arXiv subjects

Alexander Tarasevitch

Publications and source records attributed to Alexander Tarasevitch.

5 recordsLinked to original sources

arXiv2023

A modular table-top setup for ultrafast X-ray diffraction

We present a table-top setup for femtosecond time-resolved X-ray diffraction based on a Cu Kα (8.05 keV) laser driven plasma X-ray source. Due to its modular design it provides high accessibility to its individual components (e.g. X-ray optics and sample environment). The Kα-yield of the source is optimized using a pre-pulse scheme. A magnifying multilayer X-ray mirror with Montel-Helios geometry is used to collect the emitted radiation, resulting in a quasi-collimated flux of more than 105 Cu Kα photons/pulse impinging on the sample under investigation at a repetition rate of 10 Hz. A gas ionization chamber detector is placed right after the X-ray mirror and used for normalization of the diffraction signals enabling the measurement of relative signal changes of less than 1% even at the given low repetition rate. Time-resolved diffraction experiments on laser-excited epitaxial Bi films serve as an example to demonstrate the capabilities of the set-up. The set-up can also be used for Debye-Scherrer type measurements on poly-crystalline samples.

cond-mat.mtrl-sci↗

arXiv2023

A femtosecond time resolved view of vibrationally assisted electron transfer across the metal/aqueous interface

Understanding heterogeneous charge transfer is crucial if we are to build the best electrolyzers, fuel cells and photoelectrochemical water splitting devices that chemistry allows. Because the elementary processes involved have timescales ranging from femto- to milliseconds, direct simulation is not generally possible. Model Hamiltonian approaches thus have a crucial role in gaining mechanistic insight. Current generations of such theories describe a reactant(s) or product(s) that interacts with electrolyte via a single effective interaction. Such approaches thus obscure the extent to which particular solvent fluctuations influence charge transfer. Here we demonstrate experimentally that for a prototypical system, a ferrocene terminated alkane thiol self-assembled monolayer (SAM) on gold in contact with aqueous electrolyte, charge transfer from the Au to the ferrocene can be induced by vibrational excitation of the ferrocene aromatic CH. Intriguingly the energy of the aromatic CH vibration, 0.38 eV, is a large fraction of the effective solvent interaction strength inferred for the ferrocene/ferrocenium system in prior electrochemical studies: 0.85 eV. Our results thus demonstrate the coupling of charge transfer to a specific solvent motion and more generally imply that solvent may affect reduction/oxidation rates in electrocatalysis by coupling to a few distinct solvent motions. Identifying these motions is crucial in rationalizing trends in reactivity with change in electrolyte and thus in pursuing electrolyte engineering from first principles.

physics.chem-ph↗

arXiv2019

Time-resolved diffraction with an optimized short pulse laser plasma X-ray source

We present a set-up for time-resolved X-ray diffraction based on a short pulse, laser-driven plasma X-ray source. The employed modular design provides high flexibility to adapt the set-up to the specific requirements (e.g. X-ray optics, sample environment) of particular applications. The configuration discussed here has been optimized towards high angular/momentum resolution and uses K$_α$-radiation (4.51 keV) from a Ti wire-target in combination with a toroidally bent crystal for collection, monochromatization and focusing of the emitted radiation. $2\times 10^5$ Ti-K$_{\alpha1}$ photons per pulse with $10^{-4}$ relative bandwidth are delivered to the sample at 10 Hz repetition rate. This allows for high dynamic range ($10^4$) measurements of transient changes of the rocking curves of materials as for example induced by laser-triggered strain waves.

physics.ins-det↗

arXiv2019

Towards high power broad-band OPCPA at 3000 nm

High-energy femtosecond laser pulses in the mid-infrared (MIR) wavelength range are essential for a wide range of applications from strong-field physics to selectively pump and probe low energy excitations in condensed matter and molecular vibrations. Here we report a four stage optical parametric chirped pulse amplifier (OPCPA) which generates ultrashort pulses at a central wavelength of 3000 nm with 430 $μ$J energy per pulse at a bandwidth of 490 nm. Broadband emission of a Ti:sapphire oscillator seeds synchronously the four OPCPA stages at 800 nm and the pump line at 1030 nm. The first stage amplifies the 800 nm pulses in BBO using a non-collinear configuration. The second stage converts the wavelength to 1560 nm using difference frequency generation in BBO in a collinear geometry. The third stage amplifies this idler frequency non-collinearly in KTA. Finally, the fourth stage generates the 3000 nm radiation in a collinear configuration in LiIO$_3$ due the broad amplification bandwidth this crystal provides. We compress these pulses to 65 fs by transmission through sapphire. Quantitative calculations of the individual non-linear processes in all stages verify that our OPCPA architecture operates close to optimum efficiency at minimum absorption losses, which suggests that this particular design is very suitable for operation a high average power at multi kHz repetition rates.

physics.optics↗

arXiv2016

Ultrafast inhomogeneous magnetization dynamics analyzed by interface-sensitive nonlinear magneto-optics

We analyze laser-induced ultrafast, spatially inhomogeneous magnetization dynamics of epitaxial Co/Cu(001) films in a 0.4-10 nm thickness range with time-resolved magnetization-induced second harmonic generation, which probes femtosecond spin dynamics at the vacuum/Co and Co/Cu interfaces. The interference of these two contributions makes the overall signal particularly sensitive to differences in the transient magnetization redistribution between the two interfaces, i.e. ultrafast magnetization profiles in the ferromagnetic film. We find in films of up to 3 nm thickness a stronger demagnetization at the surface, because the film thickness is smaller than the effective mean free path of the spin current mediating the demagnetization, i.e. the difference between the mean free paths of the majority and minority carriers. For film thicknesses larger than 3 nm, the magnetization profile reverses, since majority spins can escape into the conducting substrate only from the interface-near region.

cond-mat.mtrl-sci↗