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Dipartimento di Fisica - Politecnico di Milano

Molecular orbital imaging by polarization-resolved high-order harmonic generation

Motivations. The visualization of the outermost orbital of a molecule is fundamental in the understanding of processes occurring during a chemical reaction or after molecular optical excitation. This goal can be achieved by tomographic techniques based on the combination of data acquired through High-order Harmonic Generation from the target molecules, allowing a visualization with extreme spatial and temporal resolution.

Thesis goal. This thesis work aims at the retrieval of the outermost orbital structure in molecules in the ground state or undergoing optical or vibrational excitation through the acquisition of high-order harmonic spectra driven by intense laser pulses, combined with the detection of the polarization state of such harmonics.

Methods. To achieve the orbital structure, a first laser pulse is focused in a gas jet containing the molecules under study inside a vacuum chamber. In this way a coherent excitation of the rotational states of the molecule is obtained, which leads to a periodic realignment of the molecules at given temporal delays (rotational revivals).

During the alignment process, a second, high intensity laser pulse induces in the molecules the generation of coherent radiation with photon energies between a few eV and hundreds of eV and a spectrum containing odd harmonics of laser radiation (generation of high-order harmonics, HHG). The harmonic spectrum strongly depends on the angle between the polarization of the driving laser and the direction of the molecular axes, which varies with the delay between the two pulses.

A typical case is shown in the figure: the spectrum of harmonics generated in a jet of N2O molecules strongly depends on the delay between the two pulses: at 19.7 ps the molecules are aligned in the direction of the laser polarization; in this case a spectral suppression appears. Around a delay of 20.1 ps the molecules are directed perpendicularly to the laser polarization and the harmonic spectral intensity increases.

This effect is related to the quantum interference between the molecular orbital and the electronic wavepacket ionized by the laser field and recolliding with the parent molecule; during this recollision, the wavepacket probes the molecule and emits coherent radiation that depends on the shape of the orbital and the collision direction.

The vast majority of molecules has anisotropic structure; this leads to the emission of harmonic radiation with elliptical polarization. The harmonic polarization state encodes further information on the molecular orbital structure that cannot be inferred by the harmonic spectra alone, since it is related to the relative phase of the emitted harmonic field along orthogonal axes. For this reason, the harmonic spectra will be also analyzed with a polarimeter that is under development.  

Thesis activities and perspectives. The thesis activities are experimental and concern:

(i) acquisition of HHG spectra combined with harmonic polarization state detection as a function of the delay between the aligning and the HHG-driving laser pulses in linear and non-linear molecules in the ground state;

(ii) exploitation of the acquired data for the retrieval of the molecular orbital through suitable reconstruction techniques and comparison of the retrieved orbital with that calculated with quantum-mechanical simulation codes;

(iii) extension of the technique to molecules either in excited vibrational state or in excited electronic state by means of a three-laser-pulse scheme (alignment + molecule excitation + high harmonic generation) for time-resolved visualization of the excited molecular state.