  Neutral particles ejected from biological material under ultra short laser
ablation have been investigated by laser post-ionization time-of-flight mass
spectrometry. It could be shown, that beside ionized species, a substantial
amount of neutral particles is ejected. A temporal study of the ablation plume
is carried out by recording neutral particle time-of- flight mass spectra as a
function of delay time between the ablation and post-ionization pulse. Close
the ablation threshold, the mechanism of ejection is found to be of
predominantely mechanical nature, driven by the relaxation of the laser-induced
pressure. In this regime of stress confinement, the ejection results in very
broad velocity distributions and extremely low velocities.
