KINETIC-STUDIES OF GROUND-STATE ATOMIC CESIUM, CS(6 S-2(1 2)), BY TIME-RESOLVED LASER-INDUCED FLUORESCENCE (CS(7 P-2(3/2)-6 S-2(1/2)) LAMBDA=455.5 NM) FOLLOWING PULSED IRRADIATION/

We present a kinetic investigation of atomic caesium in its electronic ground state at elevated temperature by laser-induced fluorescence (L IF). A detailed description is given of a new apparatus in which Cs(6( 2)S(1/2)) is generated in a stainless steel reactor from the broad-ban d pulsed irradiation of CsCl vapour at an elevated temperature and mon itored by LIF using the shorter-wavelength component of the Rydberg do ublet via the transition at lambda = 455.5 nm (Cs(7(2)P(3/2)-6(2)S(1/2 ))) using boxcar integration and computerized analysis, LIF decay prof iles for Cs(6(2)S(1/2)) in excess helium alone at various temperatures indicate diffusional loss. Decay profiles were also investigated at d ifferent total pressures with mixtures of an added reactant R of fixed relative composition f = [R]/([R] + [He]) with excess He buffer gas f rom which absolute rate data were extracted, essentially for single el evated temperatures for various added gases. The following absolute se cond-order rate constants k(R) are reported for the removal of Cs(6(2) S(1/2)) by different reactants: N2O, (1.1+/-0.4) X 10(-10) cm(3) molec ule(-1) s(-1) (T=723-753 K); CH3Cl, (5.4+/-0.2)X10(-12) cm(3) molecule (-1) s(-1) (830 K); CF3Cl, (3.0+/-0.2) X 10(-11)cm(3) molecule(-1) s(- 1) (830 K); CF2Cl2, (1.5+/-0.5) X 10(-11) cm(3) molecule(-1) s(-1) (83 0 K); CFCl3, (1.2+/-0.1) x 10(-10) cm(3) molecule(-1) s(-1) (830 K); C 2H5Cl, (1.5+/-0.5) X 10(-11) cm(3) molecule(-1) s(-1) (830 K). Whilst the collisional behaviour of Cs(6(2)S(1/2)) has been investigated by d irect monitoring in the time domain using various methods, we believe the present investigation to constitute the first study of these proce sses in the presence of the chloride reactants by LIF. With the except ion of CFCl3, where the lower value of the reaction rate constant obta ined by atomic resonance absorption spectroscopy is attributed to ther mal decomposition in the static reactor, there is reasonable agreement between the two techniques. Although the present result for k(R)(Cs N2O) using LIF is significantly larger than that obtained by resonanc e absorption, it is in accord with previous measurements using LIF, su pporting the present method for characterising kinetic data for Cs(6(2 )S(1/2)) at elevated temperatures.