KINETIC INVESTIGATION OF THE QUENCHING OF MG(3S3P-P-1(1)) ATOMS IN COLLISIONS WITH CH4 OVER THE TEMPERATURE-RANGE FROM 660 TO 850 K

Temperature dependence of the quenching efficiency of Mg(3s3p P-1(1)) by CH4 collision in the Ar bath gas has been studied over the temperat ure range of 660-850 K, using a pump-probe technique with time-resolve d laser-induced fluorescence (LIF) as detection. The obtained thermal rate coefficients are attributed to physical and chemical quenching. T he former contribution is evaluated to be less than 6.7 x 10(-11) cm(3 ) molecule(-1) s(-1), while the latter one is (0.59-1.12) x 10(-11) ex p(5.75-6.54 kcal mol(-1)/RT) cm(3) molecule(-1) s(-1) (R is gas consta nt; T is temperature). The chemical reaction dominates over the exit c hannels, especially in the low temperature. The negative temperature d ependence indicates the existence of a depth-well intermediate. The ob served depth-well energy below the reactants is about twice as small a s that evaluated theoretically. This bound state is anticipated to loc ate in the region of surface crossing between the excited and the grou nd states. We adopted several models to account for the kinetic data a s a function of temperature. The orbiting and absorbing-sphere models cannot be validly applied to the current system. The angle-dependent l ine of normals (ADLN) model takes into account the angular dependence of the threshold energy. The ADLN fit to the measured kinetic data is consistent with the insertion mechanism reported previously, which sho ws energetic preference to the end-on attack. (C) 1998 American Instit ute of Physics [S0021-9606(98)00942-8].