Strong-field photoionization in two-center atomic systems

Photoionization of an atom $A$ by a strong laser field in the presence of a spatially well-separated neighboring atom $B$ is considered. The laser field frequency is assumed to lie below the ionization potential of atom $A$ and be resonant with a dipole-allowed transition in atom $B$. In this situation, the ionization may occur either directly by multiphoton absorption from the laser field at the first atomic center or via an indirect pathway involving two-center electron-electron correlations, where the neighbor atom $B$ is first photoexcited and, afterwards, transfers its energy upon deexcitation radiationlessly to atom $A$. Considering monochromatic as well as bichromatic laser fields, we study various coupling regimes of the photoionization process and identify experimentally accessible parameter domains where the two-center channel is dominant.

Figure 1

Scheme of strong-field two-center photoionization in a bichromatic laser field. An assisting atom $B$ is first resonantly photoexcited and, subsequently, transfers the excitation energy to atom $A$. In combination with the energy of several other photons, which are simultaneously absorbed from the laser field, atom $A$ is ionized. If the amplitude of the second frequency mode is rather high, the states of atom $B$ become strongly coupled, as indicated in the picture.

Figure 2

Photoionization in a Xe-H-like system with ${\omega }_1=1.8$ eV, ${\omega }_2=10.2$ eV, and $e{A}_{02}=3.7\times {10}^{-6}$ eV (weak-field case) at interatomic distance $R=10$ Å. Various ionization channels are shown (from top to bottom): bichromatic 2CPI [thick solid blue line, see Eq. (19)], bichromatic single-center PI [dashed red line, see Eq. (29)], monochromatic 2CPI in the field ${\mathcal{A}}_2$ [thin solid black line, see Eq. (10)], monochromatic single-center PI in the field ${\mathcal{A}}_1$ [dash-dotted orange line, see Eq. (28)], and monochromatic single-center PI in the field ${\mathcal{A}}_2$ [dotted green line, see Eq. (32)].

Figure 3

Photoionization in a He-Ne-like system with ${\omega }_1=1.7$ eV, ${\omega }_2=16.85$ eV, and $e{A}_{02}=3.7\times {10}^{-5}$ eV (weak-field case) at the interatomic distance $R=5$ Å. The various ionization channels are distinguished by the same line style and color coding as in Fig. 2.

Figure 4

Photoionization in a Xe-H-like system with ${\omega }_1=0.3$ eV, ${\omega }_2=10.2$ eV, and $e{A}_{02}=3.7\times {10}^{-3}$ eV (strong-field case) at the interatomic distance $R=10$ Å. The various ionization channels are distinguished by the same line style and color coding as in Fig. 2.

Figure 5

Photoionization in a He-Ne-like system with ${\omega }_1=0.85$ eV, ${\omega }_2=16.85$ eV, and $e{A}_{02}=3.7\times {10}^{-2}$ eV (strong-field case) at the interatomic distance $R=5$ Å. The various ionization channels are distinguished by the same line style and color coding as in Fig. 2.