bombardment source 12 metastable atom bombardment gun is known from u s pat no 6 124 675 the metastable atom bombardment source comprises a metastable atom gun in which metastable species atoms or small molecules are produced and an ionization volume 24 is provided in which the decomposition products of pyrolysis collide with the metastable atom beam and are instantly ionized in this specification the term u201c metastable atom u201d includes all metastable species namely both atoms typically noble gas atoms and small gas molecules such as nitrogen which exhibit suitable properties with respect to becoming excited into a metastable state and then transferring their metastable state energy to other molecules to be ionized as described in u s pat no 6 124 675 this transfer of energy is of a precise quantum and is done with minimal exchange of kinetic energy thus resulting in ionization with little fragmentation the source 12 generates a beam of metastable atoms which is substantially free of ions due to its internal arc being curved with the anode positioned away from the beam axis because u201c in beam u201d pyrolysis is conducted within a beam of metastable species primary products radicals or molecules are produced in a cloud of metastable species leading to their ionization hence high molecular weight materials cannot be lost because they are converted to ions that are extracted from the ion volume by an electrical field the metastable atom bombardment source assembly including the ion volume is shown schematically in fig2 the metastable atom gun 12 is located at the back of the ion volume 24 and the beam of metastable.