arXiv subjects

Elnaz Zyaee

Publications and source records attributed to Elnaz Zyaee.

5 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

Beam Energy Measurement using a Bayesian Approach with the Stacked Foil Method

We present a practical method to measure the energy of proton beams at a medical cyclotron using the stacked foil technique in combination with a Bayesian inference method. By measuring the $^{48}$V activity induced in a stack of irradiated titanium foils, the proton energy can be inferred without relying on direct current or charge measurements, making the method suitable even for low-vacuum environments or air-exposed setups. This technique is further extended to configurations where the beam energy is degraded to levels around 8 MeV. A Bayesian fit of the measured activity profile allows not only for a robust energy estimation but also for a consistent treatment of uncertainties and nuisance parameters. Monte Carlo simulations are employed to validate the underlying assumptions, including the impact of energy dispersion or cross-section uncertainties. Our results demonstrate that this method provides accurate beam energy measurements across several typical experimental setups used at the Bern Medical Cyclotron. Additionally, we evaluate the sensitivity of the method to the choice of nuclear cross-section data and assess how the number of foils in the stack affects the uncertainty in the inferred beam energy.

physics.acc-ph↗

arXiv2022

Time-resolved THz Stark spectroscopy

We introduce THz Stark spectroscopy by using intense single-cycle terahertz pulses as the electric field source and monitoring the induced spectral response of an isotropic molecular ensemble with a coincident femtosecond supercontinuum pulse. THz Stark spectroscopy offers several advantages over conventional Stark spectroscopy and opens previously inaccessible perspectives. Most importantly, THz pulses oscillate faster than typical molecular rotations and consequently eliminate the requirement to freeze the samples to prevent poling effects. Hence, THz Stark spectroscopy allows for time-resolved studies at arbitrary temperatures, specifically ambient conditions more relevant to physiological or operative conditions. Moreover, dynamical field effects, e.g., higher order Stark contributions or hysteresis effects (non-Markovian behavior), can be studied on the time scales of molecular vibrations or rotations. We demonstrate THz Stark spectroscopy for two judiciously selected molecular systems and compare the results to conventional Stark spectroscopy and first principle calculations.

physics.optics↗

arXiv2022

Wideband dispersion-free THz waveguide platform

We present an integrated THz spectroscopy and sensing platform featuring low loss, vacuum-like dispersion, and strong field confinement in the fundamental mode. Its performance was characterized experimentally for frequencies between 0.1 THz and 1.5 THz. While linear THz spectroscopy and sensing gain mostly from low loss and an extended interaction length, nonlinear THz spectroscopy would also profit from the field enhancement associated to strong mode confinement. Moreover, the vacuum-like dispersion allows for a reshaping-free propagation of broadband single- to few-cycle pulses in gas-phase samples or velocity matching between THz pump and visible to infrared probe pulses. Our platform is based on a metallic structure and falls in the category of double ridged waveguides. We characterize essential waveguide properties, for instance, propagation and bending losses, but also demonstrate junctions and interferometers, essentially because those elements are prerequisites for integrated THz waveform synthesis, and hence, for coherently controlled linear and nonlinear interactions.

physics.optics↗