Séminaire Temps & Fréquence (SYRTE)
Ionization of metastable neon with strong-field pulsed laser radiation
James Calvert – LNE-SYRTE
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Current pulsed laser systems are capable of generating laser intensities upward of 10^16W/cm^2. When matter interacts with laser radiation at these intensities, a variety of nonlinear effects can be observed, such as higher harmonic generation, above-threshold ionization and multiple ionization processes. Current understanding of these effects is through the Corkum three-step model, in which a valence electron undergoes tunneling ionization from its parent atom, is driven in the oscillating laser field and returns towards the parent ion, sometimes on a collisional trajectory. Until recently, the tunnel ionization process was typically modelled using the semi-classical Ammosov-Delone-Krainov (ADK) approach in which the strong field approximation is applied to provide an ionization rate for an atom in a given electric field, with a given ionization potential defined by the electronic state of the target atom. This approach breaks down in the limit of low ionization potentials (<10eV), where over-the-barrier ionization becomes the prevalent ionization mechanism, which in turn affects the initial state of the free electron, having flow on effects in the three state model. This work examines the strong-field ionization of the metastable 3P2 state of neon (Ne*), an atom with an ionization potential of 5.1eV, with the aim to aid in characterizing the initial ionization process of the three step model and provide insight into ionization processes. In particular, it is shown that computationally expensive TDSE solutions provide a more accurate comparison to experimental data than ADK modelling. It is also shown that the spin state of the target atom has an unexplained effect on the ionization rate, with an ionization yield difference of 16% between the ms = +1/2 and ms = -1/2 states. An examination of the transverse electron momentum from electrons released by Ne* and Ar reveals different momentum distributions for circularly polarized ionization radiation. This indicates that the strong field approximation is not applicable for atomic targets with low ionization potentials.
Paris ‐ Danjon