Experimental Determination of the Lifetime for the $2p3d({}^{1}P^0)$ Helium Doubly Excited State

Two recent theoretical studies [C. Liu et al., Phys. Rev. A 64, 010501 (2001); M. Žitnik et al., ibid. 65, 032520 (2002)] predict that the fluorescence lifetimes of helium doubly excited states converging to ${\mathrm{H}\mathrm{e}}^{+}$ $N=2$ should be longer than that of the ${\mathrm{H}\mathrm{e}}^{+}$ $2p$ 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 $2p3d({}^{1}P^0)$ doubly excited state. This was determined to be $190\pm 30\mathrm{p}\mathrm{s}$ compared to 100 ps for the ${\mathrm{H}\mathrm{e}}^{+}$ $2p$ 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.

Figure 1

(a) Photon yield from the ${\mathrm{H}\mathrm{e}}^{+}$ $2p$ ion state and fit giving a lifetime of $102\pm 15\mathrm{p}\mathrm{s}$. (b) Photon yield from the $2p3d({}^{1}P^0)$ doubly excited state and fit giving a lifetime of $190\pm 30\mathrm{p}\mathrm{s}$. The solid lines are instrumental response functions of the system.

Figure 2

The four most probable cascade decay routes from the $2p3d({}^{1}P^0)$ doubly excited state. The transition probabilities shown are those calculated by Žitnik et al. [16].

Figure 3

Comparison of the Monte Carlo simulation and closed form expressions for the time resolved yields of primary and secondary fluorescence.