Temperature-dependent study of the kinetics of Ta(a4F3/2) with O2, N2O, CO2 and NO

Abstract

The gas-phase reactivity of Ta(a ⁴ F _3/2 ) with O ₂ , N ₂ O, CO ₂ and NO in the temperature range 296–548 K is reported. The room-temperature removal rate constants for the spin–orbit excited states (a ⁴ F _J , J=5/2, 7/2, 9/2) are reported for these oxidants and CH ₄ . Tantalum atoms were produced by the photodissociation of tetracarbonyl cyclopentadienyl tantalum(0) [Ta(C ₅ H ₅ )(CO) ₄ ] and detected by laser-induced fluorescence. The reaction rate constants of the a ⁴ F _3/2 ground state with O ₂ , N ₂ O, CO ₂ and NO are temperature dependent. The disappearance rates in the presence of all the reactants are independent of total pressure, indicating a bimolecular abstraction mechanism. The bimolecular rate constants are described in Arrhenius form by k(O ₂ )=(1.7±0.2) ×10 ^-10 exp(-7.8±0.4 kJ mol ^-1 /RT) cm ³ s ^-1 , k(N ₂ O)=(7.1±1.0)×10 ^-11 exp(-13.6±0.6 kJ mol ^-1 /RT) cm ³ s ^-1 , k(CO ₂ )=(1.0±0.1)×10 ^-10 exp(-26.8±0.5 kJ mol ^-1 /RT) cm ³ s ^-1 and k(NO)=(1.0±0.2)×10 ^-10 exp(-1.6±0.8 kJ mol ^-1 /RT) cm ³ s ^-1 where the uncertainties are ±2σ. The removal rates of the spin–orbit excited states with O ₂ , N ₂ O, CO ₂ and NO are spin–orbit state dependent and are generally faster than for the ground state. The a ⁴ F _3/2 ground state is unreactive with methane, although the spin–orbit excited states are quenched by methane. The a ⁴ F _5/2 , a ⁴ F _7/2 and a ⁴ F _9/2 states have second-order room-temperature removal rate constants in methane of 6.0×10 ^-13 , 9.5×10 ^-13 and 2.3×10 ^-11 cm ³ s ^-1 , respectively.