Title: Photofragment Spectra and Dynamic Imaging of Molecules
Abstract: Under a short, intense laser pulse, a molecule exhibits non-linear behavior giving rise to new observable phenomena. Interpreting these phenomena requires a good understanding of the way a strong-field modifies the properties of the molecule, both at the level of its electronic structure and at the level of its dynamics. This theme will be illustrated by the successive discussions of two processes: First, the dissociative ionization of a molecule such as H_2 under the combined effect of a single extreme ultraviolet attosecond pulse and an intense near infrared pulse [F. Kelkensberg et al, Phys. Rev. Lett. 103,123005 (2009)] will be considered. We show how this pump-probe study actually represents a transition-state spectroscopy of the strong-field dissociatiaon step, i.e. of the (probe-pulse-)dressed H_2^+ molecular ion. The way the dissociation dynamics is influenced by the duration of the near infrared probe pulse, and by the time delay between the two pulses, is discussed in terms of adiabatic versus non-adiabatic preparations and transports of time-parameterized Floquet (vibrational) resonances associated with the dissociating molecular ion. Turning next to strong-field molecular ionization, as depicted by the celebrated three-step model, we show and discuss how symmetry conservation of the Symmetry Adapted Linear Combinations (SALC) form of initial orbitals during the ionization of a symmetric molecule governs the general structure of channel-resolved photoelectron spectra, in particular of its high-energy part relevant to Laser-Induced Electron Diffraction (LIED). The issue of stability of such a symmetry-based reading is discussed by considering symmetry breaking during rotations and vibrations of the pre-aligned molecule. The concepts are illustrated mainly on results of simulations of the ionization of a $CO_2$ molecule using a Single-Active Electron (SAE) model. Many-electron wavepacket calculations performed in an attempt to go beyond the SAE approximation then show that many-orbital effects are also to be taken into account as they can reduce the stability of the photoelectron spectra with respect to molecular misalignment.
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