Picometer-Resolved Photoemission Position within the Molecule by Strong-Field Photoelectron Holography

Laser-induced tunneling ionization is one of the fundamental light-matter interaction processes. An accurate description of the tunnel-ionized electron wave packet is central to understanding and controlling subsequent electron dynamics. Because of the anisotropic molecular structure, tunneling ionization of molecules involves considerable challenges in accurately describing the tunneling electron wave packet. Up to now, some basic properties of the tunneling electron from molecules still remain unexplored. Here, we demonstrate that the tunneling electron from a molecule is not always emitted from the geometric center of the molecule along the tunnel direction. Rather, the photoemission position depends on the molecular orientation. Using a photoelectron holographic technique, we determine the photoemission position for a nitrogen molecule relative to the molecular geometric center to be $95\pm 21\mathrm{pm}$ when the molecular axis is oriented along the tunnel direction. Our Letter poses, and answers experimentally, a fundamental question as to where the molecular photoionization actually begins, which has significant implications for time-resolved probing of valence electron dynamics in molecules.

Figure 1

(a) The schematic of tunneling ionization of a ${\mathrm{N}}_2$ molecule aligned along the tunnel direction. Because of the effect of the molecular orbital, the ionized electron wave packet might be emitted from an initial coordinate (photoemission position) of $r_0$, rather than the center of the molecule. (b)–(e) The scheme of probing the photoemission position with photoelectron holographic interferometry. By adding a weak 400-nm linearly polarized laser field perpendicular to the polarization direction of a strong 800-nm field, the rescattering electron will return to the molecular geometric center with different intermediate canonical momenta along the 400-nm-field direction depending on the photoemission position (b),(d). The intermediate canonical momentum will be mapped onto the shift of the holographic fringe along the 400-nm-field direction (c),(e). The holographic patterns are simulated by the strong-field approximation in (c) and (e), in which the dashed and dash-dotted lines show the central maxima of the holographic structures without and with the weak 400-nm field, respectively. The arrow in (e) shows the shift direction of the holographic fringes.

Figure 2

(a),(b) The measured PMDs in the OTC laser fields of ${\mathrm{N}}_2$ molecules with alignment angles of (a) $\varphi =0°$ and (b) $\varphi =90°$. The central maximum of the PMD is guided by the white dashed lines. The relative phase of the OTC fields is the same for (a) and (b), corresponding to the most asymmetric fringes for the holographic structures. (c) The shift $\mathrm{\Delta }{p}_y$ of the central maximum as a function of the relative phase for the alignment angles of $\varphi =0°$ and $\varphi =90°$ at $p_x=0.4$ a.u. The solid lines show the fitting results of the experimental data with a sinusoidal function.

Figure 3

(a) $\mathcal{A}$ with respect to the final momentum $p_x$, where $\mathcal{A}$ is the amplitude of the oscillation of the fringe shift with the relative phase. For comparison, the simulated results with assuming an initial photoemission position of $r_0=2$ a.u. and $r_0=0$ are shown by the dashed blue and red curves, respectively. (b) The PMD in a single 800-nm laser field. The vertical dashed lines show the position of the classical $2U_p$ cutoff.

Figure 4

(a) The extracted photoemission position of ${\mathrm{N}}_2$ molecules for the alignment angle of $\varphi =0°$. The nuclear position relative to the molecular center ($R_0/2$) is shown by the dashed line, and the most probable photoemission position from the Wigner function distribution is shown by the blue solid line. (b),(c) The Wigner function distributions for the ${\mathrm{N}}_2$ molecules with alignment angles of (b) $\varphi =0°$ and (c) $\varphi =90°$. The red dots and blue rhombi show the molecular geometric centers and the most probable photoemission positions, respectively.