1ST OBSERVATION OF A FANO PROFILE FOLLOWING ONE-STEP AUTOIONIZATION INTO A DOUBLE PHOTOIONIZATION CONTINUUM

We have measured the double photoionization cross section of sodium at oms between the first 2s(2)2p5 2P double photoionization (52,4 eV) and 2s-single 1.3S photoionization thresholds (71.0 eV). We have also obs erved a Fano profile into the double ionization continuum resulting fr om the interference between the one-step direct double photoionization process and the resonant double Auger decay of core-excited neutral s odium in the 2s --> 3p resonance region. Profiles of absolute partial and total cross sections have been obtained in all important channels. The Fano and Starace parameters, in particular a width of 0.23 eV, ha ve been determined, allowing full characterization of the resonance.