Double-electron recombination in high-order-harmonic generation driven by spatially inhomogeneous fields

Abstract

We present theoretical studies of high-order harmonic generation (HHG) driven by plasmonic fields in two-electron atomic systems. Comparing the two-active electron and single-active electron approximation models of the negative hydrogen ion atom, we provide strong evidence that a double non-sequential two-electron recombination appears to be the main responsible for the HHG cutoff extension. Our analysis is carried out by means of a reduced one-dimensional numerical integration of the two-electron time-dependent Schrödinger equation (TDSE), and on investigations of the classical electron trajectories resulting from the Newton's equation of motion. Additional comparisons between the negative hydrogen ion and the helium atom suggest that the double recombination process depends distinctly on the atomic target. Our research paves the way to the understanding of strong field processes in multi-electronic systems driven by spatially inhomogeneous fields.PACS numbers: 42.65. Ky, 32.80.Rm, 33.20.Xx, 32.80 Qk, 42.50 Ct The incessant development of ultrafast, femtosecond (10 −15 fs) laser technology in the infrared (IR) regime opened new avenues to study a wide range of strong-field laser matter processes at their natural time scale [1][2][3]. These invaluable experimental and technological tools allowed physicists to address instrumental aspects of one of the most fundamental processes: the tunneling ionization of atoms and molecules [3]. In particular, the application of this laser technology provided a key factor for the understanding of the main mechanisms underlying the emission of coherent radiation from atoms or molecules [2, 4-6].As a matter of fact, one could say the high-order harmonic generation (HHG) process fits within the tunneling ionization regime, when the Keldysh parameter, defined by γ = Ip 2Up , is γ ≤ 1. I p and U p = E 2 0 4ω 2 0 denote here the ionization potential of the atomic target, and the electron ponderomotive potential energy in atomic units, respectively. E 0 is the peak amplitude of the laser electric field and ω 0 the carrier frequency. The so-called three-step or "simple man's" picture describes the underlying physics behind the HHG phenomenon [5]. In the first step, occurring about the maximum of the strong laser electric field, the Coulomb potential is deformed in such a way that a potential barrier is formed. Then, the electron is able to tunnel out throughout this "atomic barrier", and the atom is then ionized. In the second step, or better to say phase, once the electron is in the continuum, the electric field of the laser accelerates it. Naturally, the electron gains energy from the oscillating field, converting it into a kinetic energy. Consequently, when the electric field changes its sign, the electron reverses the direction of its motion and has a certain probability to recombine back to the ground state of the remaining ion-core. In this third step it emits its energy excess as an attosecond burst of coherent radiation, typically in the XUV or EUV spectral range. In partic...

Double-electron ionization driven by inhomogeneous fields

“…At this very low inhomogeneity degree of β = 0.005 a.u., and low IR peak intensity, this enhancement of the 2e-ionization rate is a very surprising result. Similar behaviour was previously observed in [15], where the double-electron ionization reaches higher yields leading to an enhancement in the intensity of the HHG signal. However, in that latter case a larger inhomogeneity degree of β = 0.02 a.u was used.…”

“…[14] for more details). However, since recent studies of post-ionization dynamics in spatially inhomogeneous fields [15] provides new physical effects and insights, a question arises as to the influence of spatial variation on the DI process. The aim of this work is to present a complete study of DI driven by plasmonic-enhanced spatially inhomogeneous fields with an investigation of NSDI in general, and the RESI and RIDI mechanisms in particular.…”

Attosecond physics at the nanoscale

“…To this end we employ the numerical solutions of the reduced 1D×1D-TDSE for both the two-active electron (TAE) and the single active electron (SAE) models. From these models, we plan to trace out the analogies and differences in the HHG process from these two atomic systems, a priori very similar in their intrinsic structure (Chacón et al, 2015b).…”

“…For instance, the behaviour of complex systems, e.g. multielectronic atoms and molecules, under the influence of spatial inhomogeneous fields is an unexplored area -only few attempts to tackle this problem has been recently reported (Chacón et al, 2015b;Yavuz et al, 2016Yavuz et al, , 2015. In addition, and just to name another example, it was recently demonstrated that Rydberg atoms could be a plausible alternative as a driven media (Tikman et al, 2016).…”

21st Century Nanoscience – A Handbook

“…For instance, the behaviour of complex systems, e.g. multielectronic atoms and molecules, under the influence of spatial inhomogeneous fields is an unexplored area -only few attempts to tackle this problem has been recently reported [97,137,138]. In addition, and just to name another example, it was recently demonstrated that Rydberg atoms could be a plausible alternative as a driven media [139].…”