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Journal of Physical Chemistry A, Vol.110, No.25, 7751-7754, 2006
Wavelength dependent photofragmentation patterns of tris(2,2,6,6-tetramethyl-3,5-heptanedionato)Ln (III) (Ln = Eu, Tb, Gd) in a molecular beam
Laser photoionization and ligand photodissociation in Ln(thd)(3) (Ln) Eu, Tb, Gd; thd) 2,2,6,6- tetramethyl-3,5- heptanedionato) are studied in a molecular beam via time-of-flight mass spectrometry. The fragmentation patterns are strongly wavelength dependent. With 355 nm excitation, the mass spectrum is dominated by Ln(2+), Ln(+), and LnO(+) fragments. The bare Ln ions are believed to arise from photoionization of neutral Ln atoms. The Ln atoms, in turn, are produced from the Ln( thd) 3 complex in a sequence of Ln reductions ( through ligand-to-metal charge- transfer transitions), with each reduction being accompanied by the dissociation of a neutral ligand radical. In contrast, under visible-light (410- 450 nm) excitation, a significant Ln( thd)(n)(+) signal is observed (where n = 2,3 for Ln) Tb, Gd and n = 1-3 for Ln = Eu). Thus, with visible excitation, photoionization of Ln( thd) n competes effectively with the Ln-reduction/ligand-dissociation sequence that leads to the dominant bare Ln- ion signal seen with 355 nm excitation. The fact that monoligated Ln(thd)(+) is observed only for Ln) Eu is interpreted in terms of the relative accessibility of an excited ligand-to-metal charge-transfer state from the ground electronic state of neutral Ln( thd).