Atoms halogen, photodissociation

Molecules of the type CxHyBrz produce bromine atoms upon photodissociation and oxidation, like the other halogen atoms just discussed. Unlike fluorine, the chemistry of bromine is broadly similar to that of chlorine (see Yung et al. (1980) Poulet et al. (1992) Lary (1996) and Figure 5.52) and is therefore capable of affecting the ozone budget. The production of bromine atoms is followed by the reaction... 

The photodissociation products of the homonuclear halogens in the visible and ultraviolet are now comparatively well established in view of the detailed spectroscopic studies that have been made. The strongest absorption system observed in this spectral region is associated with a transition to the 3II0u+ state which correlates with X / ) + X(2Pyz). Thus photoexcitation to the continuum associated with this state leads directly to the formation of an excited atom, while excitation to the banded region followed by predissociation will lead only to ground state atoms. 

The energy balance of photodissociation the importance of stabilization of the free radicals. When chlorobenzene or chloro-Np loses the halogen atom, a phenyl or a naphthyl radical is formed with the odd electron localized in an sp2 orbital which is orthogonal to the aromatic zr orbitals such a radical is not stabilized through resonance, unlike the benzyl- or the methyl-Np radicals for which several resonance structures can be drawn (Figure 4.32). 

In the pulsed molecular beam studies, the results were used to calculate an impact parameter for the photodissociation process. From these impact parameters it was concluded that the recoiling CN fragment did not take the lowest energy path when it was departing from the halogen atom. Rather, because of strong impulsive motion, the trajectory that the CN rsdical... 

A great deal of work has been done on the photodissociation dynamics of the halogen containing compounds. This is partially due to the production of halogen atoms and radicals that can be studied by TOF and partially due to the fact there are intense absorption bands in the ultraviolet and near ultraviolet region. These reasons, coupled with the fact that the spectroscopy of some precursor molecules is fairly well understood, have resulted in many workers looking at these compounds. Finally, some of these compounds have been studied as possible candidates for various types of lasers. 

The editor has encountered numerous studies of laser ignition, usually of explosives. These demonstrate that the ignition is normally a thermal effect, caused by heating solid particles, not photochemical, and thus the MIE is not lower than other methods. This may not obtain if the light be of a frequency (visible or uv) sufficient to excite the early steps of the explosion reaction, as, for example, photodissociation of halogens to the atomic radicals which are the start of their explosive reactions with fuels. 

In section 3.2, experiments were described in which photodissociation of the HX moiety within a weakly bound complex liberated hydrogen atoms which then went on to react with the other moiety in the complex. Alternatively, photodissociation of HX moieties within weakly bound complexes can be used to prepare halogen atom reactants. This provides advantages which derive mainly from the rapid removal of the hydrogen. 

The van der Waals attraction between Br and I2 is estimated to be 400 cm-1 by analogy with halogen/rare gas complexes (Bieler and Janda 1990 Bieler et al. 1991). This ensures that photodissociation of the HBr moiety cannot produce Br + I2 except via quenching of Br or the unlikely instance in which the hydrogen is trapped efficiently between the heavv particles. With the Br atom unable to escape from the I2 because of the Br-I2 van der Waals attraction, the system is ensured of an essentially unity quantum yield. 

The oxidation scheme for halomethanes not containing a hydrogen atom is similar to that for those which do, except that it is not initiated by tropospheric reaction with hydroxyl radicals, since the fully halogenated methanes are unreactive. Consequently, substantial amounts of CFCs and halons are transported intact up into the stratosphere, where they absorb UV radiation of short wavelength and undergo photodissociation (equation 36) to a halogen atom and a trihalomethyl radical. The halogen atom Y may enter into catalytic cycles for ozone destruction, as discussed in the introduction. 

In addition to UV/visible flash photolysis and TRIR spectroscopy, other techniques have been used for the detection of transition metal-noble gas interactions in the gas phase. The interaction of noble gases with transition metal ions has been studied in detail. A series of cationic dimeric species, ML" " (M = V, Cr, Fe, Co, Ni L = Ar, Kr, or Xe), have been detected by mass-spectroscopic methods (55-58). It should be noted that noble gas cations L+ are isoelectronic with halogen atoms, therefore, this series of complexes is not entirely unexpected. The bond dissociation energies of these unstable complexes (Table IV) were determined either from the observed diabatic dissociation thresholds obtained from their visible photodissociation spectra or from the threshold energy for collision-induced dissociation. The bond energies are found to increase linearly with the polarizability of the noble gas. 

There is no clear cage effect in the dissociation process inside the impact-heated cluster. This is contrary to experimental observations and simulations of photodissociation of molecules in clusters. This is not unexpected. For caging the dissociation products, the surrounding medium has to slow them down and to confine them so that they can recombine. No such strong cage effects are seen in our simulations. Once the halogen molecule has dissociated, the two atoms recede with hardly any noticeable hindrance. We consider that at least two factors, both unique to... 

In their study of the properties of CX4 (with X being a halogen) and their photodissociation products, Kong et alF also reported the atomic populations obtained from a natural population analysis for the Br2CBr-Br system in various... 

Ozone in the stratosphere is depleted by reactions with halogen atoms. Depletion of stratospheric ozone, commonly referred to as the ozone hole, usually occurs over the Earth s cold regions. The main source of chlorine atoms in the stratosphere is photodissociation of chlorofluorocarbon (CFC) compounds,1369 commonly called Freons, e.g. ... 

During this study, we have found that laser intensity is one of the important factors that control laser surface chemistry. At a small laser intensity, molecules adsorbed on solid surfaces dissociate into atoms and radicals. Some of these atoms or radicals react with atoms of the solid substrates. At a large laser intensity, atoms are photoablated from the solid surfaces to react with the molecules adsorbed or in the gas phase. Hence, we describe in this paragraph a) the dynamical study of UV laser photodissociation of halogen or metal-containing molecules on solid surfaces, b) reactions of atoms generated in the photodissociation of an adsorbate with solid surfaces, and c) reactions of molecules in the gas phase with the photoelectrons or metal atoms generated on intense laser irradiation of solid surfaces. 

Detailed studies of the chlorine-atom-sensitized oxidation of chloromethane, dichloromethane, 1,1,2-trichloroethane, CH2CC12, and C2C14148 have been reported. Reports on the photolysis of Freons of interest to upper-atmosphere chemistry are discussed in the last section of this Report, and laser enhancement of some halogen-containing molecular reactions is discussed in Section 11. A paper concerned with the mechanism of photodissociation of alkyl and aryl halides was discussed earlier.38 The photochemical chlorination of 1,2-dichloroethane149 and fluorination of carbonyl fluoride,150 reactions of 2 radicals,151 and the photochemical decomposition of FaO at elevated temperatures152 have been reported. 

Chemiluminescence and photoluminescence in diatomic iron oxide, Rb2, and alkali-metal dimers with halogen atoms and metal vapour-oxidant flames,202 203 lifetime measurements of selectively excited states of diatomic hydrides,204 photodissociation of alkali-metal halide vapours,206 spin-orbit relaxation of the HTe ( 2IIi) radical,20 the photodecomposition of metal carbonyl anions such as [Mn(C04)] in the vapour phase,207 and the fluorescence of Rhodamine 6G in the vapour phase 208 have been studied in recent reports. In the last study it was concluded that an insufficient concentration of the fluorescing dye could be maintained in the vapour phase to permit laser action to occur. 

A very widely employed method for the measurement of spin-orbit state-specific rate constants is the time-resolved measurement of the concentrations of individual atomic levels after formation of these species from a suitable precursor, either by flash photolysis [13], or, more recently, by laser photodissociation. The concentrations of the various atomic reactant states are monitored by atomic absorption or fluorescence spectroscopy using atomic emission sources [14], or, for spin-orbit-excited states, by observation of the spontaneous infrared emission [15-18]. Recently, Leone and co-workers have utilized gain/absoiption of a colour centre and diode infrared laser to probe the relative populations of ground and spin-orbit excited halogen atoms produced in a chemical reaction [19] and also by photodissociation [20],... 

---