Correlation between Double and Nonresonant Single Ionization

We performed simultaneous measurements of electrons and ions produced in Xe photoionization driven by an 800-nm, 100-fs laser pulse. The obtained energy and angular resolved electron spectra allow the identification of the electronic states populated during the ionization. ${\mathrm{X}\mathrm{e}}^{2+}$ ions appear at the same laser intensity as electrons emerging from a nonresonant 9-photon ionization process of Xe. Similar to optical tunneling ionization, the nonresonant ionization delivers low-energy electrons needed for the formation of ${\mathrm{X}\mathrm{e}}^{2+}$ by a backscattering process.

Figure 1

CEIS spectra from xenon. Left part: momentum electron spectrum is shown together with the ion trace recorded by a time-of-flight mass spectrometer (laser: 800 nm, 100 fs). Intensities: (a) $2.2\times {10}^{13}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$; (b) $3\times {10}^{13}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$; (c) $3.3\times {10}^{13}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$. The electron spectrum at (a) is mainly influenced by a $8+1$ REMPI process via the $4f$, $5f$, and $6f$ states. Doubly charged xenon can be found at laser intensities higher than $3\times {10}^{13}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$, where the electron spectrum is influenced by the 9-photon NR ionization process. Right part: dependency of the ionic yield as a function of the laser intensity. For comparison the result of the $S$-matrix theory for $\mathrm{X}{\mathrm{e}}^{\mathrm{+}}$ and $\mathrm{X}{\mathrm{e}}^{\mathrm{2}\mathrm{+}}$ is shown (solid line). The dashed line represents the calculation without the nonsequential part of the ionization.

Figure 2

Left: The electron yields as a function of the laser intensity from selected areas. These areas are shown in an electron image recorded at $3.8\times {10}^{13}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ (upper part). Rectangle: $e^{-}$ yields from the $9+1$ REMPI ($5g$) process. (For easier viewing, these data are moved down about 1.5 decades.) Circles: $e^{-}$ yields from the nonresonant ionization. Triangles: yields of $e^{-}$ emitted at an angle of $64°$ according to the polarization axes of the laser. The nonresonant process can be fitted with a line of a slope of about 9 and a straight line fitting the triangle has a slope of 4. Right: electron spectra as a function of the laser intensity.