Laser-induced electron diffraction: alignment defects and symmetry breaking

The fringe pattern that allows geometrical and orbital structure information to be extracted from LIED (laser-induced electron diffraction) spectra of symmetric molecules is shown to reflect a symmetry conservation principle. We show that under a field polarisation which preserves certain symmetry elements of the molecule, the symmetry character of the initial wave function is conserved during its time-evolution. We present a symmetry analysis of a deviation from a perfect alignment by decomposing the field into amajor, symmetry-determining part, and a minor, symmetry-breaking part. This decomposition leads to a corresponding factorisation of the time-evolution operator. The formalism is applied to the analysis of the robustness of LIED readings and inversions with respect to deviations from a perfect perpendicular and parallel alignment of a symmetric ABA triatomic molecule. The results indicate a particularly strong stability of the type of LIED spectra associated with the perpendicular alignment situation. [GRAPHICS]