Molecular physical chemistry (Complutense University of Madrid)

The “Molecular Physical Chemistry” Associated Unit facilitates collaboration between a group of teams with expertise in the study of the structure and dynamics of molecular systems. Collectively, they have access to advanced tools, both theoretical and experimental, which enable a far better understanding of the phenomena under investigation than would result from individual work. The research topics addressed are described in some detail below.
The fundamental objectives pursued by this Associated Unit are as follows:

• To facilitate staff mobility and exchange, both for permanent staff and trainees, between the Centres in the form of visits or short stays.
• To promote the exchange of knowledge, experimental techniques and theoretical methods between the groups.
• To facilitate the shared use of the scientific infrastructure available at the participating Centres when required for the execution of projects. To facilitate the participation of staff from the Centres in teaching activities, both formal and informal, organised by any of the requesting parties when their involvement is required. To also facilitate the participation of CSIC staff in PhD and Master’s programmes.
• To facilitate access for staff from both centres to the documentary resources available in the respective libraries and any other aspect that contributes to the maintenance and consolidation of the collaboration described.

This section covers studies of the dynamics of elementary reactions and inelastic processes using theoretical quantum mechanical (QM) and quasi-classical trajectory (QCT) methods. The cumulative reaction probability formalism is employed to study the properties of transition states and to calculate kinetic coefficients. The aim is also to intensify the study of stereodynamic properties in elementary reactions.
The comprehensive study of the dynamics of certain ion-molecule reactions relevant to interstellar chemistry models will continue. Within this line of research, particular attention will be given to: the development of the potential energy surface for the H2+H2+ reaction, the study of the classical dynamics of this reaction and its isotopic variants, the development of a quantum wave packet time-propagation programme for the study of the aforementioned reaction and its extension to the study of systems with an open-shell diatomic molecule, and the study of inelastic collisions of astrophysical interest between two diatomic molecules, one of which is open-shell, such as CH(2P)+H2. The results of this research will be used to critically evaluate the measurements published in the literature, by carrying out accurate simulations of the experimental conditions in each case. The results of some of these calculations, together with other data from the literature, will also be used to develop or improve kinetic models of cold plasmas studied experimentally at the CSIC, in which physicochemical processes of interest are simulated in various media (ionospheres, the interstellar medium and fusion reactors).
Theoretical studies of quantum control processes regarding alignment and orientation in strong fields will also continue. These studies will be of great use for the experiments on photoinitiated reactions currently being carried out, not only at the UCM (see section c) but also at other European laboratories with which we are in close contact. The dynamics of elementary processes at very low temperatures will also be studied.

Within this section, which has been progressing successfully for several years and has resulted in a number of publications, the work will focus on two lines of research: on the one hand, the interpretation and prediction of infrared spectra of solids formed by condensed mixtures of H₂O, CH₄ and CO₂, which can be used to model systems of astronomical or astrophysical interest; and secondly, in the study of aggregates formed by molecules present in the Earth’s atmosphere, such as H₂O, HNO₃, H₂SO₄, I_xO_y, and others, which are of particular relevance due to their potential involvement in atmospheric physico-chemical processes.
In both cases, the nature of the work to be carried out jointly by members of this Associated Unit will be theoretical, taking advantage of the complementary characteristics of the participating researchers’ expertise and the respective computational facilities. We will primarily use the Siesta and Gaussian software packages, with which we have extensive experience, and which are particularly well-suited for dealing with solids and smaller molecular aggregates, respectively. The results will focus on obtaining equilibrium structures of these systems and predicting their vibrational spectra and thermodynamic properties.
In general, it will be necessary to select high levels of theory to obtain high-quality results, allowing for a sufficiently accurate comparison with experimental results, surpassing previous results where available, or a prediction of similar characteristics that may be useful in future experimental measurements.

This section covers joint studies on the dynamics and control of photodissociation and molecular alignment using femtosecond laser pulses and ion and photoelectron imaging techniques. These studies aim to obtain detailed information on the dynamics and real-time control of the photodissociation of polyatomic molecules, enabling the processes of interest—generally arising from the first dissociative excited electronic state—to be distinguished from other concurrent processes related to dissociative ionisation or dissociation from highly excited electronic states. These studies will be extended to the photodissociation of cold radicals and van der Waals aggregates of organic chromophores with water and ammonia. Experiments on the coherent control of photodissociation processes will be carried out using dynamic Stark control and ultrashort laser pulse shaping techniques. Furthermore, research into molecular alignment using intense femtosecond laser pulses will be deepened, with a wide range of applications in chemical reaction dynamics, surface processing, nonlinear optics and high-intensity molecular physics. Molecular alignment in the non-adiabatic (impulsive) regime produced by an intense femtosecond laser pulse in a gaseous sample or a molecular beam will be characterised using the optical Kerr effect or through ion imaging. Furthermore, we aim to initiate a line of research involving the generation of high harmonics using ultra-intense femtosecond laser pulses in the IR region to track the dynamics of photodissociation processes.
Furthermore, the generation of molecular aggregates in laser-induced ablation plumes from solid substrates will be studied using optical techniques (laser-induced fluorescence or light scattering techniques) and time-of-flight mass spectrometry. These studies aim to obtain detailed information on the mechanisms of laser ablation and on the relationship between the composition and dynamics of the ablation plume and the properties of the plume materials deposited on a given substrate. This latter line of work is of great interest with regard to the fabrication of nanostructured materials via femtosecond pulsed laser deposition. Femtosecond laser pulses will also be used to carry out microprocessing of various solid materials, ranging from biopolymers to metals and alloys.