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Role of Atomic and Molecular Resonances in High-order Harmonic Generation from Laser-ablated Plumes.

Fareed, Muhammad Ashiq (2017). Role of Atomic and Molecular Resonances in High-order Harmonic Generation from Laser-ablated Plumes. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en sciences de l'énergie et des matériaux, 118 p.

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High-order harmonic generation (HHG) process is explained with the so-called three-step model, where the valance electron is tunnel ionized, accelerates and photorecombines with the parent ion. In this model, it is assumed that electron’s motion is not perturbed from the atomic and molecular resonances. However, several experiments have shown that in the HHG process, the electron quantum path is perturbed from the atomic and molecular resonances that change the high-order harmonic properties. As the natural resonances are of different types (e.g., autoionizing, Giant resonance), their response for HHG is also observed differently. Several theoretical models have been proposed to explain the HHG mechanism from these resonances, but, in many cases of HHG from laser-ablated plumes (LAP), the information of the exact quantum path that the electron follows in the vicinity of these resonances was uncertain because of the lack of experimental proofs. In this thesis, we use different elements containing different types of resonances and study their role in HHG. Tin, manganese and diatomic carbon are used as nonlinear media, as these elements contain strong resonance at different energies. The resonance of tin exists at energy ~ 26.35 eV, which appears from the autoionizing state (AIS) lying above the ionization threshold. Manganese contains an inner-shell giant resonance at energy ~ 51 eV that appears mainly from 3p-3d transitions. The resonance from diatomic carbon molecules could come from molecular dissociation, or involve an AIS or bound excited state. In the first step, tin is used as a nonlinear medium for HHG to study electron interaction with the autoionizing 4d105s25p state. This AIS perturbs the HHG process and increases the intensity of a single harmonic, called the resonant harmonic (RH), at energy ~26.35 eV. The RH intensity is observed to be about 20 times higher than the intensity of neighboring harmonics generated with the conventional three-step process, when driven with 0.8 μm lasers. The physics involved behind this RH generation was unknown. We investigate electron dynamics in the vicinity of AIS with tunable long-wavelength laser pulses and explain the exact mechanism involved in RH generation. Further, in past, it was assumed that during RH generation, the continuum electron is perturbed by the AIS only. However, we have found for the first time that when continuum electron accelerates in the vicinity of the AIS, this electron is actually perturbed from three states; the AIS and its two dressed states of Sn+ lying at frequency ±2Ω (where Ω is the frequency of the driving laser). The perturbation from the dressed AIS appears in the form of two satellite harmonics generated at frequency ±2Ω around the RH. The contribution of dressed AIS in satellite harmonic generation is further confirmed by solving the time-dependent Schrödinger equation (TDSE). In the second step, manganese is used as a nonlinear medium for HHG. It is well known that from manganese two high-order harmonic series are generated. The first high-order harmonic series consists of harmonics up to 48 eV. After that, the second harmonic series (SHS) starts around 51 eV and high-order harmonics up to very high energy (~156 eV) are generated, with driving laser of 0.8 μm wavelength. The physical phenomenon of this SHS generation was circumstantial and lacked experimental proof. We used different experimental schemes to investigate the physical phenomenon involved and found that the inner-shell 3p-3d transitions of Mn+ contribute to SHS generation. These results provide the routes to use inner-shell electrons for HHG and generate harmonics up to very high energy, with elements having low ionization potential (e.g., IpMn+=15.64 eV) and also to study the multi-electron dynamics in atomic media, as at least two electrons (inner-shell and valance electron) contribute in HHG. Finally, we study high-order harmonics from laser-ablated graphite plume. We observed that intensity of carbon harmonics is surprisingly high, and that these harmonics contain a strong redshift. However, the physics behind these properties was unknown. In graphite ablation, different types of carbon species can be ablated with the laser-matter interaction, and because of this, the active species responsible for HHG from laser-ablated graphite plume was unsure. Therefore, first we used time-resolved plasma emission spectroscopy to check the species contributing to HHG. From the spectroscopic results, it is revealed that laser-ablated graphite plume contains an abundance of diatomic carbon molecules. The contribution of these diatomic molecules to HHG is studied by comparing the harmonic spectra with their photoionization cross-section (PICS). A good agreement is found between the PICS of C2 with the high-order harmonic spectrum. The spectroscopic results confirm that in laser-ablated graphite plume, high-order harmonics are generated from C2, and harmonic yield is high because of the large PICS of C2 in the XUV region. For the redshift, recent theoretical models predict that this redshift could appear because of two reasons. It could be either due to the involvement of the resonant excited states or molecular dissociation might be involved in HHG. We perform different experiments and investigate the exact physical reason involved behind this redshift.

Type de document: Thèse Thèse
Directeur de mémoire/thèse: Ozaki, Tsuneyuki
Mots-clés libres: Atomic and Molecular Resonance; High-order harmonic generation (HHG); Laser-ablated Plumes; natural resonances; Tin; manganese; diatomic carbon; propagation effects
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 29 janv. 2018 21:51
Dernière modification: 29 janv. 2018 21:51
URI: http://espace.inrs.ca/id/eprint/6656

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