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 lambda = 450-470 nn 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: In + hv --> 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) S-8(7/2)0) to 4f(7)6s6p P-8(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 In (5d6s(2) D-2(3/2)0) to 5d6s6p D-2(J) levels at 21 462 (J = 5/2) and 22 125 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 similar to 2.2 eV. Resonance enhancements of similar to 10x 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.