To describe electrochemical interfacial structures and processes,many theoretical models and calculation methods have been developed.Jellium electron model is widely accepted and utilized to interpret many phenomena related to the electron spillover at the interface.However,the conceptual jellium has not been experimentally measured due to limitations of sensing and resolving the electron in solution on the scale of a few angstroms.Therefore,it is only possible to measure the distribution of the jellium by a molecule/atomic ruler.Surface-enhanced Raman scattering(SERS)is powerful to characterize molecules adsorbed on electrode surfaces with a high sensitivity,even up to the single molecule level.Recntly we have tried to push electrochemical SERS to the limit,i.e.,to expand the in-situ characterization from surface speacies to the electronic jellium at electrode-electrolyte interfaces,as shown in Fig.1a.The scale of small adsorbed molecules at a surface is about several angstroms.To push the electronic jellium to extend from metal surface into solution with a few angstroms,a very negative potential is required.Fig.1b presents the potential dependent SERS spectra of water adsorbed on Ag electrode.The intensity of bending mode of water(ca.1610 cm-1)increases distinctively as the potential is negatively moved.We performed DFT calculations on Raman spectra of water at negative charged Ag,the jellium model and the SPR distribution.The length of the jellium spillover into solutions at very negative potentials should be over 3 ?.We also studied the Au,Pt and Pd electrode systems,and surface hydrogen at Pt was used as an atomic ruler.In order to reveal the potential dependent jellium length,we also employed the organic molecular ruler to measure the jellium length on Ag.By using a relative intensity of two different functional groups,the potential dependence of the penetration length of electronic jellium could be measured.