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Circular Dichroism in Multiphoton Ionization of Resonantly Excited He Ion

Intense, circularly polarized extreme-ultraviolet and near-infrared (NIR) laser pulses are combined to double ionize atomic helium via the oriented intermediate He+(3p) resonance state. Applying angle-resolved electron spectroscopy, we find a large photon helicity dependence of the spectrum and the angular distribution of the electrons ejected from the resonance by NIR multiphoton absorption. M.Ilchen et al.  Physical Review Letters, Vol. 118 - 1, pp. 013002 (2017)

Dichroic phenomena in photoionization, i.e., the different response of a system to changes of the polarization state of the incoming light, have shown a high sensitivity to the dynamics of the underlying processes. Especially studies of the circular dichroism (CD), the different response to right- and left-circularly polarized light, have attracted much attention due to the possibility to investigate, for example, dichroic properties of electronic systems and chiral matter in general.

Dichroism in the strong field and multiphoton regime has been mainly investigated in the optical wavelength range. The extension of such studies to shorter wavelengths has become possible with the advent of free-electron lasers (FELs) and their unprecedented intensity over a large photon energy range from soft to hard x-rays. In particular, experimental investigations of the CD in two-color near-infrared (NIR)–extreme-ultraviolet (XUV) multiphoton ionization, which was predicted theoretically, only started recently with the operation of the seeded FEL FERMI, and even more recently with the installation of the Delta undulator at LCLS.

Here, we present a study where sequential double ionization and resonant two-color multiphoton ionization with circularly polarized pulses are applied together to the same system. The photoelectron spectra and the corresponding angular distributions arising from four-photon ionization of the XUV excited He+(3p) state by intense NIR pulses were investigated experimentally and theoretically for different relative helicities of the XUV and NIR radiation. The observed differences are attributed to the different angular momenta involved in the two-color multiphoton process. The most unexpected observation is related to the strong NIR intensity dependence of the CD, which is mainly caused by a helicity dependent ac Stark shift for the magnetic sublevel of the He+(3p) resonance.

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Circular Dichroism in Multiphoton Ionization of Resonantly Excited He+ Ions
Ilchen Markus, Douguet Nicolas, Mazza Tommaso, Rafipoor Amir Jones, Callegari Carlo, Finetti Paola, Plekan Oksana, Prince Kevin Charles, Demidovich Alexander, Grazioli Cesare, Avaldi Lorenzo, Bolognesi Paola, Coreno Marcello, Di Fraia Michele, Devetta Michele, Ovcharenko Yevheniy, Duesterer Stefan, Ueda Kiyoshi, Bartschat Klaus, Grum-Grzhimailo Alexei, Bozhevolnov A. V., Kazansky A. K., Kabachnik N. M., Meyer Michael
Physical Review Letters, Vol. 118 - 1, pp. 013002 (2017)
doi: 10.1103/PhysRevLett.118.013002
Last Updated on Thursday, 13 April 2017 12:51