A PLASMA DESORPTION MASS-SPECTROMETRY OF CHN-SURFACES( AND C2HN+ ION FORMATION FROM FROZEN ORGANIC)

Cyclohexane, cyclohexene, 1,3-cyclohexadiene and benzene frozen target s have been bombarded with Cf-252 fission fragments and the desorbed i ons have been analyzed by time-of-flight mass spectrometry. The behavi or of the desorption yields of the CHn+ ions (mass 12-16 u) and C2Hnion (mass 24-30 u) has been studied as a function of both the target s toichiometry and the fragment molecular structure. Because these resul ts show strong evidence of atomization followed by a recombination pro cess inside the fission fragment track, a quasi-equilibrium plasma mod el is proposed for the desorption mechanism of these species. Moreover , assuming a local thermodynamic equilibrium, the abundance of neutral and ionic species was determined. It is shown that, for appropriate c onditions of temperature and pressure inside the track, these calculat ions reproduce qualitatively the plasma desorption mass spectra of the above C-6 cyclohydrocarbons in the 12-30 u mass range.