UV photofragmentation

Rodgers, M. O., and D. D. Davis, A UV-Photofragmentation/ Laser-Induced Fluorescence Sensor for the Atmospheric Detection of HONO, Environ. Sci, Technol., 23, 1106-1112(1989). 

Figure 5. Processes involved in vacuum UV photofragmentation-laser induced fluorescence detection of ammonia.

Figure 6. Instrumental schematic for vacuum UV photofragmentation-laser induced fluorescence measurement of ammonia SHGC, second harmonic generation crystal SFMC, sum frequency mixing crystal BS, beam splitter BD, beam dump TP, turning prism CL, cylindrical lens R, reflector TD, trigger diode OSC, oscillator cell AMP, amplifier cell BE, beam expander G, grating OC, output coupler M, mirror BC, beam combiner L, lens A, aperture PD, photodiode SC, sample cell RC, reference cell FP, filter pack SAM.PMT, sample cell photomultiplier REF.PMT, reference cell photomultiplier PP, additional photomultiplier port EX, exhaust and CGI, calibration gas inlet to flow line. (Reproduced with permission from reference 15. Copyright 1990 Optical Society of America.)...

Fig. 16 Photofragmentation mass spectra of [GS+2H] (m/z = 571.36 Da) measured with the UV laser only (blue trace) and with subsequent CO2 laser excitation (red trace). The UV laser wavenumber is fixed at il.ll i. i cm (the band origin of conformer A, see Fig. 19). Two of the most abundant fragments in the UV photofragmentation mass-spectrum correspond to loss of CO and CO-NH neutral fragments. The two most abundant fi agments in the UV + CO2 mass-spectrum correspond to the loss of a CO group (w/z = 557.3 Da) and to cleavage of the C -Cp bond of a phenylalanine residue (m/z = 525.8 Da)...

Rogers, M.O., and D.D. Davis (1989), A UV-photofragmentation/laser-induced fluorescence sensor for the atmospheric detection of HONO, Environ. Set Technol, 23, 1106-1112. 

The alkyl ethers of benzoin undergo dkect photofragmentation upon absorption of uv energy at ca 360 nm to produce two free-radical intermediates. 

The extremely wide range of possible dissociation energies necessitates the use of different kinds of light source to break molecular bonds. Van der Waals molecules can be fragmented with single infrared (IR) photons whereas the fission of a chemical bond requires either a single ultraviolet (UV) or many IR photons. The photofragmentation of van der Waals molecules has become a very active field in the last decade and deserves a book in itself (Beswick and Halberstadt 1993). It is a special case of UV photodissociation and can be described by the same theoretical means. In Chapter 12 we will briefly discuss some simple aspects of IR photodissociation in order to elucidate the similarities and the differences to UV photodissociation. 

There are two general classes of photoinitiators (1) those that undergo direct photofragmentation on exposure to uv or visible light irradiation and produce active free radical intermediates and (2) those that undergo electron transfer followed by proton transfer to form a free radical species. The choice of photoinitiator is determined by the radiation source, the film thickness, the pigmentation, and the types of base resin employed. Examples of typical photoinitiator systems used to cure reactive resins are shown in Table 14.2. Benzophenone is perhaps one of the most common photoinitiators. 

Thioaldehydes with almost any substituents in the a-position can be generated according to Vedejs by photofragmentation of phenacyl sulfides (86JOC1556). This is a simple, effective, and mild reaction which can be performed in neutral media. Phenacyl sulfides are known to be unstable to visible and UV light (68CC700). Hogeveen and Smit had previously... 

Shortly after Nitzan and Brus s theoretical prediction [13] of the possibility of surface enhanced photochemistry due to plasmon interactions, Goncher and Harris [20] presented experimental evidence for enhanced photofragmentation of several aromatic molecules (pyridine, pyrazine, and benzaldehyde) on roughened silver surfaces using low intensity UV radiation (364 run). See also the more detailed, subsequent paper of Goncher, Parsons and Harris [21],... 

Other Photofragmentations - Photodissociation of tert-huty hydroperoxide at 266 nm gives OH radicals with dynamics which are similar to those found for OH from H2O2, and which are consistent with dissociation via a repulsive excited state. Rates of p-scission of the ter/-butoxy radical to acetone and methyl radicals have been determined in flash-photolysis experiments by monitoring its transient UV absorption and its laser-induced fluorescence. ... 

Photodissociation involves a direct optical excitation (usually in the UV or VUV region) from the ground (bound) electronic state to the excited (repulsive) electronic state of the molecule which then dissociates. The fundamental quantity in photofragmentation calculation is the Franck-Condon (FC) factor... 

The main classes of initiators based on photofragmentation (scission of covalent bonds) are limited to UV applications, and they include peroxides, azo compounds, benzoin derivatives, acetophenone derivatives, ketoxime esters of benzoin, tri-azines, etc. On photolysis, these classes of initiators, depending on their wavelength of sensitivity, are able to initiate the polymerization of monomers such as methyl methacrylate in negative-tone lithographic imaging. 

Previous studies show that upon absorption of a UV photon by CH3SSCH3, the primary decomposition is S-S bond scission to produce 2 SCH3 radicals [11,24,26,27]. The recent 248-nm photofragmentation study of Lee et al. [53] confirms this conclusion ... 

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