Perspectives of Attosecond Spectroscopy for the Understanding of Fundamental Electron Correlations

Abstract

The description of the electronic structure of molecules in terms of molecular orbitals is a highly successful concept in chemistry. However, it commonly fails if the electrons in a molecule are strongly correlated and cannot be treated as independent particles. Electron correlation is essential to understand inner-valence X-ray spectroscopies, it can drive ultrafast charge migration in molecules, and it is responsible for many exotic properties of strongly correlated materials. Time-resolved spectroscopy with attosecond resolution is generally capable of following electronic motion in real time and can thus provide experimental access to electron-correlation-driven phenomena. High-harmonic spectroscopy in particular uses the precisely timed laser-driven recollision of electrons to interrogate the electronic structure and dynamics of the investigated system on a sub-femtosecond timescale. In this Review, the capabilities of high-harmonic spectroscopy to follow electronic motion in molecules are discussed. Both qualitative and quantitative approaches to unraveling the detailed dynamical responses of molecular systems following ionization are presented. A new theoretical formalism for the reconstruction of correlation-driven charge migration is introduced. The importance of electron-ion entanglement and electronic coherence in the reconstruction of attosecond hole dynamics are discussed. These advances make high-harmonic spectroscopy a promising technique to decode fundamental electron correlations and to provide experimental data on the complex manifestations of multi-electron dynamics.

Probing the molecular frame of uracil and thymine with high-harmonic generation spectroscopy

“…The attosecond laser resolution relies on the nonlinear optical processes, as high-harmonic generation (HHG). [10][11][12][13] . The HHG spectra, made of the emitted harmonics of the infrared pulse frequency, encode information about the electronic structure and the dynamical effects of the systems from which the HHG signal is emitted.…”

Lanczos-based equation-of-motion coupled-cluster singles-and-doubles approach to the total photoionization cross section of valence excited states

“…We diagonalize the tridiagonal matrix T, conveniently truncated to a dimension (chain length) k, to obtain the final space pseudo-spectrum. 4. We compute the oscillator strengths between the initial state state i and the final states j from the Lanczos eigenvectors and eigenvalues of states j, and the Davidson eigenvectors and eigenvalue of state i.…”

“…With the advent of attosecond light pulses, photons lying in the ultraviolet (UV) and Xray spectral regions can now be employed as the probing light in pump-probe spectroscopy experiments reaching the attosecond time scale. [1][2][3][4][5][6][7][8][9][10][11][12] The development of new experimental techniques to access information on transient states of atoms and molecules asks for Many computational studies of the photoionization/photoabsorption cross-sections of valence and/or core electrons of atoms and molecules in their electronic ground state relying on L 2 basis sets have been reported employing either the well known Stieltjes imaging procedure [13][14][15][16][17][18] or an analytical continued procedure based on the Padé approximants [19][20][21][22] to describe the continuum part of the spectra.…”

Transient extreme ultraviolet measurement of element-specific charge transfer dynamics in multiple-material junctions

“…Recent advances in high harmonic generation have allowed the production of attosecond and longer XUV pulses in table-top instrumentation [7][8][9][10][11] . In the extreme ultraviolet (XUV) region, a photoexcited core to valence level transition is measured.…”

“…This discussion includes the difference in core-hole screening for these materials and the appropriate theoretical framework to understand their excited state dynamics. The simultaneous observation of carriers and vibrational dynamics was first achieved in the gas phase 11 . The additional many-body effects present in a solid required further theoretical development to interpret the XUV spectral changes 15 .…”