Reconstruction and control of a time-dependent two-electron wave packet

Abstract

Esta es la versión de autor del artículo publicado en: This is an author produced version of a paper published in: The concerted motion of two or more bound electrons governs atomic 1 and molecular 2,3 non-equilibrium processes and chemical reactions. It is thus a long-standing scientific dream to measure and control the dynamics of two bound and correlated electrons in the quantum regime. At least two active electrons and a nucleus are required to address such quantum three-body problem 4 for which analytical solutions do not exist, a condition that is met in the helium atom. While attosecond dynamics were previously observed for singleactive electron/hole cases 5-7 , such time-resolved observation of two-electron motion thus far remained an unaccomplished challenge. Here, we measure a 1.2-femtosecond quantum beat among low-lying doubly-excited states in helium and use it to reconstruct a correlated two-electron wave packet. Our experimental method combines attosecond transientabsorption spectroscopy 5,7-9 at unprecedented high spectral resolution (20 meV s.d. near 60 eV) with an intensity-tuneable visible laser field to couple 10-12 the quantum states from the weak-field to the strong-coupling regime. Employing the Fano resonance as a phasesensitive quantum interferometer 13 , we demonstrate the coherent control of two correlated electrons, which form the basis of most covalent molecular bonds in nature. As we show, such multi-dimensional spectroscopy experiments provide benchmark data for testing fundamental few-body quantum-dynamics theory. They also light a route for site-specific measurement and control of metastable electronic transition states that are at the heart of fundamental reactions in chemistry and biology.Electrons are bound to atoms and molecules by the Coulomb force of the nuclei. Moving between atoms, they form the basis of the molecular bond. The same Coulomb force, however, acts repulsively between the electrons. This electron-electron interaction represents a major challenge in the understanding and modelling of atomic and molecular states, their structure and in particular their dynamics 2,3,14 . Here, we focus on the 1 P sp 2,n+ series 15 of doubly-excited states in helium below the N = 2 ionization threshold. They are excited by a single-photon transition from the 1 S 1s 2 ground state by the promotion of both electrons to at least principal quantum number n = 2. The states autoionize due to electron-electron interaction and their spectroscopic signature manifests as asymmetric non-Lorentzian line shapes. The latter were first observed in the 1930s 16 and attributed 17 , by Ugo Fano, to the quantum interference of bound states with the continuum to which they are coupled (Fig. 1c,d). The coupling is described by the configuration interaction V CI with the single-ionization continuum |1s,p, where one electron is in the 1s ground state and the other one is in the continuum with kinetic energy . The magnitude of V CI determines the lifetimes of the transiently bound states. In our case,...

Reconstruction of an excited-state molecular wave packet with attosecond transient absorption spectroscopy

“…In this representation, once the spectral amplitudes c n (t) are determined at a given time t 0 , it is possible to compute their value at any subsequent time using the relation c n (t) = c n (t 0 )e −iE n (t−t 0 )/ . While this approach has been successfully used to reconstruct the dynamics of a metastable wave packet in the helium atom [22], it has limitations. Namely, to reconstruct the wave packet in space, one must know beforehand both the energies E n and the functions φ n (R).…”

“…With the availability of high-resolution VUV spectrometers [15], one can conceivably monitor the recurrence of the vibronic wave packet created close to a localized reaction center, even in solution. ATAS has already been applied successfully to selected studies of strongfield ionization [5] and dissociation [16], wave-packet motion [3,4,6,17,18], and correlated electron dynamics [19][20][21][22], as well as phase transition dynamics in condensed matter [23,24], with unprecedented energy and time resolution. As ATAS encodes both the amplitudes and phases of interacting states, reconstruction and control of an electron wave packet-in both space and time-has been achieved [22].…”

Phase-resolved two-dimensional spectroscopy of electronic wave packets by laser-induced XUV free induction decay

“…This effect was also used to spatially deflect the XFID radiation by creating a Stark-shift gradient in the medium [14]. The combination of these different physical effects and observables provides a very rich framework for ultrafast spectroscopy and strong-field physics, enabling for instance the complete reconstruction of a two-electron wavepackets [15].Nevertheless, TRAX suffers from several limitations that restrict its generalization. First, the phase sensitivity, which is a key component of TRAX, is based on the interferometric nature of the detection, which records the coherent superposition of the incoming XUV and radiated XFID light.…”

“…This effect was also used to spatially deflect the XFID radiation by creating a Stark-shift gradient in the medium [14]. The combination of these different physical effects and observables provides a very rich framework for ultrafast spectroscopy and strong-field physics, enabling for instance the complete reconstruction of a two-electron wavepackets [15].…”

Signatures and control of strong-field dynamics in a complex system

“…However, an important ingredient in approaching the ultimate goal of controlling chemistry is to get access to the phase of quantum-state coefficients and to analyze systematically the phase of the system's response. In recent work, the phase of the dipole response after excitation was measured and controlled in a simple system, namely gaseous helium (31,32). Transient absorption experiments were performed using extreme-UV attosecond pulses and 7-fs short visible to near-infrared (VIS/NIR) pulses, and the absorption was measured as a function of the time delay.…”