Ion recombination rates in rare-gas cation-halide anion systems. 2. Krypton fluoride and xenon chloride eximers

Abstract

24 32 ' ' 6 Figure 6. Adsorption of ENTMP on calcium fluoride crystals at equilibrium. Plots of r against [ENTMP].uration g = 0.4, 0.2, and 0.1 respectively. These values reflect the high adsorption affinity at low relative undersaturation in the presence of ENTMP. A similar dependence of the degree of inhibition with change in driving force has been observed for the influence of phosphonate on the dissolution rate of magnesium fluoride" and barium fluoride19 in aqueous solution. As noted 8 io ,~N T M P J mol c1 for the crystallization of gypsum by Van Rosmalen and coworkers,m the effectiveness of the HEDP as an inhibitor depends on the degree of supersaturation.In order to investigate the adsorption of phosphonate on the calcium fluoride crystals surface, adsorption equilibrium experiments were made for ENTMP on calcium fluoride crystals at u = 0. A typical adsorption isotherm is plotted in Figure 6. Assuming that the area occupied by an ENTMP molecule is 50 X m2, the fraction of the solid surface covered by adsorption molecules is only 6% at the plateau in Figure 6. At this ENTMP concentration, the dissolution rate is reduced by more than 92%. Adsorption affinity constant at equilibrium 5.2 X lo5 dm3 mol-' is in satisfactory agreement with the kinetic adsorption affinity constants los dm3 mol-'. The dissolution rate of magnesium fluoride was reduced by more than 80% when only 9% of the crystal surface was covered by HEDP molecule^.^' Moreover, the rate of crystallization may be reduced virtually to zero when only 5-7% of the crystal surface was covered by adsorbed inhibitor moIecules.2',22The emission spectroscopy/pulse radiolysis method of determining three-body ionic recombination rate constants in raregas-halogen source gas mixtures has been extended to systems where the emission is produced by both ionic and nonionic pathways. This has enabled recombination coefficient measurements to be done over a large pressure range for irradiated Kr/SF, and Xe/CFC13 gas mixtures. The rate constants measured for both these systems show the typical pressure dependence of an increase to a maximum value of -2.5 X IOl5 M-l s-l (-4 X IOd cm3 d), before the onset of the diffusion-controlled reaction. These values have been compared to the predictions of the Langevin-Harper diffusion-controlled and the Bates termolecular recombination models. The large discrepancies between theory and experiment have shown that other recombination processes dominate the ionic recombination.

Absolute Fluorescence Yields from Electron Irradiated Gases: XeBr*

“…By modeling the formation of XeBr* fluorescence via Xe 2 + /e -recombination, followed by reaction with CF 3 Br, we found that in the first 150 ns (the time taken for the directly formed XeBr* to fluoresce), less than 5% of the total fluorescence due to the ionic pathway occurred on this time scale. This result is in good agreement with a previous determination for the analogous XeCl* exciplex system, 22 where it was shown that on the time scale of direct excited state xenon reaction with CFCl 3 , the amount of fluorescence from the ionic recombination process was less than 8% of the total value. Thus, we believe that the fractions of excited state and ionic recombination reaction measured in this study are accurate to within these values.…”

“…The fitted value of k 5 ) 6.4 × 10 9 dm 3 mol -1 s -1 is in reasonable agreement with the rate constant used previously for modeling Xe 2 + /X -ionic recombination, k 5 ) 2.2 × 10 9 dm 3 mol -1 s -1 , where this latter value was determined from the ratio of the xenon three-body and two-body quenching rate constants for XeCl*. 22 By use of the above rate constants and the determined value of G o *, the excited state yield dependence on CF 3 Br pressure was calculated across the pressure range of this study. These values are shown in comparison to the experimental yield data in Figure 4, with excellent agreement observed.…”

Absolute Fluorescence Yields from Electron-Irradiated Gases. 3. XeCl* and XeI*

“…The inverse plot of these yields (Figure 1) shows a limiting linear slope at CFCl 3 pressures above 0.60 Torr. At lower pressures, the deviation from this linearity has been demonstrated 15 to be due to the ionic recombinationformed fluorescence being significant.…”

“…18,21 The relative contributions of the energy-transfer and ionic recombination processes could not be temporally resolved in this system. It has been shown previously 15 that the energytransfer process could be isolated by lowering the incident pulse energy and intensity to extremely low values and also by using very low xenon pressures. This slowed the ionic recombination reaction to such an extent that at short times (<200 ns) the fluorescence from this process was negligible.…”

Formation of the XeCl exciplex via double crossings of potential-energy curves

“…Using Xe for the buffer gas Z, Mezyk et al [27] obtained a measured rate coefficient of ~4 x 10 -12 m 3 s -1 for reaction (43), compared with a smaller value of ~1 x 10-iZm 3 S -1 calculated using the Bates formula [28], Morgan and Bates [29], employing the electrostatic tidal ionic recombination model [30] in a Monte Carlo simulation, achieved excellent agreement with the data of Mezyk et al [27]. Tidal recombination cap~ be applied to explain reaction (44).…”