Arrested relaxation

One flow system that has been designed to minimise the effects of relaxation and is commonly used in infrared chemiluminescence studies is designated the arrested relaxation system [77]. Figure 2 shows an... 

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... 

Figure 1.11 Relative rates of reaction into individual vibrational levels by arrested relaxation (AR) and measured relaxation (MR) techniques H + Cl2 -> HC1 + Cl from ref. 265 Cl + HI ->HC1 + I and Cl + DI -> DC1 + I from ref. 267. Rv are plotted against/ , the fraction of available energy which goes into product vibration.

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]. 

H -(- N2O reactions single-collision, arrested-relaxation conditions versus N2O-HX complexes N2O-HX structural properties Single-collision, arrested-relaxation conditions 1, OH - X n chemiluminescence Reactions in N20-HBr and N2O-HI complexes... 

B. H -I- CO2 Hot-Atom Reactions Under Single-Collision, Arrested-Relaxation Conditions... 

G. H -t- N2O Reactions Single-Collision, Arrested-Relaxation Conditions versus N2O-HX Complexes... 

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... 

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. 

Figure 1. Schematic of an arrested relaxation reaction vessel used at Kansas State University, taken from (24). See (11 for a drawing of the reaction vessel used by the TorotUo group.

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. 

The F + H2 reaction has been characterized thoroughly by both chemiluminescence and chemical laser techniques, and the results are shown in Table 2.2. Arrested relaxation chemiluminescence work from... 

The experimental vibrational distributions for HBr and HI were taken from ref. 60a, which used the flowing-afterglow method. The rotational distributions were taken from ref. 60b which used the arrested relaxation technique. The vibrational distribution from the latter are shifted slightly to high V levels and increased to 0.63 and 0.65, for HBr and HI, respectively. The vibrational and rotational distributions from HCl were taken from refs. 36 and 59, respectively Tamagake and Setser find = 0.57 and = 0.15. 

Three sets of arrested relaxation data have been reported for H + CI2. The vibrational distributions are essentially identical but the rotational distributions reported by Polanyi s laboratory in 1973 are somewhat broader than for the first study, e.g., increased from 0.07 to 0.09 recent work supports the higher . The for the CI2 reaction is the smallest in the H -i- X2 series. The D + CI2 reaction also has been studied, ... 

The HCl vibrational-rotational arrested relaxation distributions reported by ref. 82b are in close agreement with those given in this table for S2CI2 and SOCI2 but not for SCI2. 

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