Recent Publications

Haggag, O. S. ; Baer, R. ; Ruhman, S. ; Krylov, A. Revisiting the benzene excimer using [2, 2] paracyclophane model system: Experiment and theory. ChemRxiv 2024 Submitted. Publisher's VersionAbstract

We report high-level calculations of the excited states of [2,2]-paracyclophane (PCP), which was recently investigated experimentally by ultrafast pump-probe experiments on oriented single crystals [Haggag et al., ChemPhotoChem 6 e202200181 (2022)]. PCP, in which the orientation of the two benzene rings and their range of motion are constrained, serves as a model for studying benzene exciplex formation. The character of the excimer state and the state responsible for the brightest transition are similar to those in benzene dimer. The constrained structure of PCP allows one to focus on the most important degree of freedom, the inter-ring distance. The calculations explain the main features of the transient absorption spectral evolution. This brightest transition of the excimer is polarized along the inter-fragment axis. The absorption of light polarized in the plane of the rings reveals the presence of other absorbing states of Rydberg character, with much weaker intensities. We also report new transient absorption data obtained by a broadband 8 fs pump, which time-resolve strong modulations of the excimer absorption. The combination of theory and experiment provides a detailed picture of the evolution of the electronic structure of the PCP excimer in the course of a single molecular vibration.

Stochastic density functional theory combined with Langevin dynamics for warm dense matter
Hadad, E. R. ; Roy, A. ; Rabani, E. ; Redmer, R. ; Baer, R. Stochastic density functional theory combined with Langevin dynamics for warm dense matter. arXiv.2401.11336 Submitted. Publisher's VersionAbstract

This study overviews and extends a recently developed stochastic finite-temperature Kohn-Sham density functional theory to study warm dense matter using Langevin dynamics, specifically under periodic boundary conditions. The method's algorithmic complexity exhibits nearly linear scaling with system size and is inversely proportional to the temperature. Additionally, a novel linear-scaling stochastic approach is introduced to assess the Kubo-Greenwood conductivity, demonstrating exceptional stability for DC conductivity. Utilizing the developed tools, we investigate the equation of state, radial distribution, and electronic conductivity of Hydrogen at a temperature of 30,000K. As for the radial distribution functions, we reveal a transition of Hydrogen from gas-like to liquid-like behavior as its density exceeds 4 g/cm³. As for the electronic conductivity as a function of the density, we identified a remarkable isosbestic point at frequencies around 7eV, which may be an additional signature of a gas-liquid transition in Hydrogen at 30,000K.

Weak second-order quantum state diffusion unraveling of the Lindblad master equation
Baer, R. ; Adhikari, S. Weak second-order quantum state diffusion unraveling of the Lindblad master equation. Journal of Chemical Physics 2024, 160, 064107. Publisher's VersionAbstract

Abstract Simulating mixed-state evolution in open quantum systems is crucial for various chemical physics, quantum optics, and computer science applications. These simulations typically follow the Lindblad master equation dynamics. An alternative approach known as quantum state diffusion unraveling is based on the trajectories of pure states generated by random wave functions, which evolve according to a nonlinear Itô-Schrödinger equation (ISE). This study introduces weak first- and second-order solvers for the ISE based on directly applying the Itô-Taylor expansion with exact derivatives in the interaction picture. We tested the method on free and driven Morse oscillators coupled to a thermal environment and found that both orders allowed practical estimation with a few dozen iterations. The variance was relatively small compared to the linear unraveling and did not grow with time. The second-order solver delivers much higher accuracy and stability with bigger time steps than the first-order scheme, with a small additional workload. However, the second-order algorithm has quadratic complexity with the number of Lindblad operators as opposed to the linear complexity of the first-order algorithm.

An “inverse” harpoon mechanism
Gope, K. ; Livshits, E. ; Bittner, D. M. ; Baer, R. ; Strasser, D. An “inverse” harpoon mechanism. Science Advances 2022, 8 eabq8084. Publisher's VersionAbstract

Electron-transfer reactions are ubiquitous in chemistry and biology. The electrons quantum nature allows its transfer across long distances. In the well-known harpoon mechanism, electron-transfer results in Coulombic attraction between initially neutral reactants that leads to dramatic increase in the reaction rate. Here we present a different mechanism, in which electron-transfer from a neutral reactant to a multiply charged cation results in strong repulsion that encodes the electron-transfer distance in the kinetic energy release. 3D coincidence-imaging allows to identify such “inverse” harpoon products, predicted by non adiabatic molecular dynamics simulations to occur between H2 and HCOH2+ following double-ionization of isolated methanol molecules. Detailed comparison of measured and simulated data indicates that while the relative probability of long-range electron-transfer events is correctly predicted, theory overestimates the electron-transfer distance.

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