Quenching of the $2pnd$ ${}^1{P}^o$ doubly excited states of helium by a dc electric field

The fluorescence yield quenching of low-lying doubly excited $2pnd$ ${}^1{P}^o$ states is observed to depend strongly on a dc electric field strength and its orientation with respect to the polarization of the incoming photon beam. The reduction of the yield accompanied by the lifetime shortening is attributed to the Stark mixing with the neighboring $2sns$ ${}^1{S}^e$ states, which redirects the $2pnd$ ${}^1{P}^o$ decay to the prompt autoionization channel. For $n\ge 4$, the lifetimes decrease from several hundred picoseconds down to several tens of picoseconds when an electric field in the kV/cm range is applied parallel to the photon probe polarization. Practically no lifetime change is observed for polarization perpendicular to the electric field direction. The results of the complex-scaling calculations are in a good agreement with the experimental data.

Figure 1

Fluorescence yield measured in the region of $nc/(n+1)b$ ${}^1{P}^o$ doublets below the $N=2$ threshold. Denoted are also the energy positions of the dark $nb/(n+1)a$ ${}^1{S}^e$ doublets. The energy axes are expanded so that each minor tick represents 10 meV. The experimental energies have been shifted by 20 meV to match the calculated energy positions. Each doublet was measured under different experimental conditions and the yields are not given on the same scale.

Figure 2

The experimental scheme. The direction of external electric field $\mathrm{F}$ can be rotated around the wave vector ${\mathrm{k}}_0$ of the incident photon beam to change its angle with respect to the incoming light polarization ${\widehat{\mathrm{e}}}_0$.

Figure 3

Benchmark determination of the lifetime of the He${}^{+}$ $2p$ state in the zero electric field. Shown are the multibunch signal (a) and the signal of many bunches folded into an equivalent single-bunch response (b).

Figure 4

Influence of different field strengths on the decay time of the $4c$ state in parallel geometry $\left(\mathrm{F}\right|\left|{\widehat{\mathrm{e}}}_0\right)$. The signal of different bunches has been folded into a single bunch and normalized to 1.

Figure 5

The lifetimes of the $nc$ ${}^1{P}^o$ resonances in the external electric field for the parallel $\left(\mathrm{F}\parallel {\widehat{\mathrm{e}}}_0\right)$ and perpendicular $\left(\mathrm{F}\perp {\widehat{\mathrm{e}}}_0\right)$ geometries. The measured lifetimes (points) are compared to the calculated results (lines).

Figure 6

The relative change of the signal amplitude versus the relative change of the lifetime as a function of electric field strength [Eq. (12)] for the $4c$ ${}^1{P}^o$ state.

Figure 7

The lifetimes of the $nb$ ${}^1{P}^o$ resonances in the external electric field for the perpendicular $(\mathrm{F}\perp {\widehat{\mathrm{e}}}_0$, a) and parallel $(\mathrm{F}\parallel {\widehat{\mathrm{e}}}_0$, b) geometries. The measured lifetimes (points) are compared to the calculated results (lines).

Figure 8

The lifetimes of the $na$ ${}^1{P}^e$ resonances in the external electric field for the perpendicular $\left(\mathrm{F}\perp {\widehat{\mathrm{e}}}_0\right)$ geometry. The measured lifetimes (points) are compared to the calculated results (lines), as well as the results from [21]. The points are shifted horizontally for clarity to avoid overlapping.