Chemistry of actinide and lanthanide metal ions (M+) in laser ablation of dispersions of inorganic compounds in polyimide and polytetrafluoroethylene

Abstract

The positive ions produced during excimer laser ablation of a polyimide (PI) or polytetrafluoroethylene (PTFE) matrix containing a small concentration of a solid compound comprising an actinide (Th, U) and/or lanthanide (La, Ce, Pr, Eu, Tb, Ho, Tm, Yb, Lu) were determined by time-of-flight mass spectrometry. The polyatomic mass spectra were dominated by species of the general form, M+–L, where M+ is the actinide/lanthanide and L is some fragment ligand from the polymer. For PI, primary products were M+–CcHh with c=2 or 4 and h=0 or 1. For PTFE, the main products were generally Mf+ with f=1 or 2. Although the studied metals are largely similar in their condensed phase chemistries, rational differences were evident in the compositions of the ablation products, which often corresponded to molecules identified in high-temperature vaporization of inorganic solids. For the M+–CcHh species, the degree of hydrogenation, i.e., h=0 or 1, consistently reflected the relative thermodynamic stabilities of the binary carbide molecules, MC2, which in turn closely correlate with those of the gaseous MO. Thus, U and Ce form especially robust monoxide and dicarbide molecules and, accordingly, produced primarily the bare dicarbide, MC2+. In contrast, the monoxides and dicarbides of Eu and Tm are relatively weakly bound and their PI ablation products incorporated hydrogen to produce more substantial yields of MC2H+. The compositions of the highly ionic metal fluorides produced from PTFE ablation reflected the relative stabilities of successive metal-ion oxidation states. For example, the yield of the difluoride ion, MF2+(≈M3+F2−), relative to that of the monofluoride, MF+(≈M2+F−), was found to decrease in the order, CeF2+>HoF2+>TmF2+, which is the inverse of the ordering of the +2 to +3 ionization energies: Ce<Ho<Tm. The observation of coherent thermochemical effects in metal-ion chemistry in ablation plumes suggests quasiequilibrium high-temperature conditions. Chemical speciation in laser ablation of metal–polymer aggregates is central to the formation of novel metal-doped polymer materials by pulsed laser deposition, and may be utilized to enhance elemental specificity in direct laser ablation sampling for mass spectrometric analysis. The described techniques are, furthermore, applicable to microscale investigation of the fundamental chemistry of scarce and radioactive transuranic actinides.