Temperature dependence of ion formation in the nuclear track plasma

Secondary ion emission from organic Van der Waals solids irradiated by Cf-2 52-fission fragments was studied as a function of sample temperature. The s amples were produced by blowing a stream of gaseous ethylene and various al kanes onto a cold Cu-substrate having a temperature between 16 K and room t emperature. Mass spectra of positive as well as negative secondary ions wer e measured using a linear time-of-flight (TOF) technique. The positive ion spectra are molecule specific, they contain cluster ions and above the mole cular mass a strong pattern of reaction products. The negative ions ejected from ethylene and all alkanes investigated so far can hardly be distinguis hed, they consist mainly of CnHm- molecules with m = 0,1,2,3. Remarkable is a strong temperature dependence particularly of negative ions. With increa sing sample temperature, the rate of reactive collisions in the track plasm a seems to increase. This temperature effect is not related to a common pha se transition, - the change of yields with temperature is more gradual and is probably caused by a change of the sample density with temperature, whic h cannot be reversed. (C) 1998 Elsevier Science B.V. All rights reserved.