The reactivity of the gas-phase lanthanide atoms Ln (Ln = La−Yb with the exception of Pm) with O2 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 cm3 s-1) are described
in Arrhenius form by k[Ce(1G4)] = (3.0 ± 0.4) × 10-10 exp(−3.4 ± 1.3 kJ mol-1/RT); Pr(4I9/2), (3.1 ± 0.7)
× 10-10 exp(−5.3 ± 1.5 kJ mol-1/RT); Nd(5I4), (3.6 ± 0.3) × 10-10 exp(−6.2 ± 0.4 kJ mol-1/RT); Sm(7F0),
(2.4 ± 0.4) × 10-10 exp(−6.2 ± 1.5 kJ mol-1/RT); Eu(8S7/2), (1.7 ± 0.3) × 10-10 exp(−9.6 ± 0.7 kJ mol-1/RT); Gd(9D2), (2.7 ± 0.3) × 10-10 exp(−5.2 ± 0.8 kJ mol-1/RT); Tb(6H15/2), (3.5 ± 0.6) × 10-10 exp(−7.2
± 0.8 kJ mol-1/RT); Dy(5I8), (2.8 ± 0.6) × 10-10 exp(−9.1 ± 0.9 kJ mol-1/RT); Ho(4I15/2), (2.4 ± 0.4) ×
10-10 exp(−9.4 ± 0.8 kJ mol-1/RT); Er(3H6), (3.0 ± 0.8) × 10-10 exp(−10.6 ± 1.1 kJ mol-1/RT); Tm(2F7/2),
(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(1S0) with O2 reacts through a termolecular mechanism. The limiting low-pressure third-order
rate constants are described in Arrhenius form by k
0[Yb(1S0)] = (2.0 ± 1.3) × 10-28 exp(−9.5 ± 2.8 kJ
mol-1/RT) molecule-2 cm6 s-1.