Spectroscopy picosecond flash

Using picosecond flash spectroscopy Gupta et al. 2k) reported for 2-hydroxyphenylbenzotriazole in ethanol a short-lived transient (6 ps) followed by a transient absorption whose lifetime is estimated to be 600 ps. The authors assigned the short-lived transient to the "vertical singlet" while the long-lived transient is presumably the "proton transferred species". These measurements of transient absorptions with the picosecond flash method confirm our results derived from the fluorescence emission using the phase fluorimetric method. 

The apparatus used for picosecond flash spectroscopy on these systems has been described before(8 10). Figure 3a and b show typical transient absorption data obtained on 2-hydroxybenzophenone and the copolymer. Summary of these spectral data are given in Table 3. The transient observed at the shortest delay time (7ps) is the first excited singlet in all systems. The spectral data (at delay times > 50ps) permit placement of upper limits on triplet yields in CH2CI2 for both 2-hydroxy benzophenone itself and the copolymerized chromophore. 

Arbour, C. Sharma, D. K. Langford, C. H. Picosecond flash spectroscopy of Ti02 colloids with adsorbed dyes, J. Phys. Chem. 1990, 94, 331. 

Huppert et al. have recently applied picosecond and nanosecond flash spectroscopy to methanol solutions of 11-cis PRSB (196). Monitoring at 485 nm which is within the main absorption band of the molecule, they observe a fast (<10 ps) depletion, followed by a slow (tr 11 ns) recovery of the absorbance to a final permanent value consistent with a net (11-cis) -+ (all-trans) interconversion. In the red, between 550 and 660 nm, they observe a fast rise in absorbance (<10 ps) due to an unidentified long-lived (Td 0.5 ns) transient (X). Although unclear as to whether Tr = Td they suggested that X is an (excited-state or ground-state) isomer precursor of the final all-trans photoproduct. It was thus concluded that photoisomerization is a relatively slow process occurring in the nanosecond range. Independent of the identity of X, this conclusion does not seem to be a unique interpretation of the experimental observations noted above. In fact, these are consistent with either one of two mechanisms ... 

If an intermediate is not sufficiently stable to be isolated, it might nevertheless be formed in sufficient concentration to be detected spectroscopically. Techniques used for this purpose include UV—vis spectroscopy in stopped-flow kinetics experiments for relatively stable intermediates or IR spectroscopy in matrix isolation spectroscopy for more reactive species. For photochemical reactions, we can detect transient spectra of intermediates in the millisecond to microsecond ( conventional" flash spectroscopy) or nanosecond to picosecond or femtosecond (laser flash spectroscopy) time scale. In all cases we must be certain that the spectra observed are indeed indicative of the presence of the proposed intermediate and only the proposed intermediate. Theoretical calculations have been useful in determining the spectroscopic properties of a proposed intermediate, whether it is likely to be sufficiently stable for detection, and the t)q e of experiment most likely to detect it. In addition, kinetic studies may suggest optimum conditions for spectroscopic detection of an intermediate. ... 

With the photographic flash lamp the light pulse has a duration of several microseconds at best. The Q-switched pulsed laser provides pulses some thousand times faster, and the kinetic detection technique remains similar since photomultiplier tubes and oscilloscopes operate adequately on this time-scale. The situation is different with the spectrographic technique electronic delay units must be replaced by optical delay lines, a technique used mostly in picosecond spectroscopy. This is discussed in Chapter 8. 

In the first place, we shall take a look at the recent advances in fast reaction photochemical kinetics and spectroscopy, in particular at picosecond laser flash photolysis and femtosecond observations. Next, photophysics and photochemistry in molecular beams will be considered. Here observations are made under single molecule-single photon conditions, and these experiments provide insight into the most fundamental unimolecular gas phase reactions. 

More recently, powerful time-resolving techniques began to evolve. Nanosecond [13] and picosecond [14] flash absorption and emission spectroscopy made it possible to obtain UV spectra of transient species with very short lifetimes. 

The conventional flash photolysis setup to study photochemical reactions was drastically improved with the introduction of the pulsed laser in 1970 [17], Soon, nanosecond time resolution was achieved [13], However, the possibility to study processes faster than diffusion, happening in less than 10 10 s, was only attainable with picosecond spectroscopy. This technique has been applied since the 1980s as a routine method. There are reviews covering the special aspects of interest of their authors on this topic by Rentzepis [14a], Mataga [14b], Scaiano [18], and Peters [14c],... 

The current detailed understanding of photo-induced electron transfer processes has been advanced dramatically by the development of modern spectroscopic methods. For example, the application of time-resolved optical spectroscopy has developed from modest beginnings (flash-phyotolysis with millisecond resolution) [108,109] to the current state of the art, where laser spectroscopy with nanosecond resolution [110-113] must be considered routine, and where picosecond [114-116] or even femtosecond resolution [117] is no longer uncommon. Other spectroscopic techniques that have been applied to the study of electron transfer processes include time-resolved Raman spectroscopy [118], (time resolved) electron spin... 

Photodissociation of CO ( flash ) from the reduced enzyme after mixing with O2 ( flow ) in the dark has been the main method of initiating the reaction. Some concern has been voiced as to the possibility that this technique might introduce artifacts due to CO, but results using a rapid O2 mixing system without CO have coirfirmed the applicabihty of the flow/flash technique. Yet, it has been shown by infrared spectroscopy that upon photolysis, the heme 03-bound CO is first transferred in less than a picosecond to the nearby Cub, to which it remains bound for about a microsecond at room temperature before diffusing out of the enzyme, as also verified by EXAFS data. ... 

In purple bacteria a number of different lines of evidence led to the conclusion that bacteriopheophytin (BPh) acts as an electron carrier between the primary donor and Qa (Chapter 3). When is reduced illumination results in the photoaccumulation of reduced bacteriopheophytin, detected by its characteristic absorption changes and by an EPR signal split due to its interaction with Q Fe. At temperatures too low for rapid photoaccumulation of BPh to take place, illumination results in formation of a triplet state of the primary donor P-870 which has a polarization pattern characteristic of its formation by recombination of a radical pair. When BPh is reduced this triplet state cannot be formed. The most direct proof that BPh acts as a primary acceptor comes from the direct observation by absorption spectroscopy of BPh reduction within a few picoseconds after the flash. The BPh is reoxidized in 200 ps by electron transfer to or, if is already reduced, by recombination in 14 ns (see Chapter 3). 

The presence of a fast relaxing intermediate electron acceptor was first discovered when picosecond spectroscopy was applied to the study of photosynthetic systems. A laser flash-induced signal was detected when the primary quinone acceptor was prereduced [18,19], the lifetime of which was so short as to be observed only in the submicrosecond time range. The fast disappearance of the signal can be understood if it is considered that the rate of charge recombination in is very fast... 

Chemiluminescence has been used to measure the relative yields of excited ketones formed from self reaction of alkoxyl and alkylperoxyl radical pairs . In the photochemistry of aryl azides a dehydroazepine is detected by time resolved infra red spectroscopy and flash photolysis at room temperature . Singlet and triplet nitrenes and dehydroazepenes have also been detected in the photochemistry of 3- and 4-nitrophenyl azides . Picosecond and nanosecond laser photolysis of p-nitrophenyl acetate in aqueous media produces a triplet state of the -nitrobenzylanion and CO2 after cleavage of the rnr triplet. Absorption, emission, and reaction kinetics of dimethylsilylene produced by flash photolyses of dodecamethylcycloherasilane is another interesting study 2,... 

The direct irradiation of 1,3,5-cyclooctatriene (184) in ether or hydrocarbon solvents leads to the slow formation of two stable isomers coiresponding to disrotatory 4jr-electrocyclization (185) and bicyclo[3.1.0]pentene (186) fonnation along with small amounts of the reduced product 187 (equation 69)2 -. Conventional flash photolysis experiments later showed that, in fact, the main primary photochemical process is the formation of a short-lived stereoisomer (r = 91 ms) -, most likely identifiable as ,Z,Z-184. The transient decays to yield a second transient species (r = 23 s) identified as Z,Z-l,3,5,7-octatetraene (188), which in turn decays by electrocyclic ring closure to regenerate 184 82 (equation 70). The photochemistry of 184 has been studied on the picosecond timescale using time-resolved resonance Raman spectroscopy . 

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