Highly nonlinear, sign-varying shift of hydrogen spectral lines in dense plasmas

Hydrogen H(alpha) shift is measured in a plasma characterized by a noticeable coupling due to a high electron density (N(e)>/=2x10(18) cm(-3)) and a relatively low temperature (T=8900 K). The plasma is produced by a single laser breakdown in an underwater medium, creating a stable and radially homogeneous plasma. The results cannot be explained by the known contributions to the shift, which predict shifts by a factor of 2 greater than the experimental shifts. A contribution called dipole ionic-electronic shift (DIES) is introduced. The data presented in this work constitute the experimental discovery of this phenomenon. Indeed, the total theoretical shifts obtained taking into account the DIES are in excellent agreement with our experimental values.