Photolysis in the near

The photochemical dissociation of phosgene was first reported by Weigert in 1907 

He noted that, although the decomposition of phosgene was greatly accelerated upon irradiation (between 420 and 510 C), the position of equilibrium was not displaced, and concluded that the effect of the light was simply catalytic [2162], an observation later confirmed by Bodenstein and Onoda [219]. Less convincing was Weigert s theory of that catalytic action 

In 1930, Henri and Howell [942] incorrectly assumed that the initial step of this reaction involved the formation of CO and 2C1 . A few years later, Montgomery and Rollefson [1428] studied the kinetics of the photochemical decomposition of phosgene, and derived the rate law  

This is consistent with the following simplified mechanism [1428]  

This is also in reasonable accord with the mechanism of Lenher and Rollefson for the formation of phosgene [1226], and the full mechanism of this process is discussed at length in Section 5.1.1. The quantum yield for the process is essentially unity. This mechanism is 

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Volman has reviewed the photolysis in the near ultraviolet (995). The primary process... 

A second absorption of N2H2 starts at about 1730 A and consists of nine vibrational bands. Rotational lines are all diffuse [Trombetti (976)]. The photolysis in the near ultraviolet has been studied recently by Willis ct al. (1050). 

The photolyses of CH2I2 and CHI3 in molecular beams have been investigated by Kawasaki ct al. (560) using a broad-band polarized light source in conjunction with a mass spectrometer. The primary product of the photolysis in the near ultraviolet is the I atom. Hence, primary processes are... 

Atmospheric photolysis provides the predominant source of free radicals in the atmosphere. Photolysis of NO2 produces O atoms which form ozone ozone photolysis in the near UV produces 0( D) which reacts with H2O to produce OH radicals. A number of organic species absorb UV light and dissociate to yield organic peroxy and HO2 radicals in the presence of O2,... 

M. Fujitsuka, O. Ito, T. Yamashiro, Y. Aso and T. Otsuho, Solvent polarity dependence of photoinduced charge separation in a tetrathiophene-Cgo dyad studied by pico- and nanosecond laser flash photolysis in the near-lR region, J. Phys. Chem. A, 104, 4876-4881 (2000). 

OH reaction on a timescale of about 1 h (assuming peak daytime [OH] = 2.5 x 10 molecule cm ). Major products of the OH-initiated oxidation (Klotz et al., 1997) were the E, E- and E, Z-isomers of 2,4-hexadienedial, while phenol is a major product of the photolysis in the near UV. The rapid reaction with NO3 suggests a very rapid nighttime loss for benzene oxide/oxepin, although it is unlikely to be formed at night and its rapid daytime removal suggests little build-up of its concentration at night (Klotz et al., 1997). 

There are therefore two ways in which lasers may be used to bring about photon-assisted film formation. If the laser emits radiation in the near-ultra-violet or above, photochemical decomposition occurs in the gas phase and some unabsorbed radiation arrives at the substrate, but this latter should be a minor effect in die thin film formation. This procedure is referred to as photolysis. Alternatively, if the laser emits radiation in the infra-red, and tire photons are only feebly absorbed to raise the rotational energy levels of the gaseous... 

There are two possible structures for simple alkyl radicals. They might have sp bonding, in which case the structure would be planar, with the odd electron in ap orbital, or the bonding might be sp, which would make the structure pyramidal and place the odd electron in an sp orbital. The ESR spectra of CHs and other simple alkyl radicals as well as other evidence indicate that these radicals have planar structures.This is in accord with the known loss of optical activity when a free radical is generated at a chiral carbon. In addition, electronic spectra of the CH3 and CD3 radicals (generated by flash photolysis) in the gas phase have definitely established that under these conditions the radicals are planar or near planar. The IR spectra of CH3 trapped in solid argon led to a similar conclusion. " °... 

This brings us to U(VI) as an electron acceptor in MMCT transitions. A few examples were mentioned above already. Krol et al. have shown and used the U(V)-U(VI) MMCT transition in oxygen-deficient uranates [78]. It is situated in the near-infrared. This transition plays an important role in the quenching of the luminescence of uranates. The luminescence of U(VI)02 in solution is quenched by Mn(II) and it has been shown by flash photolysis that this is due to an excited MMCT state Mn(III)U(V) [79]. 

In comparison, photolysis of 83 in protic solvents such as methanol, ethanol, and water yields 84 as expected, but 84 forms mainly 87 rather than 85. Furthermore, in these solvents, the transient absorption (Amax 425 nm) due to 84 decays not with a second-order rate law but by biexponential decay. For example, the decay of transient absorption of 84 (A ax 420 nm) in water at pH 7 had rate constants of 2 x 10 and 3 x lO s Subsequent to the decay of 84, a transient absorption was formed with Amax 330 nm and a weak absorption band at 740 nm. However, this transient was formed much slower than 84 decayed. The absorption at 330 nm was described as a biexponential growth with rate constants of 584 and 21 s h The authors assigned this absorption to 88. Since 84 and 88 do not form and decay at the same rate, the authors theorized that 84 decays into 87, which then furnishes 88. Even though intermediate 87 does not absorb in the near UV, the authors characterized it with time-resolved IR spectroscopy. The authors demonstrated that, in hexane and a strongly acidic or basic aqueous solution, the photorelease from 83 goes through the formation of 87, whereas in near neutral aqueous solution, formation of 85 predominates. The authors concluded that the dehydration of intermediates 85 and... 

The X2L+ state has been observed in the photolysis of various cyanogen compounds in the near and vacuum ultraviolet. The B2 + and -42n states have been seen in the photolysis of cyanogen compounds in the vacuum ultraviolet [Mele and Okabe (692)]. Lifetimes of the A2U and B21 + states have been measured by Jeunehomme (532), Cook and Levy (236), Luk and Bersohn (650), Liszt and Hesser (641), and Jackson (518). These values are given in Table V-12. Quenching of the 02X state has been measured by Jackson (518) and Luk and Bersohn (650). 

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]

Linglcman (336) has found vibrationally excited CN radicals in the Hash photolysis of BrCN in the near ultraviolet. The vibrational excitation is considered to arise from the following reaction sequence... 

The (lash photolysis of Cl20 in the near ultraviolet has been studied by I dgecombe et al. (334) and Basco and Dogra (66). A mechanism proposed is... 

The CF2 radicals are produced in the vacuum ultraviolet photolysis of t F2CI2 [Rebbert and Ausloos (828)] and in the near ultraviolet photolysis it CF2Br2 and CF2HBr [Simons and Yarwood (891)]. 

Three main primary processes are found to occur in the near and vacuum ultraviolet photolysis. 

Photoilissociution. Few photochemical studies of thionyl chloride have been made. In the near ultraviolet (lash photolysis Donovan et al. (306) have found Cl and SO in absorption. Since the amounts of SO formed tire much less than those of OSCI2 decomposed, they have concluded the primary... 

Chou et al. (209) have suggested another minor process in the near ultraviolet photolysis,... 

Palmer and Padrick (790) have determined rate constants kl30 < 5 x 10"15 and k13, = 2x 10-l3cm3 molec-1 sec-1, that is, the deactivation process is more important than the chemical reaction. Mains and Lewis (659) have measured the quantum yield of methane production in low and high intensity photolysis of CH3I in the near ultraviolet. The quantum yield of CH4 is a function of pressure and ranges from 0.1 to 0.001 in the high intensity photolysis and from 0.02 to 0.05 in the low intensity photolysis. 

The photolysis of CF3I in the near ultraviolet is represented as in the case of CH3I by two primary processes... 

The photolysis of CHC13 has been performed by Yu and Wijnen (1077) in the near ultraviolet in the presence of ethane. The results of the product analysis suggest the primary process... 

The photolysis products in the near ultraviolet are H2, CO, and C02. The addition of C2H4 or 02 reduces the H2 yield to 16% of that without the scavenger but the CO and C02 yields arc only slightly reduced. 

Various transitions in the vacuum ultraviolet have been observed by Herzbcrg and Johns (467). The CH molecule dissociates by absorption of light below 3200 A. The CH absorption has been observed in the flash photolysis of diazomethane in the near ultraviolet by Herzberg and Johns (467). 

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