UV laser photolysis

A battery of sensitive techniques is being developed to probe the photofragments resulting from photolysis of metal complexes in collision free conditions. The aim is to characterize the energy content, structure and chemistry of the photoproducts. These methods rely on ultraviolet (UV) laser photolysis followed by detection methods based on UV absorption (20), chemical trapping (21,22), IR absorption (23,24.25) and ionization (5,6,26,27). 

The gas-phase reaction of nascent M(CO)s species produced by 355-nm UV laser photolysis of the parent M(CO)6,... 

Douki T., Angelov D., Cadet J., UV Laser photolysis of DNA effect of duplex stability on charge-transfer efficiency, J. Am. Chem. Soc., 2001,123, 11360-11366. 

Angelov, D., Spassky, A., Berger, M., and Cadet, J. (1997) High-intensity UV laser photolysis of DNA and purine 2 -deoxyribonucleosides formation of 8-oxopurine damage and oligonucleotide sttand cleavage as revealed by HPLC and gel electtophoresis studies. 

Cobalt complexes are the specific area of interest for a paper dealing with the technology of spinning-cell Fourier transform Raman spectroscopy. The technique uses near-IR light and, because of the rapidly spinning cell, avoids the problems of sample burning and should be most useful for the study of delicate carbonyl species. Clusters are studied somewhat differently, following UV laser photolysis, in a paper published by Belyaev et. ... 

The molecular beam, photolysis laser beam, and VUV laser beam are perpendicular to each other. However, the crossing points of the photolysis laser beam and the VUV laser beam with the molecular beam are not the same. The crossing point of the photolysis laser beam with the molecular beam is adjustable. It is 3-10 cm upstream with respect to the crossing point of the VUV laser with the molecular beam. For short distances (<3cm) between these two crossing points, the direction of the UV laser beam is changed. It is in the plane formed by molecular beam and VUV laser beam, but the angle between the photolysis laser beam and the VUV laser beam is 15 degrees. 

The photolysis of Cr(CO)6 also provides evidence for the formation of both CO (69) and Cr(CO) species (91,92) in vibrationally excited states. Since CO lasers operate on vibrational transitions of CO, they are particularly sensitive method for detecting vibrationally excited CO. It is still not clear in detail how these vibrationally excited molecules are formed during uv photolysis. For Cr(CO)6 (69,92), more CO appeared to be formed in the ground state than in the first vibrational excited state, and excited CO continued to be formed after the end of the uv laser pulse. Similarly, Fe(CO) and Cr(CO) fragments were initially generated with IR absorptions that were shifted to long wavelength (75,91). This shift was apparently due to rotationally-vibrationally excited molecules which relaxed at a rate dependent on the pressure of added buffer gas. 

Fig. 12. Transient IR difference spectra showing changes in absorbance (a) 5 / seconds, (b) 25 seconds, and (c) 1.25 mseconds after the UV flash photolysis of [CpFe(CO)2]2 in cyclohexane solution under 1 atm pressure of CO. Bands pointing upward represent an increase in absorbance (i.e., formation of a compound) and those pointing downward a decrease [i.e., depletion of starting material, (A)]. The bands are assigned as follows A, [CpFe(CO)2]2 B, CpFe(CO)2 and C, CpFe(p.-CO)3Fe(Cp). Points marked were recorded with a 12CO laser and those marked + with a 13CO laser. [Reproduced with permission from Moore et al. (61).]...

More direct evidence for the intervention of excited states of triplet carbenes in reactions in solution is obtained by spectroscopic studies. Thus, picosecond lasers make it possible to study the quenching of carbene fluorescence by various substrates in solution at room temperature. Diphenylcarbene is generated upon laser photolysis of 30 and a second UV laser pulse is time delayed by 8 ns and is used to excite the carbene, thereby producing the excited triplet DPC (Scheme 9.32). The fluorescence of DPC is then monitored with a streak camera. The fluorescence... 

The singlet-triplet splitting of NH was determined experimentally by spectroscopy of neutral NH and by negative ion photoelectron spectroscopy (PES) of the NH anion. In the latter experiment, the anion NH is prepared in the gas phase and exposed to monochromatic ultraviolet (UV)-laser hght. This photolysis leads to ejection of photoelectrons whose kinetic energies ( k) are analyzed. As the energy... 

A promising recent development in the study of nitrenium ions has been the introduction of time-resolved vibrational spectroscopy for their characterization. These methods are based on pulsed laser photolysis. However, they employ either time resolved IR (TRIR) or time-resolved resonance Raman (TRRR) spectroscopy as the mode of detection. While these detection techniques are inherently less sensitive than UV-vis absorption, they provide more detailed and readily interpretable spectral information. In fact, it is possible to directly calculate these spectra using relatively fast and inexpensive DFT and MP2 methods. Thus, spectra derived from experiment can be used to validate (or falsify) various computational treatments of nitrenium ion stmctures and reactivity. In contrast, UV-vis spectra do not lend themselves to detailed structural analysis and, moreover, calculating these spectra from first principles is still expensive and highly approximate. 

In contrast to diaryl-A3-iodanyl radicals, cyclic dialkyl-A3-iodanyl radicals seem to be intermediates in the atom-transfer [Eq. (87)]. In the laser photolysis of diiodoalkanes, formation of the cyclic hypervalent iodanyl radicals 94 was detected by UV absorption spectra as intermediates with lifetimes around 9.5 x IQ 6 s (94a), 1.4 x 10 5 s (94b), and 4.4 x IQ-6 s (94c) [162]. 

Reactions of C2 (X1 2+) and (a3 nu) Produced by Multiphoton UV Excimer Laser Photolysis... 

The laser flash photolysis technique relies on the use of a pulsed UV laser for the rapid synthesis of the reactive intermediate of interest by photochemical decomposition of a suitable stable precursor, and (most commonly) fast time-resolved UV/VIS spectrophotometry to detect the species and monitor its decay19. The absorbance-time profile so... 

A schematic of the apparatus is shown in Figure 1. OH was produced by 248 nm (or 266 nm in some experiments) pulsed laser photolysis of H2O2 and detected by observing fluorescence excited by a pulsed tunable dye laser. Fluorescence was excited in the 0H(a2e+ - X tt) 0-1 band at 282 nm and detected in the O-O and 1-1 bands at 309+5 nm. Kinetic data was obtained by electronically varying the time delay between the photolysis laser and the probe laser. Sulfide concentrations were measured in situ in the slow flow system by UV photometry at 228.8 nm. 

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