Experimental determination of the lifetime for the 2p3d(1P0) helium doubly excited state

Two recent theoretical studies [C. Liu et al., Phys. Rev. A 64, 010501 (2001); M. Zˇ itnik et al., ibid. 65, 032520 (2002)] predict that the fluorescence lifetimes of helium doubly excited states converging to He N 2 should be longer than that of the He 2p ion state. This effect is caused by the electric field of the outer electron which, through Stark mixing, gives the inner fluorescing electron some series specific, stabilizing 2s character. We have obtained the first experimental evidence that confirms this effect by measuring the lifetime of the 2p3d1P0 doubly excited state. This was determined to be 190 30 ps compared to 100 ps for the He 2p ion state. The measurements were performed using short pulses of synchrotron radiation to form doubly excited states and recording the arrival time of photons from fluorescence.