TEMPERATURE-DEPENDENT STUDY OF THE KINETICS OF TA(A F-4(3 2)) WITH O-2, N2O, CO2 AND NO/

The gas-phase reactivity of Ta(a(4)F(3/2)) with O-2, N2O, CO2 and NO i n the temperature range 296-548 K is reported. The room-temperature re moval rate constants for the spin-orbit excited states (a F-4(J), J = 5/2, 7/2, 9/2) are reported for these oxidants and CH4. Tantalum atoms were produced by the photodissociation of tetracarbonyl cyclopentadie nyl tantalum(0) [Ta(C5H5)(CO)(4)] and detected by laser-induced fluore scence. The reaction rate constants of the a F-4(3/2) ground state wit h O-2, N2O, CO2 and NO are temperature dependent. The disappearance ra tes in the presence of all the reactants are independent of total pres sure, indicating a bimolecular abstraction mechanism. The bimolecular rate constants are described in Arrhenius form by k(O-2) = (1.7 +/- 0. 2) x 10(-10) exp(-7.8 +/- 0.4 kJ mol(-1)/RT) cm(3) s(-1), k(N2O) = (7. 1 +/- 1.0) x 10(-11) exp(-13.6 +/- 0.6 kJ mol(-1)/RT) cm(3) s(-1) k(CO 2) = (1.0 +/- 0.1) x 10(-10) exp(-26.8 +/- 0.5 kJ mol(-1)/RT) cm(3) s( -1) and k(NO)=(1.0 +/- 0.2) x 10(-10) exp(-1.6 +/- 0.8 kJ mol(-1)/RT) cm(3) s(-1) where the uncertainties are +/- 2 sigma. The removal rates of the spin-orbit excited states with O-2, N2O, CO2 and NO are spin-o rbit state dependent and are generally faster than for the ground stat e. The F-4(3/2) ground state is unreactive with methane, although the spin-orbit excited states are quenched by methane. The a F-4(5/2), a F -4(7/2) and a F-4(9/2) states have second-order room-temperature remov al rate constants in methane of 6.0 x 10(-13), 9.5 x 10(-13) and 2.3 x 10(-11) cm(3) s(-1) respectively.