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).