Propensity Rule for Novel Selective Double Photoexcitation of Helium Atoms in Strong Static Electric Fields

We studied the double photoexcitation spectra of helium in a strong dc electric field and compared the results with the recent experimental data of Harries et al. [Phys. Rev. Lett. 90, 133002 (2003)]. We derived the propensity rules based on the crossing or noncrossing of energies in the Stark map to predict the selective subset of doubly excited states that are preferentially populated in such experiments. It is shown that the propensity rule is a consequence of the ubiquitous correlation properties of doubly excited states in general.

Figure 1

Hyperspherical potential curves for He ${}^{1}P^0$ states converging to the $N=2$ threshold of ${\mathrm{He}}^{+}$.

Figure 2

Comparison of calculated spectra of He in the presence of a dc electric field at $84.4\mathrm{kV}/\mathrm{cm}$ with the experimental data of Harries et al. [1]. The red (dark gray), blue (dark), and green (light gray) bars indicate these states evolved from the $a$,$b$, and $c$ field-free states, respectively, following the present propensity rule. The length of each bar measures the relative oscillator strength. The field-free ${}^{1}P^0$ resonances are also indicated at the top of the figure.

Figure 3

Stark map around the $6a{}^{1}P^0$ state. The correlation diagram is constructed from the present calculated energies at different fields following the proposed propensity rule. The purple curves are from the experimental data of Harries et al. [1]. The calculated oscillator strength for the prominent states are shown by gray bars. For details, see the text.

Figure 4

Stark map around the $7a{}^{1}P^0$ state. Others are the same as Fig. 3.