Research

Circular Dichroism of Atoms and Molecules

During the last years, studies of Circular Dichroism (CD), the different response of a sample to right- versus left-circularly polarized light, have attracted much attention due to the possibility to investigate the electronic and structural symmetry properties of matter. CD has a broad range of interest: from fundamental spin control to bio-chemistry (chiral molecules) and material science (e.g., magnetization) [1]. Dichroism in the strong field and multiphoton regime has been mainly investigated in the optical wavelength range. Free-electron lasers (FELs), with their unprecedented intensity over a large photon energy range from soft to hard X-ray extend such studies to shorter wavelengths [2]. In particular, experimental investigations of the CD in two-color near-infrared (NIR)–extreme-ultraviolet (XUV) multiphoton ionization, which was predicted theoretically [3], only started recently [4-6] with the operation of the seeded FEL FERMI.

Figure 2. a) Schematic representation of the two-color  multiphoton ionization process for He atom at XUV/NIR intensities 80mJ and 750mJ, respectively. Both XUV (48.4eV) and NIR (784 nm) pulses are left-hand circularly polarized. b) Typical VMI image cut through the 3D velocity and angular distribution obtained by Abel-inversion of the experimental image. c) Part of the photoelectron spectrum in the region of the He(1s) main line and the high-energy sidebands (SBs) measured at p/2 emission angle [4].