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DDC: 530 × Subject: Helium ×

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Now showing 1 - 3 of 3
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    Excited state distribution and spin-effects in strong-field excitation of neutral Helium
    (Bristol : IOP Publ., 2015) Zimmermann, Henri; Eilzer, Sebastian; Eichmann, Ulli
    We investigated the principal quantum number n distribution of excited states resulting from the interaction of Helium with strong, short laser pulses. We find excellent agreement with predictions of the semiclassical frustrated tunneling ionization (FTI) model [1] as well as fully quantum mechanical calculations. Furthermore, the excitation process directly populates triplet excited states due to the breakdown of the Russel-Saunders coupling scheme for high orbital angular momentum l states of Helium, which are predominantly populated in the strong laser field.
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    Hyper-domains in exchange bias micro-stripe pattern
    (Milton Park : Taylor & Francis, 2008) Theis-Bröhl, K.; Westphalen, A.; Zabel, H.; Rücker, U.; McCord, J.; Höink, V.; Schmalhorst, J.; Reiss, G.; Weis, T.; Engel, D.; Ehresmann, A.; Toperverg, B.P.
    A combination of experimental techniques, e.g. vector-MOKE magnetometry, Kerr microscopy and polarized neutron reflectometry, was applied to study the field induced evolution of the magnetization distribution over a periodic pattern of alternating exchange bias (EB) stripes. The lateral structure is imprinted into a continuous ferromagnetic/antiferromagnetic EB bilayer via laterally selective exposure to He-ion irradiation in an applied field. This creates an alternating frozen-in interfacial EB field competing with the external field in the course of the re-magnetization. It was found that in a magnetic field applied at an angle with respect to the EB axis parallel to the stripes the re-magnetization process proceeds via a variety of different stages. They include coherent rotation of magnetization towards the EB axis, precipitation of small random (ripple) domains, formation of a stripe-like alternation of the magnetization, and development of a state in which the magnetization forms large hyper-domains comprising a number of stripes. Each of those magnetic states is quantitatively characterized via the comprehensive analysis of data on specular and off-specular polarized neutron reflectivity. The results are discussed within a phenomenological model containing a few parameters, which can readily be controlled by designing systems with a desired configuration of magnetic moments of micro- and nano-elements.
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    Attosecond electron spectroscopy using a novel interferometric pump-probe technique
    (College Park, Md. : APS, 2010) Mauritsson, J.; Remetter, T.; Swoboda, M.; Klünder, K.; L'Huillier, A.; Schafer, K.J.; Ghafur, O.; Kelkensberg, F.; Siu, W.; Johnsson, P.; Vrakking, M.J.J.; Znakovskaya, I.; Uphues, T.; Zherebtsov, S.; Kling, M.F.; Lépine, F.; Benedetti, E.; Ferrari, F.; Sansone, G.; Nisoli, M.
    We present an interferometric pump-probe technique for the characterization of attosecond electron wave packets (WPs) that uses a free WP as a reference to measure a bound WP. We demonstrate our method by exciting helium atoms using an attosecond pulse (AP) with a bandwidth centered near the ionization threshold, thus creating both a bound and a free WP simultaneously. After a variable delay, the bound WP is ionized by a few-cycle infrared laser precisely synchronized to the original AP. By measuring the delay-dependent photoelectron spectrum we obtain an interferogram that contains both quantum beats as well as multipath interference. Analysis of the interferogram allows us to determine the bound WP components with a spectral resolution much better than the inverse of the AP duration. © 2010 The American Physical Society.