arXiv subjects

Vladislav Slama

Publications and source records attributed to Vladislav Slama.

4 recordsLinked to original sources

arXiv2026

Time-resolved THz Stark spectroscopy of molecules in water

Stark spectroscopy is a powerful method for probing molecular dipole moment changes, charge transfer dynamics, and polarizability under applied electric fields. Time-Resolved Terahertz Stark Spectroscopy (TRTSS), which employs intense single-cycle terahertz (THz) pulses to induce transient Stark shifts, overcomes key limitations of conventional approaches. Unlike static or low-frequency fields, THz pulses oscillate much faster than typical molecular rotation times, effectively preventing dipole reorientation and enabling measurements in solutions at ambient conditions. Here, we extend TRTSS to molecules dissolved in water, the most important polar solvent for chemical and biological systems and report the first demonstration of Stark spectroscopy in water at room temperature. Using Malachite Green and Methyl Orange as model systems, we observe clear THz-induced spectral modulations, demonstrating that TRTSS can successfully reveal THz Stark responses even in highly polar, hydrogen-bonded environments. Measured signals exhibit a combination of linear (dipole-driven) and quadratic (polarizability-driven) Stark effects in both systems, consistent with time-dependent density functional theory (TD-DFT) calculations. Comparison with TD-DFT further suggests that conformational effects can influence the extracted Stark parameters in solvated molecules.

physics.chem-ph↗

arXiv2026

An accurate theoretical framework for the optical and electronic properties of paracyclophanes

Aromatic $\pi$-stacking interactions play an important role in both natural and artificial systems, influencing processes such as charge separation in photosynthesis and charge transport in organic semiconductors. Controlling the geometry and distance between aromatic units is therefore crucial for tuning intermolecular interactions and charge-transfer efficiency. Due to their well-defined stacking geometry, paracyclophanes (PCPs) composed of two or more aromatic units connected by rigid linkers, provide an ideal platform for a systematic study of such effects. Despite extensive experimental studies of PCPs, a comprehensive and quantitatively validated theoretical description linking the structure with the electronic and optical properties is still missing. Here, we present an extensive computational and experimental investigation of the electronic and optical properties of homo-PCPs containing naphthalene diimide (NDI) or pyrene chromophores linked by bridges of varying length and rigidity. We introduce a robust methodology for an accurate simulation of the absorption and fluorescence spectra of PCPs based on a combined TD-DFT and CC2 approach, achieving excellent quantitative agreement with experiment. We also present and validate a fragment-based description of PCPs using the Frenkel exciton model. Such approach is valuable not only for interpretation of the electronic and optical properties of PCPs, but it can also significantly reduce the cost of the calculation while maintaining the accuracy of the supermolecular approach. This work establishes a quantitatively reliable framework linking structure, excitonic coupling, and charge-transfer interactions in PCPs with optical properties, providing design principles for next-generation optoelectronic materials.

physics.chem-ph↗

arXiv2025

Coupled Structural and Electronic Requirements in Alpha-FASnI3 Imposed by the Sn(II) Lone Pair

Alpha-Formamidinium-tin-iodide (alpha-FASnI3) is a leading candidate for lead-free photovoltaic applications, adopting a nearly cubic structure at room temperature, but its stability remains limited by oxidation-driven degradation. Reliable first-principles modelling of the photovoltaic alpha-phase is further complicated by inconsistent structural models and levels of theory in the literature. Here, we identify the structural and electronic requirements needed for a physically sound description of alpha-FASnI3, whose behaviour is governed by a pseudo-Jahn-Teller (PJT) instability arising from the stereochemically active Sn(II) lone pair. Using 0 K relaxations, cross-code hybrid-functional benchmarks, and finite-temperature ab initio molecular dynamics, we show that a 4x4x4 supercell with randomly oriented FA+ cations is the smallest model that removes macroscopic dipoles, preserves cubic symmetry, recovers local octahedral tilts, and captures the characteristic PJT-driven Sn off-centering. Accurate band edges and a reliable band gap require a PBE0-level hybrid functional with spin-orbit coupling to treat Sn relativistic effects, together with nonlocal dispersion (rVV10) to capture the enhanced Sn-I covalency. Finite-temperature simulations reveal that Sn off-centering remains local, <111>-oriented, and robust against thermal fluctuations, and that reproducing the experimental 300 K band gap requires a 6x6x6 supercell. These results define the essential ingredients for reliable modelling of alpha-FASnI3 and provide a rigorous foundation for studying lone-pair-driven physics in tin halide perovskites.

cond-mat.mtrl-sci↗

arXiv2024

Nano-size nature of the $\alpha$-FAPbI$_3$ by means of large-scale ab initio simulations

Formamidinium-lead-iodide (FAPbI$_3$) has established itself as the state of the art for high solar-energy conversion efficiency in perovskite-based solar cells. At room temperature, FAPbI$_3$ has a peculiar crystal structure with tetragonal symmetry where the PbI$_6$ framework is distorted from the perfect cubic structure, while the FA molecules are randomly rotated. This is well known experimentally, but the theory is still deficient in describing FAPbI$_3$ with appropriate models in which the system size is adequately taken into account. Using ab initio molecular dynamics at 300 K and first-principle calculations, we prove that, in order to obtain a proper description of the system, three factors must be satisfied simultaneously: the band gap, the minimization of structural distortion, and the zeroing out of the dipole moment. We show that the net dipole moment zeroes out as the system size increases due to PbI6 octahedra distortions rather than FA rotations. We also show that the band gap oscillations in temperature are correlated to octahedra tilting. The optimum between simulations and experimental properties indicates that FAPbI$_3$ is properly described by a system size approaching the nano-scale.

cond-mat.mtrl-sci↗