Vacuum-ultraviolet photolysis

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. 

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. 

As an example of application of semiconductor sensors for this purpose, we consider photolysis of simplest olefines (ethylene, propylene, acetylene, etc.) occurring in the range of vacuum ultraviolet. It is well-known (e.g., see [11]) that photolysis of ethylene may result in detachment of either hydrogen molecules (detached in one act) or hydrogen atoms. Hydrogen atoms subsequently associate into molecules or interact with ethylene molecules. In what follows, we consider how this problem can be solved with the help of sensors. 

The apparatus consists of a pulsed molecular beam, a pulsed ultraviolet (UV) photolysis laser beam, a pulsed vacuum ultraviolet (VUV) probe laser beam, a mass spectrometer, and a two-dimensional ion detector. The schematic diagram is shown in Fig. 1. 

E. Hayon and J. McGarvey, Flash photolysis in the vacuum ultraviolet region of sulfate, carbonate, and hydroxyl ions in aqueous solutions. J. Phys. Chem. 71, 1472-1477 (1967). 

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... 

Carbon Suboxide Photoiysis. In principle, carbon suboxide (1) can be used as a precursor to atomic carbon and two molecules of carbon monoxide as shown in Eq. 2. However, this reaction is endothermic by 141 kcal/mol and can only be realized in the vacuum ultraviolet (UV) at wavelengths that destroy most organic substrates. However, photolysis of 1 at 1470 A produces C atoms in a low-temperature matrix. The short wavelength flash photolysis of 1 coupled with atomic absorption has been used to measure the rate constants for various spin states of carbon with simple substrates. 

Br(42Pi/2) being rapidly quenched by any Br2 present.76 While Cl(32iVt) would be expected from the photolysis of Cl2, the strongest absorption transition of the excited atom at 1351.7 A (Table IV) was obscured by the molecular spectrum of undissociated Cl2 and only an absorption transition of the ground state atom at 1335.7 A (Table IV) could be detected through a window in the vacuum ultraviolet molecular spectrum.29... 

Emission from Cl(32Py2) has not been detected in flow systems40 although this excited atom has recently been observed in absorption following the vacuum ultraviolet flash photolysis of a number of chlorides, including HC1.2B The over-all rate for the reaction... 

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... 

It has now been shown that a population inversion is not achieved between the aPi/2 and 2P% states of the chlorine atom in the vacuum ultraviolet flash photolysis of CF3C1,29 and thus the construction of a chlorine atom photodissociation laser appears remote at present. 

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,... 

Though most of the oxygen in the atmosphere has been formed by photosynthesis in plants, some is produced by photolysis of water vapour in the vacuum ultraviolet region A <200 nm. Photolysis of N2, NO, N02, NHa, CO, 002 and small aliphatic hydrocarbons (alkanes) set up complex reactions in the upper atmosphere. 

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) ... 

L. Jakob, T. M. Hashem, M. M. Kantor, and A. M. Braun, Oxidative Degradation Processes by Vacuum Ultraviolet (VUV) Photolysis, Division of Environmental Chemistry, ACS Meeting, Proceedings, San Francisco, 1992. 

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. 

McNesby et al. (683) have used C2D4 as a scavenger of H atoms produced in the photolysis of H20 and NH3 in the vacuum ultraviolet. Since H atoms react extremely slowly with H20 or NH3 (< 10 16 cm3 molec-1 sec )(ref. l)they are effectively eliminated by added C2D4. Thus, the ratio of H2 with and without added C2D4 shows the extent of molecular hydrogen production in the primary process. 

The S( >) state lies 1.145 eV above the ground S(3P) state and is metastable with a lifetime of 28 sec. The S( >) state can be detected by absorption in the vacuum ultraviolet at 1667 or 1448 A (33). However, in spite of much effort S( )) atoms have not been detected by optical absorption in the vacuum ultraviolet flash photolysis of OCS because of a rapid reaction of S( D) with OCS... 

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