Photolysis lamps

The maximum yield of Cr(CO)4(C2H4)2 is achieved after ca. 30 minutes photolysis and the compound is sufficiently stabilized in scC2H4 at ambient temperature for significant quantities to remain even 12 hours after turning off the photolysis lamp. (In the conventional synthesis, trace impurities appear to catalyse the decomposition of Cr(CO)4(C2H4)2 in solution until it is purified.)... 

The reaction vessel and photolysis lamp lay within a cylindrical MgO reflector and the system could be heated to 420 K. The lamp was filled with a mixture of a few Torr of Ni in 100 Torr of Kr. Flash energies of 500-1000 J were provided by charging a 5 /iF capacitor to the appropriate voltage. The light output fell to half its peak intensity in less than 10 /is and was effectively zero after 30 jus. 

The excited state produced in absorption in this region Is presumably 1Ajj( B2), which correlates with 0( D) and C0(X E ). Experimental evidence for this process and Its quantum yield Is entirely of the product analysis variety since 0( D) has not yet been detected directly in the photolysis of CO2. This seems correct despite two studies in which observation of 0( D) was reported. Noxon (51) reported the observation of 630.0 nm radiation accompanying radiative decay of the state to the ground state.. This observation, however, has since been found by Clark and Noxon (52) to be due to scattered radiation from the photolysis lamp. In a flash-photolysis study of CO2 decomposition, Clerc and Barat (53) detected absorption at 115.2 nm and attributed it to the transition, 0(3s 2p4 ID2). The experi-... 

Two other quartz glass tubes with diameters of 230 and 244 mm are mounted (concentrically one within the other) and centrally within the inner main reactor cylinder. The smaller of these central tubes contains 12 photolysis lamps. The lamps are of 2 types 6 Philips TLA 40W (300 < A, < 450 nm A ax = 360 nm) and 6 Philips TUV 40W (main emission at A = 254 nm). The area between the glass cylinders can be cooled either by flowing coolant or dry air. 

Figure 1. Right angle reaction cell and vacuum ultraviolet photolysis lamp...

The authors are grateful for the contribution of D. D. Davis of the National Bureau of Standards, who provided valuable assistance in the design and construction of the photolysis lamps used. The authors also thank A. K. Bhattacharya, who accomplished some of the experimental measurements reported here. This work was carried out during the tenure of a National Academy of Sciences-OAR Postdoctoral Research Associateship awarded to B. M. Hughes for 1967-68. 

At low flashing frequency, [CHs ] follows the photolysis lamps closely, and in a very short time compared with the photolysis period it reaches the steady-state concentration given by equation (19). As the absorption arising from the radical... 

Flash photolysis Flash photolysis Lamp photolysis Lamp photolysis... 

B2.5.4 FLASH PHOTOLYSIS WITH FLASH LAMPS AND LASERS... 

One of the most important teclmiques for the study of gas-phase reactions is flash photolysis [8, ]. A reaction is initiated by absorption of an intense light pulse, originally generated from flash lamps (duration a=lp.s). Nowadays these have frequently been replaced by pulsed laser sources, with the shortest pulses of the order of a few femtoseconds [22, 64]. 

A recent example of laser flash-lamp photolysis is given by Hippier etal [ ], who investigated the temperature and pressure dependence of the thennal recombmation rate constant for the reaction... 

The conmron flash-lamp photolysis and often also laser-flash photolysis are based on photochemical processes that are initiated by the absorption of a photon, hv. The intensity of laser pulses can reach GW cm or even TW cm, where multiphoton processes become important. Figure B2.5.13 simnnarizes the different mechanisms of multiphoton excitation [75, 76, 112], The direct multiphoton absorption of mechanism (i) requires an odd number of photons to reach an excited atomic or molecular level in the case of strict electric dipole and parity selection rules [117],... 

The flash lamp teclmology first used to photolyse samples has since been superseded by successive generations of increasingly faster pulsed laser teclmologies, leading to a time resolution for optical perturbation metliods tliat now extends to femtoseconds. This time scale approaches tlie ultimate limit on time resolution (At) available to flash photolysis studies, tlie limit imposed by chemical bond energies (AA) tlirough tlie uncertainty principle, AAAt > 2/j. 

The photolysis of 4-substituted 2,3-dimethyl-3-isoxazolin-5-ones has been studied. Irradiation in methanol or ethanol with a 100 W high-pressure mercury lamp through a Pyrex filter of a 4-phenylthio compound produced a semithioacetal (Scheme 5). In contrast, an H, Cl or OPh moiety gave no reaction. The use of alkylthio substitution gave similar products. Cyclic compounds yielded cyclic products (Scheme 5), and the photolysis of (29) in benzene... 

Photolysis of a dioxane solution of (2) using a 250 watt quartz lamp also provides 16a,17a-methylene-20-ketones (4). ... 

The irradiations are in general performed under purified, oxygen-free nitrogen in a cylindrical irradiation apparatus with a water-cooled central sleeve into which the lamp is introduced and efficient cooling is used to keep the temperature between 10° and 0° C. Temperatures in the range of 0° to 80° do not influence the photolysis step as demonstrated in the case of a 6j -nitrite, but increase the rate of rearrangement of the nitroso derivative to the oxime. 

Nonyl aldehyde (32.66 g, 0.23 mol) and furan (200 mL, 187.2 g, 2.75 mol) were mixed in a 250-mL photolysis flask equipped with a quartz immersion well containing a Vycor filter and a 450-W Hanovia Lamp. The system was kept at -20° C with an isopropyl alcohol bath cooled by a Cryocool Immersion Cooler (CClOO). Nitrogen was bubbled throughout the duration of the reaction, and the solution was stirred vigorously. Additional furan (150 mL, 140.4 g, 2.06 mol) was added during the course of the reaction. TLC analysis indicated completion of the reaction after 20 h. After evaporation of excess furan and NMR analysis of the resultant oil (48.70 g, ca. 100%) indicated the desired photoadduct had been formed, without contamination from unreacted nonyl aldehyde. 

Tile only instance of isolation of a stable benzo-l,2-dithiete is the yellow crystalline compound 187, which was obtained by photolysis of 188 in heptane at -20°C using a medium-pressure mercury lamp [75JCS(CC)756 77JCS(P1)515],... 

A solution of dimethyl 3-acetyl-3-azatetracyclo[3.2.0.02-7.04 6]heptane-l,5-dicarboxylate (2, R1 = Ac R = H), formed by the photolysis (14 h 125-W Hg lamp under N2) of dimethyl 7-acetyl-7-azabicyclo-[2.2.1]hepta-2,5-diene-2,3-dicarboxylate (1 R1 = Ac, R2 = H 1 g, 4mmol) in Et20 (400 mL) at — 40 C, was evaporated to dryness under reduced pressure. The residue (0.7 g, 2.8 mmol) was dissolved in CHC1, and the solution heated under reflux for 1 h. Evaporation of the solvent yielded the crude product which was purified by column chromatography (silica gel, C,H2C12). The yellow fractions were collected and, after removal of the solvent, the residual oil was distilled in a sublimation apparatus to give 3 (R1 = Ac R2 = H) as a yellow oil yield 0.8 g (80%) bp 50 60 C/5 x 10 4 Torr. 

The proton NMR spectrum shows chem shifts of 6.93 5.957- (Ref 1). Photolysis with a Hg arc lamp gives N, nitrous oxide, methane, and ethane (Ref 2). It was found to produce colon and rectal carcinomas in rats after oral administration at 12mg/kg weekly, induction period 235 days (Ref 3)... 

There are countless other reactions, many like these and others rather different, but the idea in every case is the same. A sudden flash of light causes an immediate photo-excitation chemical events ensue thereafter. This technique of flash photolysis was invented and applied to certain gas-phase reactions by G. Porter and R. G. W. Nor-rish, who shared with Eigen the 1967 Nobel Prize in Chemistry. High-intensity flash lamps fired by a capacitor discharge were once the method of choice for fast photochemical excitation. Lasers, which are in general much faster, have nowadays largely supplanted flash lamps. Moreover, the laser light is monochromatic so that only the desired absorption band of the parent compound will be irradiated. 

While still slightly warm from the drying oven, the photolysis vessel with a water-jacketed quartz immersion well (Note 1) (section A of Figure 2) is charged with 500 ml. of anhydrous tetrahydrofuran (Note 2) and 10 ml. (8.05 g., 0.122 mole) of cyclopentadiene (Note 3). The solution is cooled in an ice bath and purged with dry nitrogen for 2 minutes. Then the vessel is sealed, the lamp inserted, and the solution irradiated at 0° for 30 minutes. During this period, sections B and C... 

Irradiation of cyclo-S% dissolved in CS2 by a high-pressure mercury lamp at 20 °C produces the homocycles S7, S, S12, S9, Sio, and probably S5 in concentrations decreasing in this order. Irradiation of Se in CS2 gives mainly Ss and S7 while irradiation of S7 generates Ss and S. Similarly, photolysis of S12 in CS2 yields Ss, S7, and Se [51]. For these reasons UV-Vis spectra of compounds containing S-S bonds must be recorded with caution not to trigger decomposition reactions. 

A more concentrated (1000 ppm) solution of dibenzo-p-dioxin in methanol was irradiated for 1.5 hours under a 450-watt lamp fitted with a borosilicate glass filter while nitrogen was bubbled continuously through, the solution. Unchanged starting material was recovered to the extent of 85%. The principal photolysis product again was a dark brown insoluble gum similar to that described above. Its mobility on thin layer chromatography (TLC) was very low in the benzene/ethyl acetate (4 1) solvent used to separate the other products. 

Diisopropyl methylphosphonate does not undergo direct or indirect photolysis in aquatic systems, as demonstrated by the stability of the compound in distilled water or in a natural water sample after 232 hours of reaction time with the mercury lamp filtered to exclude all wavelengths below 290 nm (Spanggord et al. 1979). 

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