[18p-C201-1~7] Ion Beam and Surface Analysis: Recent Progress in Secondary Ion Mass Spectrometry (SIMS) and Its Application to Organic Analysis

[18p-C201-5] Desorption/ionization process of organic molecules induced by molecular cluster ion beams

〇Kousuke Moritani¹ (1.Univ. of Hyogo)

Keywords:SIMS, cluster ion beam, desorption and ionization

In this decade, massive Ar cluster ion beams have been applied to projectiles for time-of-flight secondary ion mass spectrometry (TOF-SIMS). Some primary ion beam projectiles enable the detection of protein molecular ions with molecular masses exceeding several thousands daltons so far. However, in order to analyze more complicated and actual samples, such as tissue samples, the intact ion intensity should be more enhanced. Recently, water and some other molecules are applied to the massive cluster projectiles for SIMS for improving the secondary ion yields by chemical effects. The ionization probability should be influenced by the properties of the bombarding primary ion beams as well as the chemical nature of the sample surface during the collision with the primary ion. However, the ionization mechanism by liquid cluster is still unclear. We have investigated the secondary ion emission from hydrophobic and hydrophilic molecules by Ar and some molecular cluster ion beams and developed the stable molecular cluster source and equipped it with the SIMS apparatus. The molecular cluster beams were generated by bubbling Ar at constant flow rate through a reservoir filled with liquids (water, methanol, benzene, and so on) at room temperature. The cluster sizes were selected by time of flight (TOF) method and the typical size-resolution M/ΔM was 5 at Ar₁₅₀₀⁺ at the acceleration voltage of 5 kV. The sample was a thin film of amino acids, a polystyrene (PS) and a 1,4-didodecylbenzene (DDB). For the amino acid samples, the intact ions are detected as the [M+H]⁺ for both Ar cluster and hydrogen-bond cluster, such as water and methanol cluster bombardment. The water cluster projectile enhanced the yields of [M+H]⁺ species at the low energy region of 2 eV/molecule. On the other hand, for DDB molecule, intact ion was detected as the M⁺ form and the water cluster did not detected at 2eV/molecule but enhanced around 6eV/molecule. In this presentation, the desorption and ionization mechanisms for the molecular cluster ion projectiles will be discussed.