5.6.1 Dissociation

5. 6.1 Dissociation

Molecular primary particles dissociate upon reflection from a surface. Remaining charged molecules molecules can easily be identified in the TOF spectra as a new peak. Neutral molecules would appear at the same position as neutral single atoms but the neutral molecule peak is much narrower so may be separated from the wider neutral atom peak (Figure 5.6 ).

In TOF spectra acquired by reflecting particles of the MgO surface no surviving molecules either charged nor neutral could be observed even at the lowest energies investigated at JUSO (O $_{2}$ : $\approx$ 400eV per atom, H $_{2}$ : $\approx$ 200eV per atom) . There might be some remaining neutral molecules hidden in the neutral atom peaks measured but their fraction must be less than 2-3%. This limit is imposed by the fitting procedure used to calculate the charge state fractions. In the further analysis of the data I neglected the molecule fraction and assumed complete dissociation on the MgO surface.

**Figure 5.6:** TOF spectra taken from [27 ] showing a molecular neutral peak (H $_{2}$ ). A 629eV H $^{+}_{2}$ primary beam on a Al(110) target was used. No molecular peaks were ever observed when using a MgO surface.

March 2001 - Martin Wieser, Physikalisches Institut, University of Berne, Switzerland