Simultaneous versus sequential two-photon double electron ejection from He-

The 1+1 photon resonance-enhanced double ionisation of He has been observed for the first time in the interaction of a fast ion beam with a pulsed laser beam between 375 and 425 nm. Although a clear enhancement of the He+ yield is observed above the 1s3s S-3 threshold, a minimum is found at the exact location of all three photodetachment resonances located in this range. This pleads against the sequential ionisation model. The 1s3s4S S-4 resonance located below the 1s3s threshold appears to be ideally suited for the demonstration of electron correlation effects accompanying their simultaneous ejection. A careful analysis of the spectrum is needed, as spurious resonances existing in the 1+1 REMPI of He(2(3)S) and He(2(3)P) add up to the total ion yield