Light-induced processes are at the heart of a large variety of phenomena of interest in Chemistry, Biology, Engineering and Physics [1,2]. Charge transfer, redox reactivity, photosynthesis, protein folding, or electron transport efficiency in photovoltaic materials, are only a few examples of a long list of fundamental processes in Nature initialed by light [3,4]. The impressive developments on laser technologies in the last decades made it possible to obtain time-resolved images of nuclear dynamics and rearrangements occurring in the scale of a few tens of femtoseconds (1 fs = 10^-15 seconds) to picoseconds (1 ps = 10^-12 seconds), and more recently, even of the electron dynamics, triggered at the very early stages of the interaction between light and matter, and occurring at the time scale of the attoseconds (1 as = 10^-18 seconds).

Coherent laser sources capable of extracting information with high spatial resolution (atomic) and time resolution (few femtoseconds and below) are currently available in many table-top laboratories, as well as in large facilities, all over the world. These sources produce laser pulses in wide range of frequencies allowing us to track and manipulate in real time excitation and ionization processes in molecules [1,2,5-7]. A deep understanding of the underlying mechanisms of these light-induced dynamical interactions at their natural time scale are expected to open new avenues to manipulate chemical properties and reactivity, as already demonstrated by the prolific area of Femtochemistry. While in more traditional studies in photochemistry, excitation is usually triggered into a specific electronic state, the use of ultrashort pulses (of a few fs or attoseconds) can launch an electronic response that is no longer given by a stationary state of the system, but by a molecular wave packet involving several electronic states of the molecule. Consequently, stationary-state pictures based on lowest-order perturbation theory are, in most cases, inapplicable, so one is forced to solve the time-dependent Schrödinger equation.  The first step is thus to understand how to describe these light-induced electronic dynamics, for which, a proper description of both light and matter is required. Very few methodologies introduce an accurate description of the initial wave packet created by the interaction with ultrashort pulses in excitation and/or ionization. Moreover, the laser parameters (frequency, pulse duration and intensity) are of paramount relevance on the observed dynamics.

The present course aims to offer a solid background on the most fundamental theoretical aspects to describe the interaction of coherent light pulses with molecules. It offers a set of comprehensive tutorials on the description of ultrafast electron and nuclear dynamics triggered in distinct scenarios, from traditional weak-field approaches to strong field light-induced phenomena. The contents are designed to first introduce pulse characterization techniques and signal analysis employed in HHG (High-order harmonic generation) laboratories and FEL (free electron laser) facilities, followed by a solid theoretical background on weak- and strong-field approaches and phenomena induced by these ultrashort pulses. It includes practical sessions with open source codes for the description of light in time and frequency domains, and its interaction with molecules (using simplified models) using different laser parameters. An extensive tutorial and practical sessions will be provided on DFT and TDDFT approaches (OCTOPUS software) to describe the light-induced phenomena in large molecules and biomolecules.

The contents and practical sessions are designed for students at Master level or first year of PhD in the areas of Chemistry and Physics, including theoretical quantum chemists, experimental chemists and physics working with light sources, and most related areas in the field of atomic and molecular physics and chemistry and optics. The contents have been designed as an integral formation in these areas, and it is further embedded as one of the blocks for the Erasmus Mundus Master on Theoretical Chemistry and Computational Modeling. It is designed with hands-on tutorials, performed in the computer lab, exploring the first steps in photo-induced phenomena.

[1] Duris et al., Nat. Phot. 14, 30 (2020)

[2] Palacios and Martín, WIREs Comp. Molec. Sci. 10, 1 (2020)

[3] Faraji et al., Angew. Chem. Int. Ed. 55 5175 (2016);

[4] Agostini et al., WIREs Comp. Molec. Science 9, e1417 (2019)

[5] Allaria et al., Nat. Photonics 6, 699 (2012)

[6] Hassan et al., Nature 530, 66 (2016)

[7] Huang et al., Optics Express 27, 30798 (2019)