Ultrafast X-ray Photoelectron Imaging of Attosecond Electron Dynamics in Molecular Coherent Excitation

Abstract

Ultrafast photoelectron imaging allows to measure information about coherent electron dynamic processes in materials or chemical compounds on femtosecond to attosecond time scales. We show that molecular time-resolved photoelectron diffraction produced by a time-delayed soft X-ray attosecond pulse can be used to monitor the ultrafast coherent excitation induced by a resonant UV pump pulse with variable carrierenvelope phases. Asymmetric diffraction angular patterns illustrate coherent electron dynamics of charge migration with spatiotemporal resolution on the attosecond and ångstrom scale. This allows the temporal reconstruction of phases and amplitudes of electronic states and geometry of molecules as a function of time delay of the probe pulse and carrier-envelope phases of the pump pulse. Results are obtained from solutions of time-dependent Schrodinger equations of the hydrogen molecular ion, and analyzed by ultrafast photoelectron diffraction models for coherent superposition of electronic states. The present demonstration provides a guiding principle for monitoring ultrafast spatiotemporal coherent electron dynamics and imaging molecular electronic structure in complex systems by ultrafast pump−probe experiments and their dependence on carrier-envelope phases and time delays.

On the preservation of coherence in the electronic wavepacket of a neutral and rigid polyatomic molecule

“…Such pulses allow one to take real-time snapshots of ultrafast transformations of matter [4]. Pumpprobe techniques using ultrashort sub-cycle laser pulses have made it possible to control the motion of electrons in atoms [5][6][7], molecules [8][9][10][11][12][13][14][15][16][17][18][19][20], and condensed phase systems [21] such as dielectrics, raising the fundamental question in this context about the relevance of the time scale of electronic coherences for chemical dynamics evolving on a much longer time scale: are electronic wavepackets created by ultrashort pulses relevant for the making and breaking of chemical bonds in a molecule?…”

Detecting coherent core-hole wave-packet dynamics in N2 by time- and angle-resolved inner-shell photoelectron spectroscopy

“…The question on how to properly image the electron dynamics in a molecule or atom in a nonstationary electronic state has been investigated in several theoretical works, specifically, in the context of x-ray scattering, [21][22][23][24][25][26] multidimensional spectroscopy, [27][28][29] transient x-ray absorption spectroscopy, 30 electron diffraction, 31 Auger electron spectroscopy, 32,33 and molecular-frame photoelectron spectroscopy. [34][35][36][37] All these studies have addressed electron wave-packet dynamics in the outervalence electron shells. Recent improvements of x-ray free-electron lasers have made it possible to create x-ray pulses with pulse durations below 1 fs, 38 which can be focused tightly to provide extremely high intensities sufficient for x-ray multiphoton ionization.…”

Probing Attosecond Electron Coherence in Molecular Charge Migration by Ultrafast X-Ray Photoelectron Imaging

“…Angular electronic fluxes can also be used to reconstruct electron charge migration in excited benzene by preparing a coherent electronic state [37,39], which can be monitored in photoelectron momentum spectra by a time-delayed high frequency attosecond pulses [43]. It is also found that molecular photoelectron angular distributions depend on the symmetry of molecular orbitals and molecular bondings, thus allowing to monitor electron coherent and charge migration [44][45][46][47]. Most recently, nonresonant ultrafast X-ray scattering from a molecular wave packet has been used to observe an adiabatic electron transfer in molecules [48].…”