FP7-MC-IEF 235300 COCOSPEC 1

Popular description of research performed

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 mod-eled with the help of quantum mechanics, changes in re-sponse to light. But could one exploit this response to con-trol 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 de-veloped allow us to examine and predict different ways to control molecules. Our calculations yield the molecular dy-namics in full and complete detail, showing the intricate flow of energy through the molecule in real time, and repro-ducing 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 astro-physics, 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 fa-cing 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 or-ders 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 in-cluded, 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 con-trol can be seen as an excellent tool for molecular spectroscopy, providing a creative use of laser pulses and pulse se -quences 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.

Accomplishment of research objectives as presented in the original proposal

Objective: Modeling of exotic long-range states of molecular hydrogenIn the three last years a number of groups in Europe and in the US were able to observe experimentally exotic ion-pair long-range states in various molecules such as H2 and Cl2, but there was no theoretical treatment available which could help explain the properties of these exotic states. During the Marie Curie IEF COCOSPEC project, we were able to de-velop a new theory specifically adapted to describe ion-pair states in molecules, and to use this theory to explain impor -tant aspects of the experiments, including the life-times and densities of these molecular states. The first article in this series was awarded ‘Editor’s Choice for 2010’ by Journal of Chemical Physics, and the second article has just been published in Physical Review A. We are writing up two more papers on this research, one which examines ion-pair states in the molecule Cl2 specifically, and one which develops the theory further.

An exciting aspect of this work is that it gave us the opportunity to develop a more complete theoretical treatment of coupled ionization and dissociation in molecules, so to make it possible to account for dynamic effects at large internu -

Figure 1 Light-absorption and interference effects from five laser pulses driving dissociation and ionization.

clear distances. Up to now, no theory exists that treats both short and long range effects together. The new theory devel -oped is immediately useful to resolve some outstanding questions about exotic long-range ion-pair states, but will also allow better treatment of dissociative recombination – an important process in astrophysics – and improve the theoreti -cal analysis of experimental spectra from highly excited molecules.

This work has been presented at several invited talks at international conferences and invited seminars, and has met with very positive response from the community. Several high-profile publications are already in print, and two more publications are in draft form.

Objective: Coherent control in NOWe developed a computer code capable to simulate both coherent control and time-resolved dynamics (including photo-electron spectra and alignment) for a set of given molecular parameters. The planned modeling of coherent control in the NO molecule was held back by technical problems with the signs of dipole moments. Awaiting a set of reliable ab initio parameters for NO, we made a detailed study of the competition between ionization and dissociation in H2, and developed a coherent control scheme that uses a sequence of optical laser pulses to steer the dynamics of the molecules either towards ionization or dissociation. These calculations are of unprecedented detail and resolution, and allow us to trace the flow of energy in the molecule under the influence of the laser pulses, and to decipher the necessary interplay of light and molecular dynamics that leads to control. This work has been published in the journal Phys. Chem. Chem. Phys. (PCCP) and presented in several invited talks.

Objective: Coherent control of polyatomic moleculesA study of the competition between ionization and dissociation in H3, using parameters from V. Kokooline, who was a junior professor at Laboratoire Aimé Cotton during the COCOSPEC project, is being written up. As part of the work on many-atom dynamics and coherent control, we developed a code for many-body quantum dynamics based on the Cou-pled Coherent States (CCS) method. This method can treat many degrees of freedom efficiently, and could be linked with ab initio codes to run on-the-fly dynamics of photochemical processes, allowing for coherent control experiments in a wide range of molecules to be examined. We have used CCS to simulate strong field experiments. Part of this work has resulted in publications (Physical Review A) and presentations at international conferences.

New objectives established during the course of the work and new lines of research

Objective: Develop methods for ab initio simulation of strong field dynamicsQuantum dynamics methods based on coherent states (Gaussian wave pack-ets) have been proved to be useful for simulations of wave packet dynamics of nuclei, mostly in theoretical chemistry. During the COCOSPEC project we were able to show that such methods are also useful for simulating elec-tron dynamics with applications in strong field attosecond physics. In a re-cent paper (Physical Review A), we demonstrate that solving the time-de-pendent Schrödinger equation in a basis of coupled coherent states is an ef-ficient method to simulate quantum mechanics with all degrees of freedom intact. Importantly, this technique allows fully quantum simulations under realistic conditions, which is not possible with any other method. In the long run, the developed theory (named the fermion-CCS method) may make it possible to address key questions in AMO, plasma, nano and solid state physics (i.e. wherever many fermion particles interact). This work has been published in Physical Review A, and presented with invited talks at two in-ternational conferences, and a poster presentation at one international con-ference.

Objective: Demonstrate potential for new experiments using time-resolved x-ray diffractionNew sources of pulsed high-intensity x-ray radiation open the possi-bility of new types of experiments which combine spectroscopy and x-ray diffraction techniques to make ‘molecular movies’ which allow direct image of evolving electronic and molecular structures to be recorded experimentally. We have investigated the potential for such experiments and modeled experiments, which could in principle be done within a few years. This work is presently being written up for publication.

Figure 2 Momentum distributions as functions of parallel momentum compo-nents computed with the CCS for He-lium in a field of 1.5 × 1015 W/cm2.

Figure 3 Electron density from excited atomic d-orbital.