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.
The concept of molecular orbitals, one of the most successful in chemistry, commonly fails if strong electron correlation is present. This Review shows how high‐harmonic generation, the process for generating attosecond pulses in the extreme ultraviolet, can be used to spectroscopically access electron dynamics in molecules following ionization, and how the technique could be extended to identify the effects of electron correlation on dynamics.
A
bstract
We present a complete description of top quark pair production in association with a hard photon in the dilepton channel. Our calculation is accurate to NLO in QCD. It is based on matrix ...elements for
e
+
ν
e
μ
−
ν
¯
μ
b
b
¯
γ
production and includes all resonant and non-resonant diagrams, interferences, and off-shell effects of the top quarks and the
W
gauge bosons. This calculation constitutes the first full computation for top quark pair production with a final state photon in hadronic collisions at NLO in QCD. Numerical results for total and differential cross sections are presented for the LHC at a centre-of-mass energy of
s
=
13
TeV. For a few observables relevant for new physics searches, beyond some kinematic bounds, we observe shape distortions of more than 100%. In addition, we confirm that the size of the top quark off-shell effects for the total cross section is consistent with the expected uncertainties of the narrow width approximation. Results presented here are not only relevant for beyond the Standard Model physics searches but also important for precise measurements of the top-quark fiducial cross sections and top-quark properties at the LHC.
A
bstract
We present a comparative study of various approaches for modelling of the
e
+
v
e
μ
−
v
¯
μ
b
b
¯
γ
final state in
t
t
¯
γ
production at the LHC. Working at the NLO in QCD we compare the ...fully realistic description of the top quark decay chain with the one provided by the narrow-width-approximation. The former approach comprises all double, single and non-resonant diagrams, interferences, and off-shell effects of the top quarks. The latter incorporates only double resonant contributions and restricts the unstable top quarks to on-shell states. We confirm that for the integrated cross sections the finite top quark width effects are small and of the order of
O
Γ
t
/
m
t
. We show, however, that they are strongly enhanced for more exclusive observables. In addition, we investigate fractions of events where the photon is radiated either in the production or in the decay stage. We find that large fraction of isolated photons comes from radiative decays of top quarks. Based on our findings, selection criteria might be developed to reduce such contributions, that constitute a background for the measurement of the anomalous couplings in the
t
t
¯
γ
vertex.
Emission-line stars are typically surrounded by dense circumstellar material, often in form of rings or disc-like structures. Line emission from forbidden transitions trace a diversity of density and ...temperature regimes. Of particular interest are the forbidden lines of O i λλ6300, 6364 and Ca ii λλ7291, 7324. They arise in complementary, high-density environments, such as the inner-disc regions around Be supergiants. To study physical conditions traced by these lines and to investigate how common they are, we initiated a survey of emission-line stars. Here, we focus on a sample of nine Be stars in different evolutionary phases. Emission of the O i lines is one of the characteristics of Be stars. We find that four of the objects display Ca ii line emission: for the Be supergiants V1478 Cyg and 3 Pup, the kinematics obtained from the O i and Ca ii line profiles agrees with a Keplerian rotating disc scenario; the forbidden lines of the compact planetary nebula OY Gem display no kinematical broadening beyond spectral resolution; the luminous blue variable candidate V1429 Aql shows no O i lines, but the profile of its Ca ii lines suggests that the emission originates in its hot, ionized circumbinary disc. As none of the Be stars of lower mass displays Ca ii line emission, we conclude that these lines are more likely observable in massive stars with dense discs, supporting and strengthening the suggestion that their appearance requires high-density environments.
The ultrafast motion of electrons and holes after light-matter interaction is fundamental to a broad range of chemical and biophysical processes. We advanced high-harmonic spectroscopy to resolve ...spatially and temporally the migration of an electron hole immediately after ionization of iodoacetylene while simultaneously demonstrating extensive control over the process. A multidimensional approach, based on the measurement and accurate theoretical description of both even and odd harmonic orders, enabled us to reconstruct both quantum amplitudes and phases of the electronic states with a resolution of ∼100 attoseconds. We separately reconstructed quasi–field-free and laser-controlled charge migration as a function of the spatial orientation of the molecule and determined the shape of the hole created by ionization. Our technique opens the prospect of laser control over electronic primary processes.
Understanding excited carrier dynamics in semiconductors is crucial for the development of photovoltaics and efficient photonic devices. However, overlapping spectral features in optical pump-probe ...spectroscopy often render assignments of separate electron and hole carrier dynamics ambiguous. Here, ultrafast electron and hole dynamics in germanium nanocrystalline thin films are directly and simultaneously observed by ultrafast transient absorption spectroscopy in the extreme ultraviolet at the germanium M
edge. We decompose the spectra into contributions of electronic state blocking and photo-induced band shifts at a carrier density of 8 × 10
cm
. Separate electron and hole relaxation times are observed as a function of hot carrier energies. A first-order electron and hole decay of ∼1 ps suggests a Shockley-Read-Hall recombination mechanism. The simultaneous observation of electrons and holes with extreme ultraviolet transient absorption spectroscopy paves the way for investigating few- to sub-femtosecond dynamics of both holes and electrons in complex semiconductor materials and across junctions.
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