Unveiling the Inhomogeneous Nature of Strong Field Ionization in Extended Systems

Intense light-induced fragmentation of spherical clusters produces highly energetic ions with characteristic spatial distributions. By subjecting argon clusters to a wavelength tunable laser, we show that ion emission energy and anisotropy can be controlled through the wavelength-isotropic and energetic for shorter wavelengths and increasingly anisotropic at longer wavelengths. The anisotropic part of the energy spectrum, consisting of multiply charged high-energy ions, is considerably more prominent at longer wavelengths. Classical molecular dynamics simulations reveal that cluster ionization occurs inhomogeneously producing a columnlike charge distribution along the laser polarization direction. This previously unknown distribution results from the dipole response of the neutral cluster which creates an enhanced field at the surface, preferentially triggering ionization at the poles. The subsequently formed nanoplasma provides an additional wavelength-dependent ionization mechanism through collisional ionization, efficiently homogenizing the system only at short wavelengths close to resonance. Our results open the door to studying polarization induced effects in nanostructures and complex molecules and provide a missing piece in our understanding of anisotropic ion emission.