Evolution of angular distributions in two-colour, few-photon ionization of helium

Abstract

Single ionization of helium by a superposition of selected XUV high harmonics and infrared radiation has been studied by a momentum imaging technique. The measured angular distributions of photoelectrons are compared to numerical time-dependent calculations, showing very good agreement after average. The calculated angular distributions appear to depend critically on the delay between harmonic and infrared pulses on the attosecond scale, and on the relative phases and intensities of the harmonics.

Non-linear processes in the interaction of atoms and molecules with intense EUV and X-ray fields from SASE free electron lasers (FELs)

“…The spectrum is now dominated by the influence of the optical field showing an increase in the electron kinetic energy of more than 20 eV, i.e the absorption of more than 12 Ti-Sapphire laser photons. The monochromaticity of the FEL radiation gives us unprecedented access to clean sidebands, free from interference with the signal from neighbouring harmonics [75,76] or from a complicated Auger structure [77]. Hence, the intense FEL pulses mean that we can now begin to study optical multi-photon processes high up in the continuum of an atom (or indeed molecule) in an unperturbed fashion for the first time.…”

Probing the energy flow in Bessel light beams using atomic photoionization

“…As for the circular dichroism, all these (seven) dichroism signals generally depend for a localized target on the photoelectron emission angles ϑ p and ϕ p as well as on the particular sideband . For sufficiently extended targets (σ b κ 1), however, the photoionization probability and, hence, the dichroism signals above become independent of the (projection of the) orbital angular momentum or topological charge, m. This can be seen for instance from the analytical expression for the photoionization probability for infinitely extended targets (29), which is independent of m. For large targets, therefore, all the dichroism signals will depend just on the product Λ X Λ L of the two helicities and, thus, all signals with Λ X Λ L = +1 must vanish in this limit,…”

“…This circular dichroism, that is associated with some flip of the spin-angular momentum (SAM) of the incident light field, has recently been utilized, e.g., for measuring the polarization state of an ultra-violet free electron laser [26][27][28]. For molecules, in addition, the question has been raised how two-color ATI spectra are affected by the molecular symmetry and the polarization of the incident radiation [29,30].…”

Corrélations vectorielles dans la photoionisation dissociative de H2par une harmonique XUV femtoseconde sélectionnée spectralement par des miroirs multicouches

“…Dans l'approximation du recul axial [21], on peut accéder à la MFPAD, comme cela est illustré en figure 1 pour une transition perpendiculaire. Pour les expériences avec H21 [1], l'enceinte ultra-vide CIEL 2 [12] accueillait le spectromètre CV. La polarisation linéaire (P) du faisceau lumineux est parallèle à l'axe du spectromètre.…”

“…Les expériences de corrélations vectorielles (CV), mesurant en coïncidence les vecteurs vitesses d'émission de l'ion et de l'électron [10,11], fournissent les informations les plus complètes sur la dynamique moléculaire. Leur couplage avec l'utilisation de sources d'harmoniques d'ordre élevé (HHG) ouvre la voie à des études résolues en temps à l'échelle fs [12]. Dans cette direction, nous avons rapporté les premiers résultats caractérisant l'évolution de la distribution angulaire du photoélectron émis dans le référentiel moléculaire (MFPAD) pour la DPI de H 2 induite par l'harmonique 21 (H21) d'un laser Infra-Rouge (IR), d'une durée de quelques dizaines de femtosecondes.…”