Mass Spectrometry

If alkali metals such as Cs, Li, Rb, K, Na, etc. (referred as A in general) are present in the neighborhood of the probing element(M) on a sample surface, quasi-molecular ions can be formed by the attachment of these alkali ions [(MA)+ formation] in thesecondary ion mass spectrometry (SIMS) process. Formation of these MA+ molecular ions has astrong correlation to the atomicpolarizability of the element M. The emission process for the re-sputtered species M0 is decoupled from the MA+ ion formationprocess, in analogy with the ion formation in secondary neutral mass spectrometry (SNMS), resulting in a drastic decrease in the conventional ‘matrix effect’ in SIMS. Although the detection of MA+ molecular ions in SIMS has found its applicability in directmaterials quantification, it generally suffers from a low useful yield. In such cases, detection of (MA)n+ (n=2, 3,….) molecular ionsoffers a much better sensitivity (even by several orders of magnitude), as the yields of such molecular ion complexes have oftenbeen found to be higher than that of MA+ ions. The recombination coefficient of MA+ or MA2+ molecular species depends onthe electro-positivity or electro-negativity of the element M, respectively. Apart from the surfacebinding energy of the respectiveuppermost monolayer, the changes in local surface work-function have often been found to play a significant role in the emissionof these molecular ions. Although these MAn+ molecular-ion based SIMS has great relevance in the analysis of materials, acomplete understanding on the formation mechanisms of these ion-complexes is still lacking. A procedure, based on MAn+-SIMSapproach, has been proposed for the accurate germanium quantification in molecular beam epitaxy (MBE)-grown Si1−xGex alloys.The ‘matrix effect’ has been shown to be completely suppressed for all Ge concentrationsirrespective of impact Cs+ ion energies.Cesium, the fifth alkali element, is the most reactive of all the metals. The methodology hassuccessfully been applied for directquantitative composition analysis of various thin film and multilayer structures. Recent study on various ZnO-based nanostructures has successfully been correlated to their photo-catalysis and photoemission responses. The present talk will address the complex formation mechanisms of MAn+ molecular ions and potential applications of the MA+-SIMS approach in chemical analysis of low dimensionalmaterials and nanostructures.