Session N39: JCP-DCP Editors Choice

Study and control of reactive collisions in the quantum regime

In the area of cold and controlled collisions, we try to unravel the fundamental nature of chemical reactions by controlling the quantum states of the reaction partners and by carrying out the experiments at temperatures near absolute zero, e.g., in order to find out about the properties of short-lived reaction intermediates. In this temperature regime, the wave properties of matter dominate chemical reactivity. For instance, reaction products can be formed via quantum tunneling, even though the energy of the reaction partners is classically not sufficient to overcome the potential barrier. Besides a fundamental understanding of chemical reactivity, this research also allows for a better knowledge of chemical reactions in the interstellar medium, where low-temperature and low-pressure molecular clouds are exposed to high-energy cosmic radiation.

One focus of my group is on so-called chemi-ionization processes in the gas phase, in which an atom or molecule is ionized by another species in a long-lived, electronically excited (metastable) state. In this talk, I will present different means to control chemi-ionizing collisions by quantum-state preparation.   

Computing chemical potentials made easy

hemical potentials are fundamental but difficult to compute. We developed a method that is able to extract chemical potentials from equilibrium molecular dynamics (MD) simulations. This means one can run a standard MD for liquids, and determine chemical potentials

 using a simple post-processing step. As demonstrations, I will show example applications on computing the chemical potentials of NaCl/water solutions, adsorption isotherms of gas in porous materials, solubility of molecular crystals, and the conditions for diamond formation from high-pressure hydrocarbon mixtures.   

Combining Chemical Physics with Quantum Optics for Solving the Open Quantum System Dynamics of Strongly Interacting Oscillators

Modeling the non-equilibrium dissipative dynamics of strongly interacting quantized degrees of freedom is a fundamental problem in several branches of physics and chemistry. We combine the Monte Carlo wavefunction method for Markovian open quantum systems from quantum optics [1] with the multi-configuration time-dependent Hartree method (MCTDH) from chemical physics [2], to efficiently model coherent and dissipative processes of strongly interacting quantum oscillators, for applications in cavity QED with molecules [3]. The open system evolution proceeds through a sequence of deterministic state propagation steps interrupted by stochastic quantum jumps with transition probabilities that depend on the instantaneous wavefunction and the details of the system-reservoir interactions. We benchmark the proposed hybrid propagation method against exact density matrix solutions for physical systems of interest in cavity QED, demonstrating accurate results for experimentally relevant observables using a tractable number of quantum trajectories [4]. We show the potential for solving the dissipative dynamics of finite size arrays of strongly interacting quantized oscillators with high excitation densities, which is challenging for conventional density matrix propagators, and discuss applications of the method in other areas of quantum science. 

[1] Klaus Mølmer, Yvan Castin, and Jean Dalibard, Monte Carlo wave-function method in quantum optics, J. Opt. Soc. Am. B 10, 524-538, 1993.

[2] H.-D. Meyer, U. Manthe, L.S. Cederbaum, The multi-configurational time-dependent Hartree approach, Chem. Phys. Lett. 165, 73-78, 1990.

[3] F. Herrera and J. Owrutsky, Molecular polaritons for controlling chemistry with quantum optics, J.Chem. Phys. 152, 100902, 2020.

[4] J. F. Triana and F. Herrera, Open quantum dynamics of strongly coupled oscillators with multi-configuration time-dependent Hartree propagation and Markovian quantum jumps, J. Chem. Phys. 157, 194104, 2022.   

Explaining the structure sensitivity of Pt and Rh for aqueous-phase hydrogenation of phenol

Phenol is an important model compound to understand the thermocatalytic (TCH) and electrocatalytic hydrogenation (ECH) of biomass to biofuels. Although Pt and Rh are among the most studied catalysts for aqueous-phase phenol hydrogenation, the reason why certain facets are active for ECH and TCH is not fully understood. Herein, we identify the active facet of Pt and Rh catalysts for aqueous-phase hydrogenation of phenol and explain the origin of the size-dependent activity trends of Pt and Rh nanoparticles. Phenol adsorption energies extracted on the active sites of Pt and Rh nanoparticles on carbon by fitting kinetic data show that the active sites adsorb phenol weakly. We predict that the turnover frequencies (TOFs) for the hydrogenation of phenol to cyclohexanone on Pt(111) and Rh(111) terraces are higher than those on (221) stepped facets based on density functional theory modeling and mean-field microkinetic simulations. The higher activities of the (111) terraces are due to lower activation energies and weaker phenol adsorption, preventing high coverages of phenol from inhibiting hydrogen adsorption. We measure that the TOF for ECH of phenol increases as the Rh nanoparticle diameter increases from 2 to 10 nm at 298 K and −0.1 V vs the reversible hydrogen electrode, qualitatively matching prior reports for Pt nanoparticles. The increase in experimental TOFs as Pt and Rh nanoparticle diameters increase is due to a larger fraction of terraces on larger particles. These findings clarify the structure sensitivity and active site of Pt and Rh for the hydrogenation of phenol and will inform the catalyst design for the hydrogenation of bio-oils.   

Core–hole delocalization for modeling x-ray spectroscopies: A cautionary tale

In recent years, the field of X-ray spectroscopy has progressed rapidly, in particular thanks to the development of X-ray free electron lasers and the continuous improvement in time and energy resolution of time-resolved measurements. The interpretation of data collected with these advanced techniques is directly linked to theory and modeling and their ability to correctly describe the interactions between X-rays and matter. It is, therefore, important to understand the benefits and pitfalls of various approaches to compute X-ray spectra. I will discuss several common methods used to calculate steady-state X-ray absorption, emission, and photoelectron spectroscopy of molecules using first principles density- and wave function-based methods. After discussing their accuracy and precision, I will focus on the question of core-hole (CH) delocalization, i.e. the extent to which core-hole orbitals may delocalize over several atoms of symmetric and extended molecular systems, and how CH delocalization can affect the quality of calculated spectra. Capturing electronic relaxation is crucial for X-ray properties, since the strong attractive polarization of the charge density related to core-ionization or excitation is one of the main effects at play. Relaxation errors can be introduced in calculations that consider explicit CHs, if care is not given to localize the CH, or to describe electron correlation at a high level of theory. Besides discussing the effect of the delocalization-induced relaxation error (DIRE) for different X-ray spectroscopies, I will show that DIRE directly relates to the level at which exchange and correlation are included in the calculation. While a localized CH can describe orbital relaxation well, a delocalized CH with the proper symmetry requires higher levels of theory to capture the correlation effects responsible for relaxation. With the emergence of new X-ray spectroscopy software that use explicit CHs, it becomes significant to be aware of how DIRE can affect different types of X-ray spectra and mitigate it by using localized CHs, or an appropriately high theory level.