About the atypical behavior of CrO3, MoO3, and WO3 during their UV laser ablation/ionization

Pulsed-tripled Nd:YAG and excimer laser ablation of chromium, molybdenum, and tungsten trioxide at high power density has been found to generate significant yields of oxygenated cluster ions. They were analyzed by Fourier transform ion cyclotron resonance mass spectrometry. Two kinds of cluster ions were considered. The first ones are largely oxygen-deficient and their formation are the result of aggregation processes between neutrals and ions according to results obtained in the study of other transition-metal oxides. The second ones are MnO3n+ stoichiometric and MnO3n-1+ and MnO3n-1+ slightly substoichiometric species (with M = Cr, Mo, and W and n = 1-6). The production of these latter ions decreases dramatically with the power density deposited on the sample surface. This behavior is non-wavelength-dependent. Previously, Knudsen effusion mass spectrometry studies and thermodynamic considerations allowed us to determine their formation pathways. These ions come from interaction between electrons and neutral species generated by vaporization or sublimation of the sample near the laser impact or both. Electrons are emitted according to various processes during laser/matter interaction conditions.