Investigaciónhttp://hdl.handle.net/10486/1290792017-10-18T22:12:46Z2017-10-18T22:12:46ZAttosecond transient absorption spectroscopy of helium above the N=2 ionization thresholdPetersson, C. L.M.Argenti, LucaMartín, Fernandohttp://hdl.handle.net/10486/6798802017-10-18T20:57:36Z2017-07-06T00:00:00ZAttosecond transient absorption spectroscopy of helium above the N=2 ionization threshold
Petersson, C. L.M.; Argenti, Luca; Martín, Fernando
Attosecond transient absorption spectroscopy (ATAS) allows for the study of electron dynamics in atoms and molecules with attosecond time resolution. Previous works reported in the literature have made use of ATAS to image and control such dynamics in the single-channel ionization continuum of helium; in particular, in the vicinity of the doubly excited autoionizing states lying between the N=1 and N=2 thresholds. In this work, we have extended these studies to autoionizing states lying above the N=2 threshold, where several ionization channels are open. From an accurate solution of the time-dependent Schrödinger equation, we predict the appearance of pronounced one-photon beatings between the 3snp states and the adjacent 1Se and 1De resonances, as well as, more surprisingly, two-photon beatings between the 3s3p doubly excited state and the 1Po nonresonant continuum. Both effects lead to a significant distortion of the 3snp Fano profiles and to a strong variation of these profiles with the pump-probe delay, thus demonstrating control of the corresponding multichannel two-electron correlated wave packets, in the same way as reported for resonances lying below the N=2 threshold
2017-07-06T00:00:00ZControl of photoemission delay in resonant two-photon transitionsArgenti, L.Jiménez-GalánCaillat, J.Taïeb, R.Maquet, A.Martín, F.http://hdl.handle.net/10486/6798792017-10-18T20:46:57Z2017-04-26T00:00:00ZControl of photoemission delay in resonant two-photon transitions
Argenti, L.; Jiménez-Galán; Caillat, J.; Taïeb, R.; Maquet, A.; Martín, F.
The photoelectron emission time delay τ associated with one-photon absorption, which coincides with half the Wigner delay τW experienced by an electron scattered off the ionic potential, is a fundamental descriptor of the photoelectric effect. Although it is hard to access directly from experiment, it is possible to infer it from the time delay of two-photon transitions, τ(2), measured with attosecond pump-probe schemes, provided that the contribution of the probe stage can be factored out. In the absence of resonances, τ can be expressed as the energy derivative of the one-photon ionization amplitude phase, τ=∂EargDEg, and, to a good approximation, τ=τ(2)-τcc, where τcc is associated with the dipole transition between Coulomb functions. Here we show that, in the presence of a resonance, the correspondence between τ and ∂EargDEg is lost. Furthermore, while τ(2) can still be written as the energy derivative of the two-photon ionization amplitude phase, ∂EargDEg(2), it does not have any scattering counterpart. Indeed, τ(2) can be much larger than the lifetime of an intermediate resonance in the two-photon process or more negative than the lower bound imposed on scattering delays by causality. Finally, we show that τ(2) is controlled by the frequency of the probe pulse, ωIR, so that by varying ωIR, it is possible to radically alter the photoelectron group delay
2017-04-26T00:00:00ZImaging ultrafast molecular wave packets with a single chirped UV pulseJelovina, DenisFeist, JohannesMartín, FernandoPalacios, Aliciahttp://hdl.handle.net/10486/6798782017-10-18T20:50:33Z2017-04-25T00:00:00ZImaging ultrafast molecular wave packets with a single chirped UV pulse
Jelovina, Denis; Feist, Johannes; Martín, Fernando; Palacios, Alicia
We show how to emulate a conventional pump-probe scheme using a single frequency-chirped ultrashort UV pulse to obtain a time-resolved image of molecular ultrafast dynamics. The chirp introduces a spectral phase in time that encodes the delay between the pump and the probe frequencies contained in the pulse. By comparing the results of full dimensional ab initio calculations for the H2+ molecule with those of a simple sequential model, we demonstrate that, by tuning the chirp parameter, two-photon energy-differential ionization probabilities directly map the wave-packet dynamics generated in the molecule. As a result, one can also achieve a significant amount of control of the total ionization yields, with a possible enhancement by more than an order of magnitude
2017-04-25T00:00:00ZScreening effects on the electronic structure of the hydrogen molecular ionOrdóñez-Lasso, Andrés FelipeMartín, FernandoSanz-Vicario, José Luishttp://hdl.handle.net/10486/6798772017-10-18T20:46:50Z2017-01-20T00:00:00ZScreening effects on the electronic structure of the hydrogen molecular ion
Ordóñez-Lasso, Andrés Felipe; Martín, Fernando; Sanz-Vicario, José Luis
We study the effect that a statically screened Coulomb potential represented by a Debye-Hückel-Yukawa potential has in the electronic structure of the simplest molecule H2+ within the Born-Oppenheimer approximation. The method of solution is based on a two-center partial-wave expansion expressed in confocal elliptic coordinates using B-spline polynomials. General algorithms for the computation of energies, wave functions, and dipole and nonadiabatic radial matrix elements are given in detail. As it occurs in atoms, screening in simple molecules shifts the energies of bound states upwards so that, as screening increases, every bound state eventually crosses the upper ionization threshold at a critical screening value. The loss of long-range Coulomb interactions has its effect in the structure of wave functions, and consequently in the dipole and nonadiabatic matrix elements at intermediate and long internuclear distances, which determine the dynamics in external electromagnetic fields and collisional processes. Other issues related to a practical solution of the arbitrary sign problem, as well as the assignment of angular and radial nodes to the variational eigenfunctions, and the appearance of molecular shape resonances and Borromean states in H2+ as screening increases, are also addressed in this work
2017-01-20T00:00:00Z