Photolysis light sources

Such techniques imply analysis of chemical products of photolysis. Application of mass-spectrometers of various types is often hampered by a number of circumstances. These difficulties will be discussed later on. The EPR method, which is currently the most extensively employed technique, features low sensitivity and is usually used for analysis of primary fragments of photolysis. For this purpose, the radicals produced are frozen on the walls of a quartz pin and are thus accumulated inside the device. On one hand, this approach allows one to overcome the sensitivity threshold of the device. However, on the other hand, this excludes the possibility of direct kinetic measurements. The SS technique permits the use of weak light sources for detecting active particles under... 

Figure 6. Dissolution of 5pM Y-FeOOH on photolysis of a) pH 4.0, and b) pH 6.5 solutions containing either 10-1 M citrate, 10 mg/L aquatic fulvic acids, or no added organic agent. Light source simulated solar spectrum of total intensity 300 pEinsteins cm- min-1. (Reproduced from Ref. 32. Copyright 1985, American Chemical Society.)...

Figure 9. Comparison of the percentage of filterable iron obtained on photolysis of 25pM suspensions of Y-FeOOH and a-Fe203 in pH 3.0, 0.01M NaCl containing lOmM mercaptoacetic acid. Light source unfiltered 150W Xenon arc lamp.

With the invention of the laser in 1960 and the subsequent development of pulsed lasers using Q-switching (Chapter 1), monochromatic and highly-collimated light sources became available with pulse durations in the nanosecond timescale. These Q-switched pulsed lasers allow the study of photo-induced processes that occur some 103 times faster than events measured by flash lamp-based flash photolysis. 

Figure 10.7 shows a nanosecond kinetic flash photolysis apparatus. The absorbance of the sample is monitored, using a photomultiplier, by the change in the transmittance of the sample to the xenon arc lamp continuous light source. 

In the introduction to Volume 1 of this series, the founding editors, J. N. Pitts, G. S. Hammond and W. A. Noyes, Jr. noted developments in a brief span of prior years that were important for progress in photochemistry flash photolysis, nuclear magnetic resonance, and electron spin resonance. A quarter of a century later, in Volume 14 (1988), the editors noted that since then two developments had been of prime significance the emergence of the laser from an esoteric possibility to an important light source, and the evolution of computers to microcomputers in common laboratory use of data acquisition. These developments strongly influenced research on the dynamic behavior of the excited state and other transients. 

If either the protein or ligand is light sensitive then some photolysis ( photodecomposition ) may occur upon exposure to light. Under these circumstances, it is essential to shut off the excitation light source between measurements (usually by closing the excitation shutter that is provided in most instruments). The slit-width on the excitation side can also be reduced in order to minimize overall exposure (this can often be compensated by an increase in the slit-width on the emission side to increase the magnitude of emitted fluorescence). 

Fig. 12. A flash photolysis apparatus. 1, high-voltage power supply 2, 10 M12 resistor 3. high-voltage capacitor 4, coaxial cable 5, flash tube 6, vacuum system 7, reflector 8, pulsed spectroscopic light source 9, measuring cell 10, Hilger medium quartz spectrograph. (From Vallotton and Wild, Ref. ))...

Small differences in light sources have definite effects on photochemical processes. Some irradiation systems match the solar spectrum poorly, thus affecting the relative importance of the various photodissociation processes occurring in the atmosphere. Ambient variations in the nitrogen dioxide photolysis pseudo-first-order rate constant,... 

In the laser photolysis experiments the aromatic compound (4-10" M) and the nucleophile (0 04 M ) in acetonitrile-water (1 1) were irradiated with the frequency doubled pulse (100 mj, 6 ns, 347 nm) of a ruby laser. Only time-dependent absorption changes were measured (double pulsed xenon flash lamp with 10 /is continuous output as light source) absorption spectra were constructed from these measurements at 12 or 25 nm intervals. 

Photolysis of the telluroester in the presence of thiophenol isolation of benzaldehyde. A solution of the telluroester (Ar = 1-naphthyl) (15 mg, 41.7 jjmol) and thiophenol (17 mg, 154 jjmol) in CDCI3 (0.6 mL) was photolysed with a 250 W white light source at 8°C for 2 h. Preparative TLC on silica gel of the reaction mixture, eluting with ethyl acetate/hexa-nes, 1 9, gave benzaldehyde (3.5 mg, 80%). 

The continuous photolysis of the acid has also been studied at temperatures between 90-190°C. in a static system 73 the light source was a medium pressure mercury arc and irradiation was continued for 200 min. The major products isolated were C2F6 and C02, with smaller amounts of CO, H2, and CF3H C2F4 was not detected. There was a marked reduction in the yields of CO, C02, and H2 when isobutane or butene-1 were present and it was inferred that COOH radicals were produced in the system. Process (77a) was proposed as the principal process, together with a moderate contribution from process (77d), and a minor contribution from process (77c). 

Photolysis of carbethoxymethylenetriphenylphosphine in cyclohexene yields benzene, ethyl acetate, ethyl cyclohexylacetate, ethyl cyclohex-2-ene-l-acetate, phenylcyclohexane l,T-bicyclohex-2-ene (Quantum yield measured by use of a low pressure mercury lamp as a light source no yield in material) and diphenyl phosphinic acid. In this case, no triphenylphosphine is produced. On the other hand, pyrolysis of this carbethoxymethy-lene compound shows that only P=C bond fission occurs91. Using acetylmethylene-triphenylphosphine, the observed products are analogous20. However, the irradiation of... 

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... 

Since Br2 photodissociates efficiently at >400nm [94], the photolysis of both CH3CHO and N02 can be avoided by using a visible light source. Also, the Br + N02 reaction was shown to yield negligibly small concentrations of BrN02 and BrONO, presumably due to the photochemical instability of these products. 

---