Multiphoton Double Ionization of Ar in Intense Extreme Ultraviolet Laser Fields Studied by Shot-by-Shot Photoelectron Spectroscopy

Photoelectron spectroscopy has been performed to study the multiphoton double ionization of Ar in an intense extreme ultraviolet laser field ($h\nu \sim 21\mathrm{eV}$, $\sim 5\mathrm{TW}/{\mathrm{cm}}^2$), by using a free electron laser (FEL). Three distinct peaks identified in the observed photoelectron spectra clearly show that the double ionization proceeds sequentially via the formation of ${\mathrm{Ar}}^{+}$: $\mathrm{Ar}+h\nu \to {\mathrm{Ar}}^{+}+e^{-}$ and ${\mathrm{Ar}}^{+}+2h\nu \to {\mathrm{Ar}}^{2+}+e^{-}$. Shot-by-shot recording of the photoelectron spectra allows simultaneous monitoring of FEL spectrum and the multiphoton process for each FEL pulse, revealing that the two-photon ionization from ${\mathrm{Ar}}^{+}$ is significantly enhanced by intermediate resonances in ${\mathrm{Ar}}^{+}$.

Figure 1

Photoelectron spectrum of Ar obtained by 15 000 FEL shots at a photon energy setting to 21.4 eV. The photoelectron kinetic energies expected in two-photon ionization of ${\mathrm{Ar}}^{+}$ $3p^{-1}$ into ${\mathrm{Ar}}^{2+}$ $3p^{-2}$, as well as those expected in one-photon ionization of Ar into ${\mathrm{Ar}}^{+}$ $3p^{-1}$, are indicated. Here, the long and short bars correspond to the energies relevant to the ${}^{2}P_{3/2}$ and ${}^{2}P_{1/2}$ levels of ${\mathrm{Ar}}^{+}$ $3p^{-1}$, respectively, as the final or initial state of each ionization step. A faint peak (less than 0.1% of the ${\mathrm{Ar}}^{+}$ $3p^{-1}$ peak intensity) is identified around 27 eV (not shown) due to the above-threshold ionization. In the inset, the mean photon energies determined from the ${\mathrm{Ar}}^{+}$ $3p^{-1}$ peak energies in the individual single-shot spectra are plotted against the intensities of the corresponding FEL shots which are monitored by a photoion yield detector.

Figure 2

(a) Intensity of the ${\mathrm{Ar}}^{+}$ photoelectron peak and total intensity of the ${\mathrm{Ar}}^{2+}$ peaks as a function of the FEL power, derived from the shot-by-shot analysis of the photoelectron spectra at a FEL setting to 21.4 eV photon energy. In order to cover the wide intensity range, the average FEL power is varied using a gas attenuator. The curves show the fits with a set of rate equations formulated for the sequential three-photon double ionization mediated by ${\mathrm{Ar}}^{+}$ resonances. The absolute FEL intensity is determined by the ${\mathrm{Ar}}^{+}$ photoelectron intensity curve with a fit using the reported absorption cross section [22]. (b) Energy diagram depicting sequential and nonsequential double ionization due to three-photon absorption.

Figure 3

Photoelectron spectra in the range of the ${\mathrm{Ar}}^{2+}$ $3p^{-2}$ peaks, derived from a single-shot-basis analysis. In the analysis, the single-shot photoelectron spectra obtained at three different nominal FEL photon energies of 21.0, 21.2, and 21.4 eV were sorted by the mean photon energies determined from the ${\mathrm{Ar}}^{+}$ $3p^{-1}$ peak energies, and then averaged within narrow photon energy ranges that divide the whole photon energy range into 13 segments.

Figure 4

Contour map showing the ${\mathrm{Ar}}^{2+}$ $3p^{-2}$ photoelectron profiles as a function of mean photon energy. The diagonal red (dotted) lines describe the kinetic energies for the two-photon absorption from the ${\mathrm{Ar}}^{+}$ $3p^{-1}$ spin-orbit levels. The white (solid) lines denote the energies of photoelectrons emitted on one-photon ionization from ${\mathrm{Ar}}^{+*}$ states, where the ${}^{2}D_{3/2,5/2}$ and ${}^{2}P_{1/2,3/2}$ levels of ${\mathrm{Ar}}^{+*}$ $[3p^{-2}({}^{1}D)]3d$ are assumed as the initial states for ${\mathrm{Ar}}^{2+}({}^{3}P)$ and ${\mathrm{Ar}}^{2+}({}^{1}D,{}^{1}S)$, respectively. Top panel shows the intensity evolution with photon energy for the formation of the individual ${\mathrm{Ar}}^{2+}$ levels, compared to the calculated oscillator strengths for the ${\mathrm{Ar}}^{+}$ $3p^{-1}$ (${}^{2}P_{1/2,3/2}$) to ${\mathrm{Ar}}^{+*}$ $[3p^{-2}({}^{1}D)]3d$ transitions [20].