Title: Extracting spectroscopic molecular parameters from short pulse photo-electron angular distributions,
Abstract: Using a quantum wave packet simulation including the nuclear and electronic
degrees of freedom, we investigate the femtosecond and picosecond energy- and
angle-resolved photoelectron spectra of the E($^1\Sigma_g^+$) electronic state
of Li$_2$. We find that the angular distributions of the emitted photoelectrons
depend strongly on the pulse duration in the regime of ultrashort laser pulses.
This effect is illustrated by the extraction of a time-dependent asymmetry
parameter whose variation with pulse duration can be explained by an incoherent
average over different ion rotational quantum numbers. We then derive for the
variation of the asymmetry parameter a simple analytical formula, which can be
used to extract the asymptotic CW asymmetry parameters of individual
transitions from measurements performed with ultra-short pulses.