Arrested relaxation experiments

Fluorine atoms usually are produced by a microwave discharge through CF or SF. Figure 1 shows a schematic diagram of a vessel with liquid nitrogen cooled inner walls that has been used for "arrested relaxation" experiments. Such a vessel arrests vibrational relaxation and partially arrests rotational relaxation. The maximum fraction of the surface should be cooled for arrested relaxation studies with HF because collisions of HfJj with warm surfaces result in some relaxation with subsequent reentry of those molecules into the cone of sight of observation. Mirrors usually are placed inside the vessel to more efficiently collect the Infrared radiation. 

Rotational relaxation of HF is only partially stopped even in the best "arrested relaxation" experiments. In order to estimate the initial rotational distributions, some correction for relaxation is necessary. There is experimental (77,78,32) and theoretical (79) support for collisional deactivation transition probabilities of the form. 

Within the last few years, the problems arising from relaxation and the resultant difficulty of estimating values of Rv for the lower vibrational levels have been resolved by carrying out experiments using both the arrested-relaxation and the measured-relaxation techniques. The Rv that have been derived in these two sets of experiments are plotted in Figure 1.11 against / , the fraction of available energy in the HC1 vibration. The relative rates... 

The arrested-relaxation method has been applied [227,228] to the reactions of F + H2 and F + D2, and the measured-relaxation technique to F + H2 [229, 230]. These values of / ., and Rv are particularly important since they can be compared with the results of molecular beam and chemical laser experiments (see Table 1.4), and the agreement is satisfactory. The HF vibrational and rotational degrees of freedom absorb approximately 67% and 6% of the total energy and once again there is a marked parallelism between the results for a reaction and its isotopic analog. Preliminary measurements on other reactions producting HF have been reported by Jonathan et al. [230]. 

In many experiments only partially detailed information is obtained. For example, i.r. chemiluminescence experiments under conditions of arrested relaxation can provide rovibrational state distributions of reaction products. Where, as has usually been the case, the reagents are thermally equilibrated, the corresponding rate constants are defined by... 

Similar experiments at Pt (136) showed again substantial pseudocapacitance (Fig. 21C) but, additionally, unusually large apparent values of 6, above 1. This was attributed to significant sorption of H not only on but just below the metal surface, in the near-surface region. The behavior at Pt is characteristically different from that at Ni or Ni-Mo composites insofar as the potential relaxation transients (Fig. 21 A) show arrests corresponding to desorption of appreciable coverages by ad- and absorbed H. At Pd, similar but much more marked effects of this kind are observed and tend to confirm the interpretation for Pt that some H is absorbed. The results for Pt also... 

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