QUENCHING OF O₂(b¹∑⁺_g)

Abstract

We have been making measurements of the rate of reactions of Oz(blC:) in a discharge flow system. The mechanisms and kinetics of the reactions involving this species have been outlined by Ogryzlo and co-workers.'*2 The 'C: state is continuously being formed in the tube by the disproportionation of two lowerlying Oz(alA,) molecules:The (IX;) is quenched either on the wall or by an added quencher:Our principal measurement is the relative intensity of the forbidden emission of the I Z : state at 7,619 A. The '2: concentration (and emission intensity) is usually that calculated from Reactions 1 through 3 using the steady-state approximation.A conventional Stern-Volmer plot of intensity as a function of quencher pressure yields only a ratio, ka/kw. The problem is to determine kw accurately so that kQ can be obtained. The decay of 7,619 A intensity along the length of the flow tube is characteristic of the decay of the 'A, source in this steady-state situation. A previous determination' of kw has necessitated a knowledge of ko and absolute intensity measurements. We have developed a new direct method for measuring kw in these systems. The technique involves placing a wire spiral in the flow (FIGURE la). With proper choice of size and material (silver plated on platinum or aluminum) the wire will preferentially quench the 'C: state more than it quenches the 'A: state. This quenching disrupts the steady-state nature of the 'Xi kinetics at one point in the tube. Below the spiral the 'C: concentration grows back into a new steady-state value (see FiGURE 1 b). The rate of this relaxation is characteristic of, and can be used to calculate, kw. From the analysis of this type of data, a value of the surface deactivation efficiency for Pyrex@ glass of about 0.02 has been obtained. This value is close to that observed by Izod and Wayne.3 This kw combined with Stern-Volmer data has yielded a kQ for H 2 0 of 4 X lo-'' cc sec-', which is intermediate between values of 20 x lo-'* reported by Sthul and Welge4* and 1 x obtained by Ogryzlo's group.' Bimolecular quenching rates can also be obtained directly with the wire spiral. In the presence of an added quencher the relaxation period is shorter, being characterized by a pseudo first-order rate constant k = (kw + ka[Q]). The slope of a plot of this rate constant versus [Q] is kQ. We get values of kQ by this method that agree with the Stern-Volmer data.

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