TD-PICO-MF · Time-Delay PhotoIonization of Core-Orbitals in the Molecular Frame

Time-Delay PhotoIonization of Core-Orbitals in the Molecular Frame

The recent development of broadband coherent extreme ultraviolet sources makes it possible to measure dynamics with attosecond resolution (1 attosecond = 10^-18 s). This time scale is associated with the electron dynamics and more particularly to the electronic correlation in various systems, from atoms and molecules to solids. The investigation of the electronic dynamics of atomic and molecular systems requires remarkably precise tools in order to follow the motion of the electronic wave packets. The corresponding timescale is often related, in Bohr’atomic theory, to the time for a ground state electron in a hydrogen atom to accomplish a full revolution around the nucleus, namely 150 attoseconds. This time scale was experimentally reached at the beginning of the 21st century when light pulses with durations of few hundreds of attosecond and photon energies ranging from few eV’s to hundreds of eV’s were generated through a physical process termed high-harmonic generation (HHG). These novel pulses triggered the emergence of the attosecond research field, which aimed at unravelling the physical processes that are occurring within this time and energy scale. In addition to the fundamental interest to study ultrafast dynamics triggered/probed by the interaction of photons with matter, the attosecond domain has also opened up the possibility to directly control the rapid electronic motion inside a molecule, as well as the subsequent nuclear dynamics such as bond breaking and/or formation giving promising opportunities for future applications in chemistry and biology. These ultrashort light pulses are described as a series of XUV attosecond pulses, and are referred to as Attosecond Pulses Train (APT). They are generated by focusing an intense laser field (10^14W/cm2) on an atomic gas target. The non-linear electronic response of the medium causes the generation of the APT, which is described in the frequency domain as a comb of odd-order harmonics of the generating field. When the generating laser field duration is sufficiently short (few cycles), only an isolated attosecond pulse is created. By photoionizing a target gas with attosecond pulses combined with a phase locked infrared (IR) field, it is possible to retrieve fundamental quantities such as the delay of photoemission of photoelectrons from different atomic orbitals. In fact, the delay of photoemission is an extremely valuable observable in the identification and the study of autoionizing states, Cooper minima and shape resonances. One of the major techniques to access the delay of photoemission is named RABBIT. In this case, the dressing IR field used is at low intensity (10^11W/cm2) and has the same frequency as the field used to generate the harmonics. Scanning the delay between the IR pulse and the XUV APT shows the formation and disappearance of energy channels named sidebands that oscillates at twice the frequency of the IR pulse (Figure 1 (a)). These sidebands arise from two-photon transitions (XUV±IR) and their oscillations are theoretically described as the interference between two quantum paths: the first with the absorption of an XUV photon and the emission of an IR photon and the second with the absorption of one XUV and one IR photon. Each sideband thus gives access to the spectral variation of the scattering phase, directly related to the delay of photoemission (given by the spectral derivative of the scattering phase). The vast majority of RABBIT studies have focused on atomic systems, which hold spherical symmetry with respect to the probing process. It is only over the past few years that studies were directed toward molecular systems expanding the field toward physical chemistry. To that regard, the HO initiated this trend toward molecular systems with the N2 molecule. Molecular photoionization in general presents much richer phenomena compared to atomic systems as the probing process depends on the relative alignment of the molecule with respect to the XUV polarization axis. Yet, most RABBIT studies have been performed in randomly aligned molecules. In this arrangement, the measured quantities only represent an average over all possible orientations, thus losing most of the structural information of the molecule under study. This restriction can be lifted in some special cases involving dissociative ionization using coincidence detection in a COLTRIMS to retrieve the molecular frame. However, the latter can only be retrieved for fast dissociative ionization process such that the initial structure can be correctly inferred. Another way to access the molecular frame is achieved with the prealignment of the molecular system prior to the ionization step. It then becomes possible to investigate the ionization process directly in the molecular frame from any accessible cationic states. Molecular alignment can be achieved through impulsive alignment by a weak non-resonant light pulse and is now a recognized technique to access the molecular frame. The electric field of the laser pulse interacts with the induced dipole of the molecule producing a torque on the molecule described as the formation of a rotational wave packet. In the case of linear and symmetric top molecules, it leads to revivals of the alignment after the laser pulse at time delays related to the moment of inertia of the system. By implementing this methodology, 3D-angularly resolved photodetection scheme experiments within the molecular frame in a field-free environment can be achieved. The TD-PICO-MF project aims at developing a detailed understanding of electronic correlation effects in photoemission by mapping the molecular cationic potential experienced by the electron during its scattering. The approach relies on the combination of attosecond photoionization of an atomic core-orbital with laser-induced control of molecular systems in order to resolve the photoemission process both spatially and temporally. By merging these two domains, both the electron’s initial localization and final momentum are known with respect to the molecular axis with an attosecond time resolution. Therefore, a global picture of the evolution of the cationic molecular potential during a chemical reaction becomes accessible. In particular, the research project proposes to study, as a prototype, the photoemission from the 4d core-orbitals of the iodine atom in iodine monochloride (ICl) molecules in gas phase. The measurements will be realized during different laser controlled configurations of the molecular system e.g., during its alignment, orientation and dissociation. The expected impact is to gain a global picture of the influence of the molecular cationic potential during a chemical reaction and to evidence the role of electronic correlation through the measurement of 3D-scattering phase maps in the molecular frame.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз