Multiple Scales in the Fine Structure of the Isoscalar Giant Quadrupole Resonance in ²⁰⁸Pb

The fine structure of the isoscalar giant quadrupole resonance in ^{208}Pb, observed in high-resolution (p,p') and (e,e') experiments, is studied using the entropy index method. In a novel way, it enables to determine the number of scales present in the spectra and their magnitude. We find intermediate scales of fluctuations around 1.1 MeV, 460 keV and 125 keV for an excitation energy region 0 - 12 MeV. A comparison with scales extracted from second RPA calculations, which are in good agreement with experiment, shows that they arise from the internal mixing of collective motion with two particle-two hole components of the nuclear wavefunction.