Double differential fragmentation cross section measurements of H2n+1+ions,n⩽7

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Abstract
Pursuing previous integral fragmentation cross section measurements of hydrogen cluster ions with He as scattering partner [1], double differential fragmentation cross sections against Ar have been measured for ions in the mass range from H5 + to H15 +. The ions are formed by electron bombardment of neutral clusters in a nozzle beam and extracted from the ionizing region at an energy of 300 eV; directly in front of the scattering region they are decelerated to a laboratory energy E 0 between 6·8 and 75 eV. In the scattering region the primary (parent) ion beam is attenuated and fragment (daughter) ions are formed. The detection of the parent and daughter ions occurs as function of their laboratory deflection angle as well as their laboratory velocity. It is found that both the angle distribution and the velocity distribution broaden symmetrically with an increase of the number of H2 molecules lost in the fragmentation process. For example, at an energy E 0 = 6·8 eV the values for the full width at half maximum (FWHM) of the angular distributions in case of H13 + and H5 + (parent ion H15 +) amount to 1·8° and 8·4°, respectively. The FWHM for the velocity distributions are 200 m s-1 and 1500 m s-1, respectively. Except for the fragment H5 + from parent H15 + and H13 +, where a 5 per cent shift to lower velocities is observed at E 0=6·8 eV, the mean velocity of the other fragment ions equals the velocity of their parent ion, within the experimental accuracy of 2 per cent. The measured fragment angle distributions (proportional to dσ m,n dΩ) are converted to Im,n (θ)-curves (proportional to dσ m,n dθ) by multiplication by sin θ. For Ar and H2 as scattering partner it is found that the normalized distributions Im,n (θ) follow a universal curve if plotted against θ√E 0. Further, the FWHM of the time of flight measurements (TOF) decreases proportional to E 0 -1. From both these findings we shall argue, that, in a system travelling with the cluster, the fragments are distributed independently of E 0. A broadening of the angle distribution of the attenuated parent ions is hardly noticeable. The total sum of the integrated fragment intensities equals the loss of intensity of the parent ion, within the experimental accuracy of 20 per cent. Both observations lead to the conclusion that elastic scattering is unimportant.

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