COCOSPEC · Simulating Coherent Control with Spectroscopic Accuracy

Simulating Coherent Control with Spectroscopic Accuracy

Molecules are fundamental building blocks of the world around us and consist of atoms bound together by shared electrons. The particles which constitute a molecule, the atomic nuclei and the electrons, are in constant and never-ending motion. This internal dynamics, which can be modeled with the help of quantum mechanics, changes in response to light. But could one exploit this response to control molecules by subjecting them to specific types of light? A new field of research called coherent control is based on this premise. The idea is to use light as a chemical reagent to control, for instance, the outcome of chemical reactions. The models of molecules that my colleagues and I have developed allow us to examine and predict different ways to control molecules. Our calculations yield the molecular dynamics in full and complete detail, showing the intricate flow of energy through the molecule in real time, and reproducing the complicated energy-resolved spectra with high accuracy, thus revealing which types of motion dominate the dynamics. Thanks to the precision of our models, our predictions can be directly compared to experiments, and their usefulness extends to completely different fields of research. For instance, we are able to explain important processes in combustion and plasma chemistry, in atmospheric chemistry (with all its implications for global warming) and in astrophysics, including processes that ultimately lead to the birth of new stars. One exotic type of molecules that we have developed new theory for were recently discovered in laboratories in the Netherlands, Switzerland and the US. These molecules begin life as, for instance, normal hydrogen molecules (H2), but are pumped with energy from lasers so that they become very large, with the distance between the two (bound) atoms reaching almost macroscopic dimensions. When the two atoms approach each other during the vibrational motion, an electron is squeezed out instead, and orbits the molecule at great distance. A good way to think about it is that the molecule tethers on the brink of dissociation (bond-breaking) and ionization (removal of an electron). There is much we still do not understand about these molecules, but our new theory, developed during the Marie Curie IEF, has already helped explain many of the exotic properties and observations of these molecules. The dynamics of competing ionization and dissociation in a diatomic molecule embodies many of the key challenges facing molecular spectroscopy, such as strong non-adiabatic couplings between electronic and nuclear motion, energy flow between different degrees of freedom (electronic, vibrational, rotational), delicately balanced interference effects between ionization and dissociation continua, complex (overlapping) resonances and internal time-scales spanning orders of magnitude. We have used recently developed time-dependent Multichannel Quantum Defect Theory (MQDT) to obtain complementary time and frequency domain perspectives on the complex dynamics in H2. MQDT is used to solve the stationary, time-independent, Schrödinger equation for the molecular Hamiltonian with all degrees of freedom included, which in turn provides a highly adapted and converged basis for the solution of the time-dependent Schrödinger equation. The calculations yield the molecular dynamics in full detail, providing both a detailed picture of energy flow in real time, and reproducing the complicated energy- resolved spectra with high accuracy. In this context coherent control can be seen as an excellent tool for molecular spectroscopy, providing a creative use of laser pulses and pulse sequences to study molecules, in close analogy to NMR. The results shed light not only on the control mechanisms, but also on the fundamental photodynamics of the ubiquitous H2 molecule.

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