Observation of Photoion Backward Emission in Photoionization of He and ${\mathrm{N}}_2$

We experimentally investigate the effects of the linear photon momentum on the momentum distributions of photoions and photoelectrons generated in one-photon ionization in an energy range of $300\mathrm{eV}\le E_{\gamma }\le 40\mathrm{keV}$. Our results show that for each ionization event the photon momentum is imparted onto the photoion, which is essentially the system’s center of mass. Nevertheless, the mean value of the ion momentum distribution along the light propagation direction is backward-directed by $-3/5$ times the photon momentum. These results experimentally confirm a 90-year-old prediction.

Figure 1

Mean value of electron (red) and ion (blue) momenta along the light propagation axis after one-photon ionization. Horizontal axis: Photon momentum $k_{\gamma }=E_{\gamma }/c$ (bottom scale) and photon energy (top scale). Circles: He photoionization (circularly polarized light); squares: ${\mathrm{N}}_2$ $K$-shell photoionization followed by Auger decay, where the mean value corresponds to the sum of both $N^{+}$ fragments adjusted for the Auger electron (linearly polarized light). The lines show Eqs. (1) and (2), respectively. Note that in order to compare the He and the ${\mathrm{N}}_2$ data sets, $I_P$ was approximated as zero in this figure. The error bars are asymmetric only because the $y$ axis is logarithmic.

Figure 2

Momentum distribution of ${\mathrm{He}}^{+}$ ions from single ionization by circularly polarized photons with $E_{\gamma }=300$, 600, 1125, and 1775 eV (from inner to outer ring). Horizontal axis: momentum component parallel to light propagation axis. Vertical axis: momentum perpendicular to the light propagation. The lower half is a mirror image of the upper half of the figure. The red circles are centered at the origin with a radius of the electron momentum $k^e=\sqrt{2(E_{\gamma }-I_P)}$. The blue outer circle is forward shifted by the corresponding photon momentum $k_{\gamma }=0.476\mathrm{a}.\mathrm{u}.$. The data from the different photon energies are arbitrarily normalized to match the color scale.

Figure 3

Electron angular distributions after single ionization of He obtained from the measured ion momentum distributions shown in Fig. 2, using ${\mathit{k}}^e=-{\mathit{k}}^{\mathrm{ion}}+{\mathit{k}}_{\gamma }$. (a) $E_{\gamma }=300\mathrm{eV}$, (b) $E_{\gamma }=1775\mathrm{eV}$. Red line: Fit using Eq. (3). (c) Nondipole parameter $\gamma$ from fit of Eq. (3) to the data. Red line: Calculations from Ref. [30].