Resonant Laser Ablation of Lanthanides: Eu and Lu Resonances in the 450−470 nm Region

Resonant laser ablation (RLA) of solids with a pulsed dye laser achieves wavelength-specific enhancements in yields of particular metal ions (M + ) by resonant photoionization or photoexcitation of atoms in the ablation plume. In the present study, RLA was performed for λ = 450−470 nm on solids comprising selected lanthanide elements. Enhancements in yields of Eu + and Lu + occurred at wavelengths corresponding to one-photon resonant absorptions for the neutral atoms: Ln + h ν → Ln*. The excited Ln* were subsequently preferentially ionized by the quasi-thermal collisional processes normally operative in the ablation plume: Ln* + kT → Ln + + e - . The excessive atomic absorption line widths of up to 40 cm - 1 (fwhm) were attributed primarily to collisional and Stark broadening in the dense plume. The strongest Eu + resonances correspond to transitions from ground Eu (4f 7 6s 2 8 S 7/2 0 ) to 4f 7 6s6p 8 P J levels at 21 445 ( J = 5 / 2 ), 21 605 ( J = 7 / 2 ), and 21 761 cm - 1 ( J = 9 / 2 ). Primary Lu + resonances were for excitation from ground Lu (5d6s 2 2 D 3/2 0 ) to 5d6s6p 2 D J levels at 21 462 ( J = 5 / 2 ) and 22125 cm - 1 ( J = 3 / 2 ). Other Ln + resonances were assigned to atomic transitions originating from levels as high as 7476 cm - 1 for Lu and 17 945 cm - 1 for Eu, which suggests a significant population of ablated atoms with internal energies up to ∼2.2 eV. Resonance enhancements of ∼10× were observed for Ln + compared with concurrently ablated nonresonant ions; optimization of such parameters as wavelength, irradiance, and ion extraction should result in substantially greater selectivity. The selective enhancement of Ln + increases the analytical utility of laser ablation mass spectrometry; the derived spectroscopic information complements conventional atomic absorption/emission techniques and can illuminate the role of excited state atoms in the ablation process.