Few-Photon Multiple Ionization of ${\mathbf{N}}_2$ by Extreme Ultraviolet Free-Electron Laser Radiation

Few-photon multiple ionization of ${\mathrm{N}}_2$ was studied differentially in a reaction microscope using 44 eV, $\sim 25\mathrm{fs}$, intense ($\sim {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$) photon pulses from FLASH. Sequential ionization is observed to dominate. For various intermediate charge states $\mathrm{N}_2{}^{n+}$ we find a considerable excess of photons absorbed compared to the minimum number that would energetically be required. Photoionization of aligned $\mathrm{N}_2{}^{n+}$ ions, produced by photon absorption in sequential steps, is explored and few-photon absorption pathways are traced by inspecting kinetic energy releases and fragment-ion angular distributions.

Figure 1

Illustration of the dominant dissociative multiple ionization pathways of ${\mathrm{N}}_2$ and the KER spectra of coincident ionic fragments ${\mathrm{N}}^{+}/{\mathrm{N}}^{+}$ and ${\mathrm{N}}^{+}/{\mathrm{N}}^{2+}$ for double (blue and violet) and triple (green) ionization, respectively. Potential curves are taken from [10, 15, 22, 27]. Three $\mathrm{N}_2{}^{2+}$ dissociation states are clearly visible in the KER spectra plotted in the right of this figure, marked as (a), (b), and (c), respectively. The KER spectrum labeled $d$ corresponds to dissociation of the ${}^2\Pi _u$ state of $\mathrm{N}_2{}^{3+}$ [22]. The Frank-Condon regime (vertical shaded band) and two-photon transitions (vertical arrows) are sketched.

Figure 2

Ion yields as a function of the laser intensity $I$. Solid points and solid lines denote the present measurements and fits ($\log Y=n·\log I+\log {\sigma }_n$), respectively.

Figure 3

FIAD polar density plots differential in the KER for various fragmentation channels. $\theta$ and the radius represent the angle with respect to the polarization axis (horizontal) of the linearly polarized light and the KERs, respectively. Curves display the KER spectra integrated over all fragment emission angles $\theta$. (a) noncoincident ${\mathrm{N}}^{+}$ fragments. (b) ${\mathrm{N}}^{+}+{\mathrm{N}}^{+}$ coincidences. (c) noncoincident ${\mathrm{N}}^{2+}$ ions.

Figure 4

FIADs for selected KER regimes (dissociation states).