Rate constants have been measured by the flowing afterglow technique at 300 °K for the quenching of Ar(3P2), Ar(3P0), Kr(3P2), and Xe(3P2) by a large number of small molecules. For the same reagent, the magnitudes of the cross-sections usually increase in the series Ar(3P2), Ar(3P0), Kr(3P2), and Xe(3P2). The Ar(3P2) and Ar(3P0) data are compared to results in the literature for these states and to data for Ar(3P1) and Ar(1P1). The set of thermal quenching cross sections are used to test the correlations between the magnitudes of the cross sections and properties of the reagents as predicted by the orbiting, absorbing-sphere, golden rule, and curve-crossing mechanisms for quenching. The best correlation is between the cross sections and the C6 coefficient. The analysis supports the proposition that the orbiting-controlled, curve-crossing model is the general mechanism governing the magnitude of the thermal cross sections for quenching of the metastable states. This model explains the very large quenching cross sections of F2 and OF2 (relative to other molecules composed of first row elements) because covalent–ionic curve crossing occurs outside the conventional orbiting radius. The validity of the simple van der Waals dispersion forces as being the dominant entrance channel interaction between the excited state rare gas atoms and the reagents is discussed.

Reactive quenching studies of Xe (6s,3 P 2) metastable atoms by chlorine containing molecules

The decay rates of Ar(3P2), Ar(3P0), Kr(3P2), and Xe(3P2) metastable states in argon carrier gas have been measured in a flowing afterglow apparatus. From analysis of the dependence of the pseudo-first-order decay constants upon pressure, the diffusion, two-body, and three-body rate constants have been assigned for each metastable state in Ar. The rate constants found for Ar(3P2) agree with the values accepted in the literature for these processes. The two-body rate constants for Kr(3P2) and Xe(3P2) are a factor of 4 lower than for Ar(3P2). The three-body rate constants for Kr(3P2) and Xe(3P2) are reduced by factors of 10 and 100, respectively, relative to the Ar(3P2) rate constant. The reduction in three-body constants for Kr(3P2)+2Ar and Xe(3P2)+2Ar is consistent with the reduced binding energy of KrAr* and XeAr*. The diffusion and three-body coefficients for Ar(3P0) are equal to those for Ar(3P2), but the two-body rate constant is 2.5 times larger than for Ar(3P2). The results for each of the three processes are discussed in terms of physical models.

The flowing afterglow technique was used to study the reactions of Xe(3P2), Kr(3P2), and Ar(3P2) metastable atoms with small fluorine containing molecules. Fluorides from Groups III through VIII (XeF2) were examined. Although all the fluorides have large quenching rate constants, only F2, XeF2, some interhalogen fluorides, and small molecules with the OF or NF bond have high branching ratios for XeF* or KrF* excimer formation. The branching ratio measurements were made via comparison to the XeCl* and KrCl* emission intensities from reaction of Xe(3P2) and Kr(3P2) with Cl2, which were adopted as reference reactions. Within experimental error, the branching ratios are unity for Kr(3P2) and Xe(3P2) with Cl2, F2, and OF2. Increasing the argon pressure from 1–40 torr gives extensive vibrational relaxation but no electronic quenching of the lowest energy excimer state of XeF*, XeCl*, KrCl*, or KrF*. Increasing pressure also reduces the intensity of the secondary emission system of KrCl* and XeCl* which implies collisional transfer from the (2Π3/2) to the 2Σ+ excimer state. The occurrence of vibrational relaxation suggests a spontaneous radiative lifetime of ∼50 nsec. A higher energy excimer state (2Π1/2) was identified for XeF*, XeCl*, XeBr*, XeI*, KrF*, and KrCl*. Discussion is presented regarding the chemical dynamics and chemiluminescence of these reaction systems.

The XeCl* emission spectrum has been used to study the reactions of Xe  (6s, 3P2) metastable atoms with 20 chlorine containing reagents. Although the total quenching rate constants are large, only Cl2, the mixed halogens and Group VI chlorides have high product branching fractions for XeCl* excimer formation. From analysis of the low pressure XeCl* emission spectra, the ratios for XeCl  (III,1/2 or B,1/2) and XeCl  (II,3/2 or C,3/2) formation were determined. Using results presented in the following paper, the vibrational energy distributions of the XeCl* molecules also can be estimated. A wide difference, depending upon reagent, is observed for the vibrational energy disposal. The short wavelength limit of the XeCl  (B–X) emission can be used to assign upper limits to D°0  (R–Cl) and this work gives D°0  (SCl–Dl) ?44.7; D°0  (ClS2–Cl) ?46.2, D°0  (SOCl–Cl) ?53.9; D°0  (SO2–Cl) ?58.5, and D°0  (PCl2–Cl) ?77.7 kcal mole−1. For purposes of comparison, the XeCl* emission spectra also were obtained from the reaction of Xe  (6s, 3P1) resonance atoms with Cl2, PCl2, CCl4, and COCl2. The branching ratios and energy disposal for Xe  ( 3P2) and Xe  ( 3P1) reactions appear to be qualitatively similar. Based upon all of these data, the dynamics of the reactions between Xe  ( 3P2) and the chlorine containing reagents are discussed and compared to the models in the literature used previously for the reactions of alkali metal atoms with halogen containing molecules. Characteristic XeI* and XeBr* spectra from Xe  ( 3P2) atom reactions are included in an Appendix.