arXiv subjects

Ioannis Makos

Publications and source records attributed to Ioannis Makos.

5 recordsLinked to original sources

arXiv2026

Disentangling the Effect of Ionic Coupling and Multiple Interfering Terms in Attosecond Molecular Interferometry

Attosecond interferometry in a two-color field is central to attosecond metrology and spectroscopy. In this technique, a photoelectron wave packet is released when a single photon from an extreme ultraviolet comb is absorbed. The wave packet then either emits or absorbs one or more near-infrared photons, leading to the formation of sidebands of the main photoelectron peaks. This picture applies well to atoms and assumes that the near-infrared laser pulse only acts on the photoelectron leaving the parent ion. The effect of the near-infrared pulse on the electronic structure of the cation is not considered, since the field usually cannot induce transitions between its electronic levels. Here, we demonstrate how dynamics induced by the near-infrared field in the cation can significantly impact the amplitude and phases of the sideband signal of the photoelectrons associated with specific dissociative channels of CO$_2$ molecules. This coupling of the near-infrared field with the molecular cation opens a third quantum pathway contributing to the signal measured in attosecond interferometry. Through angle- and energy-resolved characterization of the sideband oscillations, we observe reduction of interference amplitude over specific energy range upon angle integration. By comparison with theoretical predictions, we can isolate the contributions of specific interfering pathways to the two-color multi-pathway photoionization process. The scheme investigated in our work is general, and our observations highlight the importance of the additional pathway for accurately interpreting attosecond interferometry experiments involving molecules and more complex quantum systems.

physics.atom-ph↗

arXiv2026

Breakdown of the isotropic asymptotic approximation in two-colour photoionisation

The Wigner delay is defined as the energy derivative of the scattering phase of a particle in a given potential, unveiling the time taken (or gained) due to the interaction. The characterisation of this delay plays a central role in attosecond science, where the time resolution allows to gain information on the time interval required for a photoelectron to be emitted into the continuum after the absorption of a single photon. Attosecond interferometric techniques, based on two-colour (extreme ultraviolet and near-infrared) photoionisation schemes, cannot provide a direct measurement of the Wigner delay, because the low-frequency photon contributes with an additional delay, which is imprinted on the outgoing photoelectron. The isolation of the Wigner delay is usually achieved by appealing to the asymptotic approximation, which assumes that the two-photon delay is separable into a Wigner and a near-infrared-induced phase and provides a universal analytical expression for the latter. In this study, we introduce a self-referencing approach based on the implementation of non-consecutive extreme ultraviolet harmonics, in order to test the validity of the asymptotic approximation. We demonstrate its breakdown by observing a deviation of a few tens of milliradians (corresponding to a few attoseconds) between its predictions and the experimentally measured phases of the sideband oscillations generated in our scheme, in agreement with full-dimensional simulations.

physics.atom-ph↗

arXiv2024

Anisotropy parameters for two-color photoionization phases in randomly oriented molecules: theory and experiment in methane and deuteromethane

We present a combined theoretical and experimental work investigating the angle-resolved phases of the photoionization process driven by a two-color field consisting of an attosecond pulse train and an infrared pulse in an ensemble of randomly oriented molecules. We derive a general form for the two-color photoelectron (and time-delay) angular distribution valid also in the case of chiral molecules and when relative polarizations of the photons contributing to the attosecond photoelectron interferometer differ. We show a comparison between the experimental data and the theoretical predictions in an ensemble of methane and deuteromethane molecules, discussing the effect of nuclear dynamics on the photoionization phases. Finally, we demonstrate that the oscillating component and the phase of the two-color signal can be fitted using complex asymmetry parameters, in perfect analogy with the atomic case.

physics.chem-ph↗

arXiv2023

Isotopic effects in molecular attosecond photoelectron interferometry

Isotopic substitution in molecular systems can affect fundamental molecular properties including the energy position and spacing of electronic, vibrational and rotational levels, thus modifying the dynamics associated to their coherent superposition. In extreme ultraviolet spectroscopy, the photoelectron leaving the molecule after the absorption of a single photon can trigger an ultrafast nuclear motion in the cation, which can lead, eventually, to molecular fragmentation. This dynamics depends on the mass of the constituents of the cation, thus showing, in general, a significant isotopic dependence. In time-resolved attosecond photoelectron interferometry, the absorption of the extreme ultraviolet photon is accompanied by the exchange of an additional quantum of energy (typically in the infrared spectral range) with the photoelectron-photoion system, offering the opportunity to investigate in time the influence of isotopic substitution on the characteristics of the photoionisation dynamics. Here we show that attosecond photoelectron interferometry is sensitive to isotopic substitution by investigating the two-color photoionisation spectra measured in a mixture of methane (CH$_4$) and deuteromethane (CD$_4$). The isotopic dependence manifests itself in the modification of the amplitude and contrast of the oscillations of the photoelectron peaks generated in the two-color field with the two isotopologues. The observed effects are interpreted considering the differences in the time evolution of the nuclear autocorrelation functions in the two molecules.

physics.atom-ph↗

arXiv2022

Ultrastable, high-repetition-rate attosecond beamline for time-resolved XUV-IR coincidence spectroscopy

The implementation of attosecond photoelectron-photoion coincidence spectroscopy for the investigation of atomic and molecular dynamics calls for a high-repetition-rate driving source combined with experimental setups characterized by excellent stability for data acquisition over time intervals ranging from a few hours up to a few days. This requirement is crucial for the investigation of processes characterized by low cross sections and for the characterization of fully differential photoelectron(s) and photoion(s) angular and energy distributions. We demonstrate that the implementation of industrial-grade lasers, combined with a careful design of the delay line implemented in the pump-probe setup, allows one to reach ultrastable experimental conditions leading to an error in the estimation of the time delays of only 12 as. This result opens new possibilities for the investigation of attosecond dynamics in simple quantum systems.

physics.atom-ph↗