Molecular-Frame 3D Photoelectron Momentum Distributions by Tomographic Reconstruction

Naphthalene molecules are fixed in space by a laser field and rotated, in 2° steps, over 180°. For each orientation, they are ionized by an intense, circularly polarized femtosecond laser pulse, and the 2D projection of the photoelectron momentum distribution is recorded. The molecular-frame 3D momentum distribution is obtained by tomographic reconstruction from all 90 projections. It reveals an anisotropic electron distribution, angularly shifted in the polarization plane, that is not accessible by the 2D momentum images. Our theoretical analysis shows that the magnitude of the angular shift is very sensitive to the exact form of the laser-modified molecular potential.

Figure 1

(a) Schematic of the experimental setup showing a naphthalene molecule aligned by the elliptically polarized YAG pulse (red ellipse) and ionized by the circularly polarized probe pulse (red spiral). The VMI spectrometer projects the photoelectron momentum distribution onto the detector, and the 2D electron images are recorded for 90 alignments of the molecule over a 180° interval obtained by rotating the YAG polarization ellipse in steps of 2° around the $x$ axis. Examples of electron images for the major axis of the ellipse parallel to (vertical) and perpendicular to (horizontal) the detector are shown in (b) and (c), respectively.

Figure 2

Tomographic reconstruction of the molecular frame 3D photoelectron momentum distribution, represented by an isosurface, when naphthalene molecules are ionized by a RCP probe pulse. The momenta at the isosurface value of 0.035 are given in atomic units (a.u.). The result obtained with a LCP probe pulse is almost identical except that the angular shift of the isosurface in the field polarization plane $y^{\prime }$ and $z^{\prime }$ is in the opposite direction.

Figure 3

Projections of the 3D momentum distribution onto the polarization plane from 3D aligned naphthalene molecules ionized with a RCP probe pulse. The five different projections correspond to five different $p_{x^{\prime }}$ intervals: (a) 0.117–0.194 a.u., (b) 0.194–0.272 a.u., (c) 0.272–0.350 a.u., (d) 0.350–0.427 a.u., (e) 0.427–0.583 a.u. (f) The geometry of the projections with respect to the molecule and the probe pulse polarization. The probe pulse propagates into the paper.

Figure 4

Numerical values of the experimental (symbols) and theoretical (curves) angular shifts of the electron momentum distribution in the polarization plane as a function of probe pulse intensity for naphthalene. Full curve: Perfectly aligned molecules, full ionic potential; Dashed curve: Perfectly aligned, only Coulomb potential. Dotted curve: Nonperfectly aligned molecules (see text), full ionic potential. The experimental points are plotted for both LCP and RCP probe pulses. See text for details.