Theory of controlled molecules

We develop theoretical methods and computational tools for investigations of the nuclear and electronic dynamics of polyatomic molecules that are based on the software packages RICHMOL, TROVE, PECD, and DYSON. Our focus is on accurate methods that are quantitatively predictive but at the same time applicable to complex molecular systems encountered in real-world situations. Our current primary research interests are:

• variational simulations of nuclear motion dynamics of weakly-bound molecular complexes with an emphasis on the dissociation
• simulations of photoelectron imaging techniques, such as photoelectron circular dichroism and laser induced electron diffraction.

We are also interested in high-resolution spectroscopy of molecules, especially the study of forbidden transitions and symmetry-breaking phenomena like ortho-para nuclear spin interactions and molecular chirality.

Photoelectron dynamics

Strong-field ionization is a versatile and powerful tool to visualize and ultimately control structural changes taking place during chemical reactions and biological processes. Accurate quantum-mechanical simulations of strong field ionization processes are extremely helpful for the analysis of observations, providing, in some cases, the only means of molecular structure retrieval from such experiments. Using quantum-mechanical and semi-classical approaches, we develop computationally efficient solvers for laser-induced strong-field ionization that accurately describes the phenomenon of photoelectron circular dichroism and self-diffraction imaging, including ionization, long-range photoelectron propagation in the laser field, and rescattering with target molecular ions.

Machine learning

Machine learning has made a significant inroad into the natural sciences in recent years, providing efficient computational technology of highly expressive parametric functions. In two DASHH-funded projects, we are developing deep neural network algorithms for calculating highly excited and quasi-bound vibrational states of molecules, as well as long-range excursion dynamics of photoelectrons for modelling of imaging experiments.

We also apply machine learning approaches for calculating molecular potential energy surfaces (PESs). Calculating potentials for weakly-bound complexes is especially difficult and computationally expensive due to the loosely bound nature of the intermolecular interactions, which results in a complicated shape of the potentials with local minima and saddle points. In our recently published article, "Active learning of potential-energy surfaces of weakly-bound complexes with regression-tree ensembles," we proposed and investigated an active learning algorithm that allows for the construction of intermolecular PES of pyrrole-water dimer with reduced computational costs without sacrificing accuracy.

Laser control of molecules

Laser-controlled rotational-vibrational molecular dynamics is a subject of active research in physics and chemistry. In particular, the control of molecular spatial alignment and orientation is highly leveraged in many ultrafast imaging experiments and stereochemistry studies to reduce the blurring of observables and increase the experimental resolution. Laser-field control of chiral molecules is of particular interest because of the challenges of detecting the enantiomeric excess and handiness in chiral mixtures at ultrafast timescales as well as chiral purification and discrimination.
Through comprehensive theoretical predictions, we provide essential analysis for laser alignment experiments of complex molecules and develop new methodologies for realistic laser experiments to detect chirality, spatially separate enantiomers, and induce chirality through spontaneous symmetry breaking.

Precision spectroscopy

Recent advances in high-intensity tunable laser sources and improvements in detection sensitivity enable laboratory observations of ultra-weak transitions in molecules, such as due to molecular quadrupole moment or nuclear spin flips. Using variational methods TROVE and RICHMOL we are making predictions of such transitions in small molecules with unprecedented accuracy. As an example, we made the first comprehensive variational predictions of forbidden nuclear spin ortho-para transitions in water molecule. Our calculated linelist of quadrupole transitions has enabled the first experimental detection of such transitions in gas phase water at room temperature. Our predicted quadrupole transitions in CO₂ were found in the atmosphere of Mars and detected in the laboratory.

Previous project: Nuclear-spin effects in laser alignment

We developed a robust variational method, which was implemented in the Richmol computational package, to account for the quadrupole as well as weaker spin-spin and spin-rotation interactions in the laser-induced dynamics of molecules. In several studies, we showed that the impact of nuclear quadrupole interactions on postpulse (field-free) molecular dynamics is minimal during the first revivals. However, over longer time scales, the effect is completely detrimental and is highly influenced by laser intensity. Read more here