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Classical 3-D trajectory investigation of bromine and iodine atom recombination reactions in He, Ar, and Xe, performed earlier, are extended, using an improved sampling technique, to include a larger number of trajectories and a wider temperature range (200–1500 K). The three body potential energy surfaces used were assumed to be nearly additive, but otherwise were defined by the existing molecular beam and spectroscopic data and contained essentially no arbitrary parameters. The agreement between computed and experimental rate constants is reasonable, and is best if the third body is heavy and reaction proceeds via a bound complex, such as IXe. Orbiting inert gas – recombining atom intermediate dimers, XM*, where X = I or Br, contribute to the overall recombination reaction via XM* + X → X2 + M reaction, provided M is heavy. If M = He, this reaction path is negligible at all temperatures studied, again provided that X = I or Br.

Flash photolysis of I2in a large excess of O2was studied between 300 and 1173 K. Between 573 and 1173 K the rate of atomic recombination was measured without interference of side reactions. Its rate constant was found to be . Between 573 and 873 K there is an induction period lasting several hundred microseconds during which only a small net change in the iodine molecule concentration takes place. This induction period is consistent with the reaction Beow 573 K, the overall recombination rate constant has a temperature dependent negative temperature coefficient and is enhanced when the flash includes radiation in the neighborhood of 200 nm. Evidently this occurs because the flash photolysis of O2creates new efficient third bodies, such as O3and O2(1Δg), which accelerate the recombination reaction either directly or via an IO intermediate. The mechanism for the formation of IO during the flash photolysis of I2in an excess of O2was studied. It was shown that the reactions O(3P) or , where O3* refers to an electronically or vibrationally excited state of O3, account for the formation of IO under our experimental conditions. IO may also possibly be formed via the reaction . On the other hand, reactions between excited or ground state I atoms and O2suggested by earlier workers do not occur. Thus, all experimental complications observed are traceable to the presence of O2photolysis products and to the reaction of I(2P1/2) with O2(3Σg−).

The Egyptian tombs at Beni Hasan (ca. 2100 B.C.) have developed an ubiquitous, progressive grey surface layer which disfigures their painted surfaces. Recently, we published a series of X-ray photoelectron spectra of samples from these tombs and proposed a mechanism for surface layer formation. It involves CO2-saturated condensed water which gradually erodes the mural substrate. However, no direct evidence for this erosion was available. To test the proposed mechanism, light microscopy was used to study cross-sections of the murals. Areas of repeated solution and precipitation, in the form of dendritic structures were discovered in the subsurface regions, especially in yellow pigmented samples. In red pigmented samples, this degradation is not seen to the same extent. A series of electron microprobe analyses of the surface and subsurface regions of small painted fragments also were performed. Porous, heterogenous subsurface regions with pockets of phosphates have been found, predominantly in the yellow samples. These findings support the mechanism proposed earlier. Pore size measurements, estimated by photographic measurements, indicate that subsurface alteration took place before the bulk of the surface deposit layer had formed and that the existing surface deposit protects, as well as obscures, the painted walls. The surface layer development described here has some aspects common to other natural systems, i.e. the interaction of CO2in water with carbonate-bearing rocks and deposits. However, at Beni Hasan there is effectively no source of groundwater and the painted surfaces are sheltered from direct exposure to the environment. Therefore the Beni Hasan deterioration has unique mechanistic features.

Although certain classes of reactions are known to occur via 4-centre transition states, there has been no irrefutable proof that in the gas phase addition reactions also proceed via a 4-centre concerted process. Results of recent information theory calculations on the HF(υ) + CH2CF2system indicate that it should be possible to observe reaction products for HF(υ = 5). Results are presented here on experiments attempting to achieve laser-induced 4-centre addition of HF to several unsaturated molecules (C2H2, C2H4, C4H6, CH2CF2, and CF2CClF). A mixture of HF and co-reactant was irradiated by a dye-laser tuned photo-acoustically to the fifth vibrational level of HF, and the resulting mixture was analysed by gas chromatography and mass spectrometry. There was no evidence of any laser-induced products. An upper bound was deduced for the HF + CH2CF2addition rate constant at 610 K: .

Recombination rate constants for iodine atoms in CF4, SF6, CO2, Ne, and Kr were studied by flash photolysis. The results can be summar ized as follows:These rate constants are compared with those available in the literature, and analyzed, qualitatively, in terms of current theories of recombination reactions.

Computational studies of Br2shock wave dissociation in argon were conducted, and the possibility of vibrational relaxation–dissociation coupling, stronger than previously suspected (2), was investigated. In order for such a coupling to occur, the vibrational temperature in the lower vibrational levels of Br2must be lower than that in the higher vibrational levels. The approximate shapes of vibrational distribution functions were obtained, and corresponding shock wave profiles were computed. The calculations predict an appreciable difference between the translational and vibrational temperatures of the dissociating Br2. In order to check this conclusion, the Br2absorption coefficients at 440 mμ, measured at equilibrium by heating Br2in a furnace from 300 to 1250°K, were compared to the absorption coefficients measured in shock waves.

Dissociation of Br2in Ar was studied at 3500 K using classical 3-D trajectory technique, and compared with earlier trajectory calculations. Some of the assumptions used previously were eliminated, while others were studied in some detail. The one-way flux, equilibrium rate coefficient, obtained from over 8400 trajectories, was found to be over an order of magnitude larger than the experimental rate constant. This was taken as an indication that at high temperatures the nonequilibrium effects are important in dissociation reactions. In order to understand these effects better, additional calculations using an improved set of assumptions were performed. The calculated dissociation rate constant for Br2 + Ar → 2Br + Ar reaction, which accounted for nonequilibrium effects, agrees reasonably well with experimental results.