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We explore how the spectral phase of attosecond pulse trains influences the optical cross section in transient absorption (TA) spectroscopy. The interaction of extreme ultraviolet (XUV) and time-delayed near-infrared (NIR) fields with an atomic or molecular system governs the dynamics. As already shown in RABBITT experiments (Reconstruction of Attosecond Beating by Interference of Two-Photon Transitions), the spectral phase of the XUV pulses can be extracted from the photoionization spectrum as a function of the time delay. Similarly, this XUV phase imprints itself on delay-dependent optical cross-section oscillations. With a perturbative analytical approach and by simulating the quantum dynamics both in a few-level model and via solving the time-dependent Schrödinger equation for atomic hydrogen, we reveal the similarity between the spectral phase in RABBITT and TA spectroscopy. Published by the American Physical Society2025 

We demonstrate a left-right asymmetry control of the photoelectron angular distribution in multiphoton ionization of Li atoms by a bichromatic laser field. By delaying the fundamental (780 nm) and its second harmonic relative to each other in steps of 130 attoseconds, we can vary the relative phase between the two laser fields with subwavelength accuracy and thereby steer the ejected electrons. Good agreement is found between the measurements and calculations at the appropriate intensities of the two harmonics. Published by the American Physical Society2024 

Following up on a previous paper on two-color photoionization of Ar(3p) [D. Bharti et al., Phys. Rev. A 103, 022834 (2021)], we present measurements and calculations for a modified three-sideband (3-SB) version of the reconstruction of attosecond beating by interference of two-photon transitions (RABBITT) configuration applied to He(1s). The 3-SB RABBITT approach allows us to explore interference effects between pathways involving different orders of transitions within the continuum. The relative differences in the retrieved oscillation phases of the three sidebands provide insights into the continuum-continuum transitions. The ground state of helium has zero orbital angular momentum, which simplifies the analysis of oscillation phases and their angle dependence within the three sidebands. We find qualitative agreement between our experimental results and the theoretical predictions for many cases but also observe some significant quantitative discrepancies. 

AbstractWe report on a series of detailed Breit-Pauli and Dirac B-spline R-matrix (DBSR) differential cross section (DCS) calculations for excitation of the$$5\,^2\textrm{S}_{1/2} \rightarrow 5\,^2\textrm{P}_{1/2}$$ $5{}^2{\text{S}}_{1/2}\to 5{}^2{\text{P}}_{1/2}$and$$5\,^2\textrm{S}_{1/2}\rightarrow 5\,^2\textrm{P}_{3/2}$$ $5{}^2{\text{S}}_{1/2}\to 5{}^2{\text{P}}_{3/2}$states in rubidium by 40 eV incident electrons. The early BP computations shown here were carried out with both 5 states and 12 states, while the DBSR models coupled 150 and 325 states, respectively. We also report corresponding results from a limited set of DCS measurements on the unresolved$$5\,^2\textrm{P}_{1/2,3/2}$$ $5{}^2{\text{P}}_{1/2,3/2}$states, with the experimental data being restricted to the scattered electron angular range 2–$$10^\circ $$ ${10}^{\circ }$. Typically, good agreement is found between our calculated and measured DCS for excitation of the unresolved$$5\,^2\textrm{P}_{1/2,3/2}$$ $5{}^2{\text{P}}_{1/2,3/2}$states, with best accord being found between the DBSR predictions and the measured data. The present theoretical and experimental results are also compared with predictions from earlier 40 eV calculations using the nonrelativistic Distorted-Wave Born Approximation and a Relativistic Distorted-Wave model. Graphic abstract