RMT calculation of the photoelectron momentum distribution in the polarisation plane for F- in a two-cycle, 800~nm pulse. The extremely short pulse employed constrains the ionisation to a single pulse peak. The main feature is a peak of the photoelectron momentum which, in the simplified 'strong-field' picture, should occur on the horizontal axis. However, the peak is clearly shifted, appearing at a negative angle of around -12°. This demonstrates that some additional, multi-electron effect is influencing the trajectory of the outgoing electron. However, because this is a negative ion we may rule out the influence of the long-range Coulomb potential which is present during the ionisation of noble gas targets: the photo-detached electron 'sees' only the neutral fluorine atom. In the associated paper, we propose that instead, the contraction of the electron cloud in response to the photo-detachment is the source of the outgoing electron's momentum shift.