Photon-Momentum-Induced Molecular Dynamics in Photoionization of ${\mathrm{N}}_2$ at $h\nu =40\mathrm{keV}$

We investigate $K$-shell ionization of ${\mathrm{N}}_2$ at 40 keV photon energy. Using a cold target recoil ion momentum spectroscopy reaction microscope, we determine the vector momenta of the photoelectron, the Auger electron, and both ${\mathrm{N}}^{+}$ fragments. These fully differential data show that the dissociation process of the ${\mathrm{N}}_2^{2+}$ ion is significantly modified not only by the recoil momentum of the photoelectron but also by the photon momentum and the momentum of the emitted Auger electron. We find that the recoil energy introduced by the photon and the photoelectron momentum is partitioned with a ratio of approximately $30:70$ between the Auger electron and fragment ion kinetic energies, respectively. We also observe that the photon momentum induces an additional rotation of the molecular ion.

Figure 1

Kinetic energy release of fragmentation of ${\mathrm{N}}_2^{2+}$ into ${\mathrm{N}}^{+}+{\mathrm{N}}^{+}$ after photoabsorption. The graph for $K$-shell photoionization at 419 eV photon energy is taken from Ref. [17]. (Red line) This Letter. ${\mathrm{N}}_2^{2+}$ produced by Compton scattering at $h\nu =40\mathrm{keV}$. (Blue line) This Letter. ${\mathrm{N}}_2^{2+}$ produced by $K$-shell photoionization with photons of $h\nu =40\mathrm{keV}$. The data are all normalized to the highest peak.

Figure 2

Position of the maximum in the KER distribution between 10 and 12 eV for Compton scattering and $K$-shell photoionization of ${\mathrm{N}}_2$ at $h\nu =40\mathrm{keV}$ as a function of $\cos \vartheta$, where $\vartheta$ is the angle (a) ${\vartheta }_{k_{ep},k_{\gamma }}$ or (b) ${\vartheta }_{k_{eA},k_{\mathrm{sum}}}$. See the text for an explanation (including of the shaded area). Black lines, the KER peak position for Compton scattering (${\mathrm{KER}}_0$); magenta points, measured data for the KER peak position for $K$-shell photoionization; red lines, expectation for $\mathrm{\Delta }\mathrm{KER}+{\mathrm{KER}}_0$, where $\mathrm{\Delta }\mathrm{KER}$ is taken from (a) Eq. (3) or (b) Eq. (4); blue line, $0.7\times \mathrm{\Delta }\mathrm{KER}+{\mathrm{KER}}_0$ [expectation from Eq. (3), assuming that only 70% of the maximum recoil energy is imparted on the ionic fragments].

Figure 3

Fragmentation of ${\mathrm{N}}_2$ by $K$-shell photoionization. Displayed are the components $x$ and $y$ of the relative momentum ${\overrightarrow{p}}_{\mathrm{rel}}$ (see the text). (a)–(d) Different emission directions of the photoelectron (gray lines). The red arrows indicate the photoelectron recoil, the blue arrows the sum of photoelectron recoil and the photon momentum.