Measurement of asymmetric electron cloud in cluster nano-plasma

A nanocluster ionized by a femtosecond pulse of wavelength larger than the cluster size is known to set up an electron cloud that oscillates with respect to the ion cloud. Most theoretical frameworks on understanding cluster ionization are built on this dipole oscillation model. Here, we probe this dipole by pump-probe absorption spectroscopy. A linearly polarized pump pulse sets up the dipole and the polarization of the probe pulse that comes after a prescribed time is varied, and the differential changes in the absorption are measured. The polarization dependence of the probe pulse absorption gives a direct measure of the asymmetry and ellipticity in the dielectric permittivity. Our results show that orthogonally polarized pump-probe pulses absorb about 18% less compared to the parallel case at the peak of the linear plasma resonance. A dipole excitation model compared to the experimental observations is used to decipher the asymmetry in the transient electron cloud.