Selective production of the doubly excited $2p^2$ (${}^{1}D$) state in He-like Ar${}^{16+}$ ions by resonant coherent excitation

Selective production of the doubly excited state in 464 MeV/u He-like Ar${}^{16+}$ ions was accomplished using three-dimensional resonant coherent excitation in a thin foil of silicon crystal. Through a coherent interaction with the periodic crystal field, the Ar${}^{16+}$ ions were resonantly excited sequentially from the ground state to the $1s2p$ (${}^{1}P$) state and then $2p^2$ (${}^{1}D$) state in a way analogous to two-color x-ray laser excitation. The periodic crystal fields of 10${}^{11}$ V/m in the projectile's frame are equivalent to the fields generated by x-ray lasers with photon energies of 3139.55 and 3286.95 eV and an energy flux of 10${}^{16}$ W/cm${}^2$. We confirmed the doubly excited state formation through observation of the subsequent collisional ionization, Auger ionization, and radiative deexcitation.

Figure 1

Energy levels of He-like Ar${}^{16+}$ and H-like Ar${}^{17+}$ [8] with their rates (in s${}^{-1}$) for radiative deexcitation [9], collisional ionization [10], and the Auger ionization [11].

Figure 2

(a) Schematic layout of the experimental setup and detailed geometry of the electron analyzer. (b) The resonance conditions for RCE of $1s^2-1s2p$ (blue line) and $1s2p-2p^2$ (red line), respectively. The dots are the conditions used in the measurements for the NR, SR, and DR conditions. (c) The charge-state distribution of transmitted Ar ions at the 2D-PSD observed under the NR, SR, and DR conditions.

Figure 3

Energy-momentum distribution of the observed electrons under the (a) NR, (b) SR, and (c) DR conditions. The dashed line represents the theoretical dispersion relationship of the electron energy and momentum. The projections onto the momentum axis, i.e., the electron-momentum spectra, are shown in a linear scale at the right side of the contour maps. The electron-momentum spectrum under the DR condition is also shown in a logarithmic scale in (d) with a result of Monte Carlo simulation. The electron yields are corrected against fluctuation of the beam intensity.

Figure 4

P frame x-ray spectra emitted from the projectile ions under the NR, SR, and DR conditions. Two-component Gaussian curves are fitted to the spectra and are shown as solid lines to guide the eye.