arXiv subjects

Musahid Ahmed

Publications and source records attributed to Musahid Ahmed.

4 recordsLinked to original sources

arXiv2026

Chemical potentials from structure factors: II. Charged multi-component mixtures

The chemical potentials of charged multi-component mixtures are central to electrolyte thermodynamics, but remain difficult to compute from atomistic simulations. The S0 method enables computing chemical potentials of mixtures from equilibrium molecular dynamics simulations. Here we extend the S0 method to charged mixtures by combining the composition-space framework developed in Part I: Neutral Multi-component Mixtures with a Coulombic treatment of the small-wavenumber limits of static structure factors. This approach separates thermodynamically relevant neutral composition fluctuations from forbidden macroscopic charge fluctuations, and further accounts for charge-neutrality constraints. We use the method to compute the chemical potentials of multiple-halide aqueous salt solutions, elucidating the ion-specific thermodynamic effects. We also calculate the mixing free energies of molten salt mixtures, demonstrating the importance of correctly describing long-wavelength electrostatic correlations.

cond-mat.stat-mech↗

arXiv2026

Chemical potentials from structure factors: I. Neutral multi-component mixtures

The chemical potentials of multi-component mixtures underlie many physical and chemical phenomena, but remain challenging to compute. The S0 method enables the computation of chemical potentials from equilibrium molecular dynamics simulations, by leveraging the thermodynamic relationship between particle number fluctuations and derivatives of chemical potentials, followed by numerical integration along different compositions. Here we generalize the S0 method from two-component mixtures to neutral multi-component mixtures. We first extend the statistical mechanical formalism to high-dimensional compositional space, and then introduce a Gaussian process integration scheme combined with active learning to efficiently integrate chemical potentials and sample diverse compositions. We use this method to compute the mixing free energies of a molten metal alloy, and the solubilities of two paracetamol polymorphs in water-ethanol solvents. The extended S0 method provides a practical and scalable route for computing chemical potentials in neutral bulk multi-component mixtures from atomistic simulations.

physics.chem-ph↗

arXiv2026

Rotational Coherence Dominates Early-Time Dynamics and Produces Long-Time Revivals in the S2 State of Azulene

The ultrafast dynamics of azulene have been debated for decades, with reported picosecond decay constants variously attributed to intramolecular vibrational redistribution (IVR), internal conversion, or rotational dephasing. Using polarization and femtosecond time-resolved Resonance Enhanced Multi-photon Ionization Spectroscopy with a nanosecond delay window, we disentangle this long-standing inconsistency and show that the early 2-5 ps decay component arises entirely from rotational dephasing of an excited-state wavepacket. Identical time constants extracted from the decay of the parallel signal and rise of the perpendicular signal across multiple vibronic origins provide an unambiguous rotational anisotropy signature, eliminating the need for IVR-based interpretations. Extending the measurement window to 1.3 ns reveals well-structured J-type and C-type rotational coherence revivals in S2 azulene on top of the well-documented fluorescence decay, demonstrating that both the short- and long-time dynamics contain information about the coherent rotational dynamics. These results show that azulene, and by extension polycyclic aromatic hydrocarbons (PAH), can sustain structured rotational coherence deep into the nanosecond regime, positioning PAHs as model systems for quantum-coherent wavepacket dynamics and providing a framework for probing coherence, decoherence, and rotational control in electronically rich molecular systems.

physics.chem-ph↗

arXiv2019

Velocity map imaging of inelastic and elastic low energy electron scattering in organic nanoparticles

Electron transport is of fundamental importance, and has application in a variety of fields. Different scattering mechanisms affect electron transport in solids. It is important to comprehensively understand these mechanisms and their scattering cross-sections to predict electron transport properties. Whereas electron transport is well understood for high kinetic energy (KE) electrons, there are inconsistencies in the low KE regime. In this work, velocity map imaging soft X-ray photoelectron spectroscopy is applied to unsupported organic nanoparticles to extract experimental values of inelastic and elastic mean free paths. The obtained data is used to calculate corresponding scattering cross-sections. The data demonstrates a decrease of the Inelastic Mean Free Path and increase of the Elastic Mean Free Path with increasing electron KE between 10-50 eV.

physics.chem-ph↗