Temperature-Dependent Rate Constants for the Reactions of Gas-Phase Lanthanides with O2

The reactivity of the gas-phase lanthanide atoms Ln (Ln = La−Yb with the exception of Pm) with O 2 is reported. Lanthanide atoms were produced by the photodissociation of [Ln(TMHD) 3 ] and detected by laser-induced fluorescence. For all the lanthanides studied with the exception of Yb, the reaction mechanism is bimolecular abstraction of an oxygen atom. The bimolecular rate constants (in molecule -1 cm 3 s -1 ) are described in Arrhenius form by k [Ce( 1 G 4 )] = (3.0 ± 0.4) × 10 -10 exp(−3.4 ± 1.3 kJ mol -1 / RT ); Pr( 4 I 9/2 ), (3.1 ± 0.7) × 10 -10 exp(−5.3 ± 1.5 kJ mol -1 / RT ); Nd( 5 I 4 ), (3.6 ± 0.3) × 10 -10 exp(−6.2 ± 0.4 kJ mol -1 / RT ); Sm( 7 F 0 ), (2.4 ± 0.4) × 10 -10 exp(−6.2 ± 1.5 kJ mol -1 / RT ); Eu( 8 S 7/2 ), (1.7 ± 0.3) × 10 -10 exp(−9.6 ± 0.7 kJ mol -1 / RT ); Gd( 9 D 2 ), (2.7 ± 0.3) × 10 -10 exp(−5.2 ± 0.8 kJ mol -1 / RT ); Tb( 6 H 15/2 ), (3.5 ± 0.6) × 10 -10 exp(−7.2 ± 0.8 kJ mol -1 / RT ); Dy( 5 I 8 ), (2.8 ± 0.6) × 10 -10 exp(−9.1 ± 0.9 kJ mol -1 / RT ); Ho( 4 I 15/2 ), (2.4 ± 0.4) × 10 -10 exp(−9.4 ± 0.8 kJ mol -1 / RT ); Er( 3 H 6 ), (3.0 ± 0.8) × 10 -10 exp(−10.6 ± 1.1 kJ mol -1 / RT ); Tm( 2 F 7/2 ), (2.9 ± 0.2) × 10 -10 exp(−11.1 ± 0.4 kJ mol -1 / RT ), where the uncertainties represent ±2σ. The reaction barriers are found to correlate to the energy required to promote an electron out of the 6s subshell. The reaction of Yb( 1 S 0 ) with O 2 reacts through a termolecular mechanism. The limiting low-pressure third-order rate constants are described in Arrhenius form by k 0 [Yb( 1 S 0 )] = (2.0 ± 1.3) × 10 -28 exp(−9.5 ± 2.8 kJ mol -1 / RT ) molecule -2 cm 6 s -1 .