Ultraviolet photolysis

Subsequent reactions depend on conditions. Ultraviolet photolysis of isolated molecules in an inert matrix yields the radicals CIO and CIOO. At room temperature, photolysis of diy gaseous CIO2 yields CI2, O2, and some CIO3 which either dimerizes or is further photolysed to CI2 and O2 ... 

Enhancement of the total butene yield is observed when various additives whose ionization potential falls below about 9.4 e.v. are present during ethylene radiolysis (35). This is consistent with the above interpretation (Figure 2). In the vacuum ultraviolet photolysis of cyclobutane the yield of butenes varies with the ionization potential of the additives in the same way as observed here (12). The maximum enhancement corresponds closely to the yield of C4H8+, as expected from our mechanism. 

On the other hand, the formation of ethylene was ascribed mainly to the unimolecular decomposition of a neutral excited propane molecule. These interpretations were later confirmed (4) by examining the effect of an applied electrical field on the neutral products in the radiolysis of propane. The yields of those products which were originally ascribed to ion-molecule reactions remained unchanged when the field strength was increased in the saturation current region while the yields of hydrocarbon products, which were ascribed to the decomposition of neutral excited propane molecules, increased several fold because of increased excitation by electron impact. In various recent radiolysis 14,17,18,34) and photoionization studies 26) of hydrocarbons, the origins of products from ion-molecule reactions or neutral excited molecule decompositions have been determined using the applied field technique. However, because of recent advances in vacuum ultraviolet photolysis and ion-molecule reaction kinetics, the technique used in the above studies has become somewhat superfluous. 

D. L. Phillips, W.H. Fang, and X. Zheng, Isodiiodomethane is the methylene transfer agent in cyclopropanation reactions with olefins using ultraviolet photolysis of diiodomethane in solutions a density functional theory investigation of the reactions of isodiiodomethane, iodomethyl radical, and iodomethyl cation with ethylene. J. Am. Chem. Soc. 123(18), 4197-4203 (2001). 

The effort required to establish identity of a nitrosamine in an environmental sample depends on the nature of the problem and the specificity of the primary detection system. TEA response is much stronger evidence of identity than response from a flame ionization or nitrogen-specific detector. If TEA response is supported by chemical (9) or ultraviolet photolysis (8) supporting data, identification is adequate for many... 

Less work has been done on the time-resolved IR spectroscopy of Cr(CO)6 in the gas phase (69,91,92). Broadly, the results were similar to those with Fe(CO)5. Ultraviolet photolysis (249 nm) provides evidence for several CO loss products (92) tentatively assigned as Cr(CO)5 and Cr(CO)4. 

Thermolysis of Fe(C0)5, f°r example, gives pyrophoric, finely divided iron powder ( 19) ultraviolet photolysis (20) yields Fe2(C0)9, via the intermediate Fe(C0)4 multiphoton infrared photolysis in the gas-phase (2M) >(22) yields isolated Fe atoms. Multiple ligand... 

Diazepines (150) can be prepared by ultraviolet photolysis of iV-iminopyridinium ylids (151) but N—N bond homolysis in the ylid substrates presents a problem in regard to the synthesis of N-l substituted... 

A knowledge of the kinetics of the decomposition of ozone is essential for the understanding of the chemistry of some important processes which occur in earth s atmosphere. Yet, in spite of numerous studies and the structural simplicity of ozone, the mechanism of its ultraviolet photolysis is still uncertain. Electronically and vibrationally excited species are involved in ozone decomposition and the current knowledge of the chemical behavior of such intermediates is still in its infancy. 

In spite of differences of opinion over the nature of the secondary processes in the ultraviolet photolysis, there is now abundant evidence339-341,344,345 that at wavelengths shorter than 3200 A 0(1Z)2) atoms are produced in the primary process and disappear in a very rapid335 reaction... 

The vacuum ultraviolet photolysis of HI has been studied by Martin and Willard16 using the 1849 A mercury lines as the exciting radiation. They estimated the molar extinction coefficient at 1849 A to be between 110 and 150 in fair agreement with Romand s3 experimental value of 125. The hydrogen atoms produced have sufficient energy to cause the reaction... 

Ultraviolet photolysis of ozone (there is a small background level of ozone in the troposphere as a result of downward transport from the stratosphere) ... 

A chemically pumped CO2 laser oscillating at 10 p was reported by GrossIn this system vibrationally excited COj molecules are produced by inelastic collisions with vibrationally excited DF which was formed by ultraviolet photolysis of a F2O-D2 mixture with a Xe flashlamp, producing free fluorine atoms which could react with Dj... 

Rahn, T., H. Zhang, M. Wahlen, and G. A. Blake, Stable Isotope Fractionation during Ultraviolet Photolysis of N20, Geophys. Res. Lett., 25, 4489-4492 (1998). 

Other flow techniques were developed in subsequent years, but towards the end of the 1960s it was recognized that detectable concentrations of reactive radicals could be obtained in a static sample by ultraviolet photolysis of... 

Although a population inversion for the two states of the bromine atom has been observed following the vacuum ultraviolet photolysis of CF3Br,76 it has not yet proved possible to meet the threshold conditions for stimulated emission. The work reported in Ref. 76 showed that the intensity of the... 

The reaction of 01D above can be shown to be important as the ultraviolet photolysis of N02, which gives rise to 01D,62 results in the production of IO in the presence of CF3I.26 The abstraction of atomic oxygen from N02 by I(52Ph) would be an endothermic process. Thus although reaction (99) appears to be established, its contribution following the photolysis of CF3/03 mixtures may not be quantitatively assessed. The importance of reaction (98) therefore remains in doubt. 

The primary process in the vacuum ultraviolet photolysis of methylene iodide has been studied by Style and Ward,12 who observed that irradiation with light of wavelength 1250-2000 A. excites the fluorescence spectrum of iodine. Attempts to observe any appreciable delay between light absorption and fluorescence were unsuccessful, and the intensity of fluorescence was directly proportional to the light intensity and the pressure of methylene iodide. It was concluded that the excited I2 was produced in the primary process,... 

The latter path is favored energetically and is quite similar to other known reactions, such as methane photolysis. Vacuum ultraviolet photolysis (1236 A.) of CH488 is believed to yield CH2 and H2 as the major primary process, although other primary steps occur to some extent. 

VEEL spectra of surface phenyl groups have been obtained by ultraviolet photolysis of phenyl chloride on Ag(lll) at 300 K (211) and by thermal decomposition of phenyl iodide on Cu(lll) (212). The spectra are similar, and the strengths of the absorptions from the out-of-plane yCH modes, at 740 and 725 cm-1 respectively, show that the phenyl group is also more near to parallel than to perpendicular to the metal surfaces, again probably because of a combination of cr- and 77-bonding. 

The primary step in the ultraviolet photolysis of ozone almost certainly gives rise to an electronically excited, 1D, oxygen atom (see, for example, references 62 and 63) and if there is conservation of spin, then the product oxygen molecule should also be a singlet ... 

Young and Black18 believe, on the basis of the kinetic behavior of their system, that all O D) produced in the primary photolytic step reacts with 02 according to reaction (20). On the other hand, Izod and Wayne, from absolute measurements of [02(1E9+)] and [0(3P)], suggest that only one Oa(1S9+) molecule is produced in reaction (20) for about 500 0(1D) atoms deactivated. The calculation assumes that [0(a/>)] is equal to twice [0(1Z))] formed initially, although it seems improbable that the assumption is greatly in error for the vacuum ultraviolet photolysis of oxygen. For the overall deactivation of 0(1Z>) by 02,... 

An alternative approach to a study of the reactions of 02(1Efl+) is to work under conditions where [02(1A9)] is not in excess compared with [02(1Sa+)] as it is in discharge systems. Young and Black18 demonstrated that the vacuum ultraviolet photolysis of oxygen leads to the formation of 02(1Sfl+) by the energy-transfer process (Sect. IV-E) ... 

These equations demonstrate the link that is expected between HO and R02 in the troposphere. Hydroxyl radicals are formed in processes initiated by photolysis of various precursors, for example, the ultraviolet photolysis of... 

Vacuum ultraviolet photolysis of acetylene results in formation of triplet C2, as evidenced by its characteristic emission.139 Presumably, triplet acetylene is first formed and decomposes to C2 and H2. Saturated hydrocarbons undergo radiolytic reactions, but the relative importance of excited states versus ionized states has not yet been established with any certainty. 

The result that d>Nj < 1 in vacuum ultraviolet photolysis indicates that some electronically excited states produced may not dissociate immediately. This is supported by (he results that emission bands from several excited NO states have been observed with 1470 A excitation [Young et al. (1070)]. On the other hand, Stuhl and Niki (950) have found that O atoms are produced with light of wavelengths above 1600 A in the pressure region where the reaction of an electronically excited NO can be neglected. It is likely that both the production of excited NO and dissociation take place in the vacuum ultraviolet. 

The OH(/42L + ) has been seen in the vacuum ultraviolet photolysis of water, hydrogen peroxide, and nitric acid. The OH(.4 214) emission produced from OH(A"2n) by light absorption has been extensively used to measure OH( A2n> reaction rates [for example, Stuhl and Niki (951)]. 

The SO(X3L ) is formed as a primary product of the photolysis ofS02 below 2190 A. The SO(/l3n, B3 ) states have been found in the vacuum ultraviolet photolysis of OSCI2 [Okabe (768)]. 

The CS(X I + ) has been seen in the flash photolysis of CS2 in the near ultraviolet. The CS(A n) has been observed in the vacuum ultraviolet photolysis of CS2 (769) and SCCI2 (774). Fluorescence from the CS((t3n) state has been observed in the photolysis ofCS2 in the 1250 to 1400 A region of absorption. The lifetime and quenching rates of CS([Pg.39]

Ashfold and Simons (47a) have recently shown that both CN(/l2ri) and (B E+) states are formed in the vacuum ultraviolet photolysis of BrCN. At the low pressure limit CN(B3 + ) shows a vibrational population inversion at the 1236 A photolysis (a maximum at o = 2), while at higher pressures the population shows a monotonic decrease with an increase of o observed before by Mole and Okabe (692). They attribute the pressure elTect to the collisionally induced inicrsystcm crossing between the /t2Il (o > 10) and neighboring B2Z + (o > 0) levels. Because of the long radiative life of A2n sialc ( - 7 /isec) (532), it is susceptible to collisions even at a pressure of 10 mtorr. 

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