COLLISIONAL ACTIVATION OF LARGE IONS - ENERGY-LOSSES AND AN IMPULSIVECOLLISION-THEORY OF ENERGY-TRANSFER

The translational energy losses, Delta E, experienced by keV beams of singly charged ions of high masses (> 1000 u) in collisions with eithe r an inert-gas atom or hydrogen molecule depend on the mass of the tar get gas employed. There is no evidence of a dependence on the ionisati on energy of the target gas. Delta Es are similar with He and D-2 targ ets, and both are larger than those with Ar. This behaviour is found w ith organic ions composed of light atoms and with inorganic cluster io ns composed of heavy atoms. The measured Delta Es are consistent with internal energy uptake, Q, occurring via direct momentum transfer in a n impulsive collision. It is concluded that the Q taken up by an ion i s dependent upon the masses of its constituent atoms. In collision wit h He, a light-atom ion of a given molecular mass takes up more Q than does a heavy-atom ion of similar molecular mass under the same experim ental conditions, but the Delta E can be similar in the two cases.