arXiv subjects

Miltcho Danailov

Publications and source records attributed to Miltcho Danailov.

13 recordsLinked to original sources

arXiv2026

X-ray driven displacive excitation of coherent phonons

Modulating electron-phonon coupling offers a route to control structural displacements and tune material functionality. Valence-to-conduction band transitions, however, provide limited leverage over the driving force. Here, we demonstrate coherent lattice dynamics in trigonal tellurium using free-electron laser pulses tuned to the Te N4,5-edge. Over a broad fluence range, the oscillation amplitude obeys the displacive excitation of coherent phonons framework, extended to core resonance with twice the driving efficiency of a visible pump. Ab initio calculations decompose the force into competing multiband contributions, inaccessible to optical excitation, whose balance shifts as carriers relax. Tunable extreme-ultraviolet and X-ray pulses thus open a regime in which the displacive response is set by band-dependent coupling to the lattice, not by the number and temperature of the photocarriers alone.

cond-mat.mtrl-sci↗

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↗

arXiv2025

H2-roaming dynamics in the formation of H3+ following two-photon double ionization of ethanol and aminoethanol

Roaming reactions involving a neutral fragment of a molecule that transiently wanders around another fragment before forming a new bond are intriguing and peculiar pathways for molecular rearrangement. Such reactions can occur for example upon double ionization of small organic molecules, and have recently sparked much scientific interest. We have studied the dynamics of the H$_2$-roaming reaction leading to the formation of H$_3^+$ after two-photon double ionization of ethanol and 2-aminoethanol, using an XUV-UV pump-probe scheme. For ethanol, we find dynamics similar to previous studies employing different pump-probe schemes, indicating the independence of the observed dynamics from the method of ionization and the photon energy of the disruptive probe pulse. Surprisingly, we do not observe a kinetic isotope effect in ethanol-D$_6$, in contrast to previous experiments on methanol where such an effect was observed. This distinction indicates fundamental differences in the energetics of the reaction pathways as compared to the methanol molecule. The larger number of possible roaming pathways compared to methanol complicates the analysis considerably. In contrast to previous studies, we additionally analyze a broad range of dissociative ionization products, which feature distinct dynamics from that of H$_{3}^{+}$ and allow initial insight into the action of the disruptive UV-probe pulse.

physics.chem-ph↗

arXiv2024

Strong-field quantum control in the extreme ultraviolet using pulse shaping

Tailored light-matter interactions in the strong coupling regime enable the manipulation and control of quantum systems with up to unit efficiency, with applications ranging from quantum information to photochemistry. While strong light-matter interactions are readily induced at the valence electron level using long-wavelength radiation, comparable phenomena have been only recently observed with short wavelengths, accessing highly-excited multi-electron and inner-shell electron states. However, the quantum control of strong-field processes at short wavelengths has not been possible, so far, due to the lack of pulse shaping technologies in the extreme ultraviolet (XUV) and X-ray domain. Here, exploiting pulse shaping of the seeded free-electron laser (FEL) FERMI, we demonstrate the strong-field quantum control of ultrafast Rabi dynamics in helium atoms with high fidelity. Our approach unravels a strong dressing of the ionization continuum, otherwise elusive to experimental observables. The latter is exploited to achieve control of the total ionization rate, with prospective applications in many XUV and soft X-ray experiments. Leveraging recent advances in intense few-femtosecond to attosecond XUV to soft X-ray light sources, our results open an avenue to the efficient manipulation and selective control of core electron processes and electron correlation phenomena in real time.

physics.atom-ph↗

arXiv2022

Extreme ultraviolet wave packet interferometry of the autoionizing HeNe dimer

Femtosecond extreme ultraviolet wave packet interferometry (XUV-WPI) was applied to study resonant inter-atomic Coulombic decay (ICD) in the HeNe dimer. The high demands on phase stability and sensitivity for vibronic XUV-WPI of molecular-beam targets are met using an XUV phase-cycling scheme. The detected quantum interferences exhibit vibronic dephasing and rephasing signatures along with an ultrafast decoherence assigned to the ICD process. A Fourier analysis reveals the molecular absorption spectrum with high resolution. The demonstrated experiment shows a promising route for the real-time analysis of ultrafast ICD processes with both high temporal and spectral resolution.

physics.atm-clus↗

arXiv2022

Time-resolved Ultrafast Interatomic Coulombic Decay in Superexcited Sodium-doped Helium Nanodroplets

The autoionization dynamics of superexcited superfluid He nanodroplets doped with Na atoms is studied by extreme-ultraviolet (XUV) time-resolved electron spectroscopy. Following excitation into the higher-lying droplet absorption band, the droplet relaxes into the lowest metastable atomic $1s2s$ $^{1,\,3}$S states from which Interatomic Coulombic Decay (ICD) takes places either between two excited He atoms or between an excited He atom and a Na atom attached to the droplet surface. Four main ICD channels are identified and their time constants are determined by varying the delay between the XUV pulse and a UV pulse that ionizes the initial excited state and thereby quenches ICD. The time constants for the different channels all fall in the range $\sim$1~ps indicating that the ICD dynamics are mainly determined by the droplet environment. A periodic modulation of the transient ICD signals is tentatively attributed to the oscillation of the bubble forming around the localized He excitation. The ICD efficiency depends on the total number of excited states in a droplet rather than the density of excited states pointing to a collective enhancement of ICD.

physics.atm-clus↗

arXiv2021

Improved stabilization scheme for extreme ultraviolet quantum interference experiments

Interferometric pump-probe experiments in the extreme ultraviolet (XUV) domain are experimentally very challenging due to the high phase stability required between the XUV pulses. Recently, an efficient phase stabilization scheme was introduced for seeded XUV free electron lasers (FELs) combining shot-to-shot phase modulation with lock-in detection. This method stabilized the seed laser beampath on the fundamental ultraviolet wavelength to a high degree. Here, we extend this scheme including the stabilization of the XUV beampath, incorporating phase fluctuations from the FEL high gain harmonic generation process. Our analysis reveals a clear signal improvement with the new method compared to the previous stabilization scheme.

physics.atom-ph↗

arXiv2021

All-optical spin injection in silicon revealed by element specific time-resolved Kerr effect

Understanding how a spin current flows across metal-semiconductor interfaces at pico- and femtosecond timescales has implications for ultrafast spintronics, data processing and storage applications. However, the possibility to directly access the propagation of spin currents on such time scales has been hampered by the simultaneous lack of both ultrafast element specific magnetic sensitive probes and tailored metal-semiconductor interfaces. Here, by means of free electron laser-based element sensitive Kerr spectroscopy, we report direct experimental evidence of spin currents across a Ni/Si interface in the form of different magnetodynamics at the Ni M2,3 and Si L2,3 absorption edges. This further allows us to calculate the propagation velocity of the spin current in silicon, which is on the order of 0.2 nm/fs.

cond-mat.mtrl-sci↗

arXiv2021

Unravelling the Full Relaxation Dynamics of Superexcited Helium Nanodroplets

The relaxation dynamics of superexcited superfluid He nanodroplets is thoroughly investigated by means of extreme-ultraviolet (XUV) femtosecond electron and ion spectroscopy complemented by time-dependent density functional theory (TDDFT). Three main paths leading to the emission of electrons and ions are identified: Droplet autoionization, pump-probe photoionization, and autoionization induced by re-excitation of droplets relaxing into levels below the droplet ionization threshold. The most abundant product of both droplet autoionization and photoionization is He$_2^+$, whereas the delayed appearance of He$^+$ is indicative of the ejection of excited He atoms from the droplets. The state-resolved time-dependent photoelectron spectra reveal that intermediate excited states of the droplets are populated in the course of the relaxation, terminating in the lowest-lying metastable singlet and triplet He atomic states. The slightly faster relaxation of the triplet state compared to the singlet state is in agreement with the simulation showing faster formation of a bubble around a He atom in the triplet state.

physics.atm-clus↗

arXiv2020

Attosecond Pulse-shaping using a seeded free-electron laser

Attosecond pulses are fundamental for the investigation of valence and core-electron dynamics on their natural timescale. At present the reproducible generation and characterisation of attosecond waveforms has been demonstrated only through the process of high-order harmonic generation. Several methods for the shaping of attosecond waveforms have been proposed, including metallic filters, multilayer mirrors and manipulation of the driving field. However, none of these approaches allow for the flexible manipulation of the temporal characteristics of the attosecond waveforms, and they suffer from the low conversion efficiency of the high-order harmonic generation process. Free Electron Lasers, on the contrary, deliver femtosecond, extreme ultraviolet and X-ray pulses with energies ranging from tens of $\mathrmμ$J to a few mJ. Recent experiments have shown that they can generate sub-fs spikes, but with temporal characteristics that change shot-to-shot. Here we show the first demonstration of reproducible generation of high energy ($\mathrmμ$J level) attosecond waveforms using a seeded Free Electron Laser. We demonstrate amplitude and phase manipulation of the harmonic components of an attosecond pulse train in combination with a novel approach for its temporal reconstruction. The results presented here open the way to perform attosecond time-resolved experiments with Free Electron Lasers.

physics.optics↗

arXiv2020

Tracking Attosecond Electronic Coherences Using Phase-Manipulated Extreme Ultraviolet Pulses

The recent development of ultrafast extreme ultraviolet (XUV) coherent light sources bears great potential for a better understanding of the structure and dynamics of matter. Promising routes are advanced coherent control and nonlinear spectroscopy schemes in the XUV energy range, yielding unprecedented spatial and temporal resolution. However, their implementation has been hampered by the experimental challenge of generating XUV pulse sequences with precisely controlled timing and phase properties. In particular, direct control and manipulation of the phase of individual pulses within a XUV pulse sequence opens exciting possibilities for coherent control and multidimensional spectroscopy, but has not been accomplished. Here, we overcome these constraints in a highly time-stabilized and phase-modulated XUV-pump, XUV-probe experiment, which directly probes the evolution and dephasing of an inner subshell electronic coherence. This approach, avoiding any XUV optics for direct pulse manipulation, opens up extensive applications of advanced nonlinear optics and spectroscopy at XUV wavelengths.

physics.atom-ph↗

arXiv2020

High-Gain Harmonic Generation with temporally overlapping seed pulses and application to ultrafast spectroscopy

Collinear double-pulse seeding of the High-Gain Harmonic Generation (HGHG) process in a free-electron laser (FEL) is a promising approach to facilitate various coherent nonlinear spectroscopy schemes in the extreme ultraviolet (XUV) spectral range. However, in collinear arrangements using a single nonlinear medium, temporally overlapping seed pulses may introduce nonlinear mixing signals that compromise the experiment at short time delays. Here, we investigate these effects in detail by extending the analysis described in a recent publication (Wituschek et al., Nat. Commun., 11, 883, 2020). High-order fringe-resolved autocorrelation and wave-packet interferometry experiments at photon energies > $23\,$eV are performed, accompanied by numerical simulations. It turns out that both the autocorrelation and the wave-packet interferometry data are very sensitive to saturation effects and can thus be used to characterize saturation in the HGHG process. Our results further imply that time-resolved spectroscopy experiments are feasible even for time delays smaller than the seed pulse duration.

physics.atom-ph↗

arXiv2017

First FAMU observation of muon transfer from mu-p atoms to higher-Z elements

The FAMU experiment aims to accurately measure the hyperfine splitting of the ground state of the muonic hydrogen atom. A measurement of the transfer rate of muons from hydrogen to heavier gases is necessary for this purpose. In June 2014, within a preliminary experiment, a pressurized gas-target was exposed to the pulsed low-energy muon beam at the RIKEN RAL muon facility (Rutherford Appleton Laboratory, UK). The main goal of the test was the characterization of both the noise induced by the pulsed beam and the X-ray detectors. The apparatus, to some extent rudimental, has served admirably to this task. Technical results have been published that prove the validity of the choices made and pave the way for the next steps. This paper presents the results of physical relevance of measurements of the muon transfer rate to carbon dioxide, oxygen, and argon from non-thermalized excited mu-p atoms. The analysis methodology and the approach to the systematics errors are useful for the subsequent study of the transfer rate as function of the kinetic energy of the mu-p currently under way.

physics.atom-ph↗