitor and control ultrafast electronic evolution above the ionisation threshold. Here we illustrate, from a theoretical point of view, the effects coherent superpositions of 1 P o doubly excited states in the helium atom have on channel-resolved photoelectron spectra as well as on the transient absorption spectrum of the atom in the extreme ultraviolet region, when they are created by a single-attosecond pulse in the presence of a strong few-cycle near-infrared/visible pulse which acts as a probe. Interference fringes varying rapidly with the pump-probe time delay are visible in both photoelectron and transient absorption spectra. From such fringes, the wave packet itself can conceivably be reconstructed. Conversely, all observables are modulated by the characteristic beating periods of the wave packet, so that control of partial ionisation yields, branching ratios, and light absorption or amplification can be achievedThe research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement No 290853, the European COST Actions CM0702 and CM1204, the ERA-Chemistry project No PIM2010EEC-00751, the Marie Curie ITN CORINF, and the MICINN projects No.s FIS2010-15127 and CSD 2007-00010 (Spain

Two-photon finite-pulse model for resonant transitions in attosecond experiments

We present an analytical model capable of describing two-photon ionization of atoms with attosecond pulses in the presence of intermediate and final isolated autoionizing states. The model is based on the finite-pulse formulation of second-order time-dependent perturbation theory. It approximates the intermediate and final states with Fano's theory for resonant continua, and it depends on a small set of atomic parameters that can either be obtained from separate \emph{ab initio} calculations, or be extracted from few selected experiments. We use the model to compute the two-photon resonant photoelectron spectrum of helium below the N=2 threshold for the RABITT (Reconstruction of Attosecond Beating by Interference of Two-photon Transitions) pump-probe scheme, in which an XUV attosecond pulse train is used in association to a weak IR probe, obtaining results in quantitative agreement with those from accurate \emph{ab initio} simulations. In particular, we show that: i) Use of finite pulses results in a homogeneous red shift of the RABITT beating frequency, as well as a resonant modulation of the beating frequency in proximity of intermediate autoionizing states; ii) The phase of resonant two-photon amplitudes generally experiences a continuous excursion as a function of the intermediate detuning, with either zero or $2\pi$ overall variation.Comment: 23 pages, 13 figure

Modulation of attosecond beating in resonant two-photon ionization

We present a theoretical study of the photoelectron attosecond beating at the basis of RABBIT (Reconstruction of Attosecond Beating By Interference of Two-photon transitions) in the presence of autoionizing states. We show that, as a harmonic traverses a resonance, its sidebands exhibit a peaked phase shift as well as a modulation of the beating frequency itself. Furthermore, the beating between two resonant paths persists even when the pump and the probe pulses do not overlap, thus providing a sensitive non-holographic interferometric means to reconstruct coherent metastable wave packets. We characterize these phenomena quantitatively with a general finite-pulse analytical model that accounts for the effect of both intermediate and final resonances on two-photon processes, at a negligible computational cost. The model predictions are in excellent agreement with those of accurate ab initio calculations for the helium atom in the region of the N=2 doubly excited states

Autoionizing states of atomic boron

We present a B-spline K-matrix method for three-active-electron atoms in the presence of a polarizable core, with which it is possible to compute multichannel single-ionization scattering states with good accuracy. We illustrate the capabilities of the method by computing the parameters of several autoionizing states of the boron atom, with 2Se, 2Pe,o, and 2De symmetry, up to at least the 2p2 (1S) excitation threshold of the B II parent ion, as well as selected portions of the photoionization cross section from the ground state. Our results exhibit remarkable gauge consistency, they significantly extend the existing sparse record of data for the boron atom, and they are in good agreement with the few experimental and theoretical data available in the literature. These results open the way to extend to three-active-electron systems the spectral analysis of correlated wave packets in terms of accurate scattering states that has already been demonstrated for two-electron atoms in Argenti and LindrothL.A. acknowledges support from the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC grant XCHEM 290853, the MINECO project no. FIS2013-42002-R, the ERA-Chemistry Project PIM2010EEC-00751, the European grant MC-ITN CORINF and the European COST Action XLIC CM1204

Hybrid Gaussian-B-spline basis for the electronic continuum: Photoionization of atomic hydrogen

As a first step towards meeting the recent demand for new computational tools capable of reproducing molecular-ionization continua in a wide energy range, we introduce a hybrid Gaussian-B-spline basis (GABS) that combines short-range Gaussian functions, compatible with standard quantum-chemistry computational codes, with B splines, a basis appropriate to represent electronic continua. We illustrate the performance of the GABS hybrid basis for the hydrogen atom by solving both the time-independent and the time-dependent Schrödinger equation for a few representative cases. The results are in excellent agreement with those obtained with a purely B-spline basis, with analytical results, when available, and with recent above-threshold ionization spectra from the literature. In the latter case, we report fully differential photoelectron distributions which offer further insight into the process of above-threshold ionization at different wavelengthsWork supported by the European Research Council under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC Grant Agreement No. 290853, European COST Action No. CM1204 XLIC, and MICINN Project No. FIS2010-1512

Investigation of the ionization of neon by an attosecond XUV pulse with the time-dependent Schrödinger equation

We investigate theoretically the single ionization of neon by an attosecond XUV pulse, aiming at a better understanding of the outgoing electron wave-packet in the early stages of its detachment. To do so, we integrate the one-electron time-dependent Schrödinger equation numerically. The non-local interaction with the spectator electrons in the time-dependent hamiltonian is accounted for with a configuration-averaged effective Hartree-Fock potentia

The B-spline K-matrix Method in Atomic Physics

In the course of this thesis the B-spline K-matrix method, a theoretical technique capable of reproducing the single ionization continuum of atoms, was developed. Two systems were addressed in particular: helium and boron, as representatives of two- and three-active-electron atoms. Some of the findings presented here resulted in original cont ributions to the scientific literature. Total and partial photoionization cros s sections and asymmetry parameters of the fundamental helium state were examined up to the sixth ionization threshold, yielding the first ab initio reproduction of the first intruder state effects below N=4 threshold, the first reproduction of dipole asymmetry parameters below N=6 threshold and of nondipole anisotropy param eter \u3b3 below N=2 threshold. Most of the material presented here relies on the multiple ba sis implementation of the B-spline K-matrix method which allows the reprod uction of almost arbitrarily excited metastable satellites below a prescri bed threshold. The new technique, when used to investigate helium triplet states, yielded the most accurate and extensive existing characterization of triplet me tastable states up to the fifth ionization threshold. Within more than 1700 natural and unnatural S, P and D doubly excited states, eleven intruder states were discov ered, entirely unknown before. At least two experimental groups, at ELETTRA in Trieste and at BESSY II in Berlin, are recently tackling the problem of measuring the m etastable 2^3S helium photoionization cross section. We therefore undertook a parallel theoretical investigation of the photoionization process of both the fundame ntal and the excited ^3S helium states. The latter prelude to the future investigati on of the radiative decay of doubly excited states and already revealed interesting p eculiar features. To this purpose, we devised an extension of the K-matrix method to treat the atom-radiation interaction non perturbatively. A general three electron package has been developed and appl ied to study boron resonances. Specific formulas were derived to obtain arbitrary tensorial one-particle and scalar two-particle matrix elements between three electron states on non-orthogonal basis as required for an efficient exploitation of B-splines. A detailed study of the B-spline effective completeness led to a general result which assure that B-spline based methods are well conditioned for a large class of knot grids