Ionization Probability in Molecular Secondary Ion Mass Spectrometry: Protonation Efficiency of Sputtered Guanine Molecules Studied by Laser Postionization

The prospect of improved secondary ion yields for secondary ion mass spectrometry (SIMS) experiments drives innovation of new primary ion sources, instrumentation, and postionization techniques. An important factor affecting the detection sensitivity in molecular SIMS and other desorption techniques as well is believed to be the poor ionization probability of a sputtered molecule, a value which is often assumed to be as low as 10^-5 but at present is basically unknown. In order to estimate how much headroom there is for future developments toward strategies aimed at enhancing the ionization probability, we study the protonation efficiency of sputtered guanine molecules for formation of [M + H]⁺ secondary ions using strong field laser postionization (LPI) to detect the corresponding neutral molecules. In order to allow a quantitative comparison of secondary ion and neutral yields, the postionization signal is corrected for undersampling of the principally detectable plume of sputtered neutral particles by the focused laser beam. It is shown that the protonation probability of molecular guanine desorbed from a clean film under bombardment with 20 keV C₆₀ cluster projectiles is of the order of 1-2 × 10^-3, with some remaining uncertainty arising from laser-induced fragmentation and possible differences in the emission velocity distributions of neutral and ionized molecules. Moreover, we find that the postionization signal can in principle be boosted by 2 orders of magnitude if a more powerful ionization laser is employed.