Secondary ion mass spectrometry of low-temperature solids

Secondary ion mass spectra obtained by bombardment of low-temperature solids such as the rare gases, nitrogen, oxygen, nitrogen oxides, and others with heavy ions of keV energies contain many cluster ion peaks, often with unusual compositions. A mechanism which accounts for the results in a qualitative way is described. It is proposed that it has general mechanistic implications for keV particle-induced desorption mass spectrometry in spite of the obvious difference between the low-temperature solids and the more usual molecular solids. The mechanism has three principal features. First, it is recognized that keV-energy atoms and ions are quite efficient in causing ionization, secondary electron formation, ionizing fragmentation and electronic excitation of molecules within the molecular solid. These processes occur in addition to homolytic bond cleavage within the collision cascade region and lead to local charging within the insulating solid. Some of the damage centers in the solid are ejected essentially immediately from one of the surface layers during the collision cascade period of the impact event. Second, since the chemical nature of the primary damage centers is the same as in MeV-energy bombardment, so are the chemical reactions and charge transfer processes leading to secondary damage, at least in a qualitative sense. Some of the secondary damage products are ejected during the thermal spike regime which follows the collision cascade. The relative importance of the two batches of the secondary ions is a function of the nature of the bombarding ion. Third, polar and/or polarizable molecules will be the best at aggregating with the various resulting charged species during the ejection from the thermal spike region into the vacuum and the best at staying associated with the charged species during subsequent gas-phase fragmentation of the resulting metastable cluster ions in an “evaporating fractionation” process.