1 Molecular Movies from Molecular Frame Photoelectron Angular
11
(C 2 Σ g , I P = 19.4 eV)). When the ionization by the XUV laser pulse is preceded
by the IR alignment laser, appreciable changes occur in the measured photoelectron momentum distributions. This is reflected in the experimental contour plot
(left side) shown in Fig. 1.4b, where the afore-mentioned differential photoelectron kinetic energy and angular distribution is plotted as a function of the kinetic
energy and the angle of ejection of the photoelectron with respect to the laser polarization axis. The differential photoelectron kinetic energy and angular distribution shows all the rings that are visible in the 2D slice in Fig. 1.4a, and moreover
shows that the differences between the measurements for aligned and anti-aligned
molecules sensitively depend both on the orbital that is ionized and the electron kinetic energy. To begin with, the influence of the ionized orbital manifests itself in
the total photoelectron yield. The yield of electrons from the HOMO and HOMO1 orbitals is suppressed when the molecules are aligned compared to when the
molecules are anti-aligned, whereas the yield of photoelectrons corresponding to
the HOMO-3 increases when the molecules are aligned. Ionization of the HOMO2 favors aligned molecules at low photoelectron kinetic energies, but this behavior
reverses above a kinetic energy of 15 eV, when anti-aligned molecules ionize more
efficiently.
The dependence of the ionization on the alignment/anti-alignment of the molecular sample informs about the perpendicular resp. parallel character of the ionizing
transition. When the photoionization occurs by means of a parallel transition the
ionization efficiency of molecules that are aligned parallel to the laser polarization
axis will be higher than that of molecules that are anti-aligned. In this case, the
symmetry of the final (molecular ion + electron) state will be Σ u . Similarly, when
the photo-ionization occurs by means of a perpendicular transition, the ionization
efficiency of molecules that are aligned perpendicular to the laser polarization axis
will be higher than that of molecules that are aligned along the polarization axis
and the symmetry of the final (molecular ion + electron) state will be Π u . Based on
the experimental data the conclusion can be drawn that the HOMO and HOMO-1
of CO 2 ionize by means of a perpendicular transition, and the HOMO-3 by means
of a parallel transition. The ionization of the HOMO-2 is predominantly parallel at
low energies (up to a photoelectron kinetic energy of 15 eV) and then changes to
predominantly perpendicular.
The experimental results can be well-reproduced by an electron-molecule quantum scattering method that was previously also successfully applied to calculate
MFPADs recorded with synchrotron radiation [34, 35]. This method is based on the
multichannel Schwinger configuration interaction method (MCSCI), where the initial state and the final ionic states are represented as configuration interaction (CI)
wave functions. Calculated differential photoelectron kinetic energy and angular
distributions (making use of the alignment distributions provided by the experimental fits of the time-dependent molecular alignment, see Fig. 1.3) are shown in the
contour plot shown on the right side of Fig. 1.4b, as well as in Figs. 1.4c–f, where
the theoretical differential photoelectron kinetic energy and angular distributions are
plotted separately for the four most important orbitals that contribute to the ionization signal. The overall agreement between the experimental and theoretical data is
very satisfactory.
11
(C 2 Σ g , I P = 19.4 eV)). When the ionization by the XUV laser pulse is preceded
by the IR alignment laser, appreciable changes occur in the measured photoelectron momentum distributions. This is reflected in the experimental contour plot
(left side) shown in Fig. 1.4b, where the afore-mentioned differential photoelectron kinetic energy and angular distribution is plotted as a function of the kinetic
energy and the angle of ejection of the photoelectron with respect to the laser polarization axis. The differential photoelectron kinetic energy and angular distribution shows all the rings that are visible in the 2D slice in Fig. 1.4a, and moreover
shows that the differences between the measurements for aligned and anti-aligned
molecules sensitively depend both on the orbital that is ionized and the electron kinetic energy. To begin with, the influence of the ionized orbital manifests itself in
the total photoelectron yield. The yield of electrons from the HOMO and HOMO1 orbitals is suppressed when the molecules are aligned compared to when the
molecules are anti-aligned, whereas the yield of photoelectrons corresponding to
the HOMO-3 increases when the molecules are aligned. Ionization of the HOMO2 favors aligned molecules at low photoelectron kinetic energies, but this behavior
reverses above a kinetic energy of 15 eV, when anti-aligned molecules ionize more
efficiently.
The dependence of the ionization on the alignment/anti-alignment of the molecular sample informs about the perpendicular resp. parallel character of the ionizing
transition. When the photoionization occurs by means of a parallel transition the
ionization efficiency of molecules that are aligned parallel to the laser polarization
axis will be higher than that of molecules that are anti-aligned. In this case, the
symmetry of the final (molecular ion + electron) state will be Σ u . Similarly, when
the photo-ionization occurs by means of a perpendicular transition, the ionization
efficiency of molecules that are aligned perpendicular to the laser polarization axis
will be higher than that of molecules that are aligned along the polarization axis
and the symmetry of the final (molecular ion + electron) state will be Π u . Based on
the experimental data the conclusion can be drawn that the HOMO and HOMO-1
of CO 2 ionize by means of a perpendicular transition, and the HOMO-3 by means
of a parallel transition. The ionization of the HOMO-2 is predominantly parallel at
low energies (up to a photoelectron kinetic energy of 15 eV) and then changes to
predominantly perpendicular.
The experimental results can be well-reproduced by an electron-molecule quantum scattering method that was previously also successfully applied to calculate
MFPADs recorded with synchrotron radiation [34, 35]. This method is based on the
multichannel Schwinger configuration interaction method (MCSCI), where the initial state and the final ionic states are represented as configuration interaction (CI)
wave functions. Calculated differential photoelectron kinetic energy and angular
distributions (making use of the alignment distributions provided by the experimental fits of the time-dependent molecular alignment, see Fig. 1.3) are shown in the
contour plot shown on the right side of Fig. 1.4b, as well as in Figs. 1.4c–f, where
the theoretical differential photoelectron kinetic energy and angular distributions are
plotted separately for the four most important orbitals that contribute to the ionization signal. The overall agreement between the experimental and theoretical data is
very satisfactory.