Laser photolysis pulsed lasers

By means of the sub-microsecond time resolution achieved by employing a pulsed laser photolysis/pulsed laser induced fluorescence technique, Hynes and wine are able to obtain k14a, k.14a and k and obtain an overall expression for kj4 as a function of temperature and [O2]- Their result in 700 torr of air is again substantially lower than the result from three competitive studies (Table IV)... 

We have employed a pulsed laser photolysis - pulsed laser induced fluorescence technique to carry out direct, real time studies of OH reactions with DMS and DMS-dfc in N2, air, and O2 buffer gases. Both temperature and pressure dependencies have been investigated. We find that the observed rate constant (kQbs = d[0H]/[0H] [DMS]dt) depends on the O2 concentration. Our results are consistent with a mechanism which includes an abstraction route, a reversible addition route, and an adduct + O2 reaction which competes with adduct decomposition under atmospheric conditions. 

Donohoue et al. [31] has reported two other kinetic data sets for Cl and Br reactions using a pulsed laser photolysis-pulsed laser induced fluorescence spectroscopy. These data sets are obtained using pseudo-first order conditions with respect to halogens or mercury and experiments were performed at a broad range of temperatures. The authors of these studies indicate an uncertainty estimation of 50% in the rate coefficients due to the determination of absolute concentrations of chlorine and bromine atoms [31]. Sumner et al. [20] reinvestigated both reactions using a 17.3 m environmental chambers equipped with fluorescent lamps and sun lamps to mimic environmental reactions, and evaluated the rate constants... 

A reeent re-evaluation of the rate coefficient and the branching ratio has been made by Williams et al. (2001) using the pulsed laser photolysis-pulsed laser induced fluorescence (PLP-PLIF) teehnique. The effective rate coefficient for the reaction of OH -1- DMS and OH + DMS-db was determined as a function of O2 partial pressure at 600 Torr total pressure in N2/O2 mixtures the temperature was 240 K for DMS and 240, 261, and 298 K for DMS-db. This new work shows that at low temperatures the currently recommended expression underestimates both the effective rate coefficient for die reaction and also the branching ratio between addition and abstraction. For example, at 261 K a branching ratio of 3.6 was obtained as opposed to a value of 2.8 based on the work of Hynes et al. (1986). At 240 K the discrepancy increases between a measured value of 7.8 and a value of 3.9 using the extrapolated values from the 1986 work of Hynes et al. (the branching ratio is defined here as (kobs-kia)/kia). In addition, at 240 K the expression for Us in 1 atm air based on the work of Hynes et al. (1986) predicts a value which is a factor of 2 lower then the value measured at... 

Combustion processes are driven by energy-releasing chemical reactions. Detailed knowledge of the chemical kinetics of these individual reactive steps is required input to combustion models. For more than a decade, elementary gas-phase reaction kinetics has been successfully studied with the flash photolysis/resonance fluorescence technique (1-8). Typically, following broadband photolysis of a molecular precursor, reactant decays have been measured under pseudo-first-order kinetic conditions with cw resonance lamp excitation of free radical fluorescence. Increased utilization of laser probes in kinetic studies is exemplified by the recent pulsed-laser photolysis/pulsed-laser-induced fluorescence experiments of McDonald, Lin and coworkers (9-13). 

D Ottone, L., Campuzano-Jost, P., Bauer, D., Hynes, A.J. A pulsed laser photolysis - pulsed laser induced fluorescence study of the kinetics of the gas-phase reaction of OH with NO2. J. Phys. Chem. A 105, 10538-10543 (2001)... 

ULTRASONICS I LASER PHOTOLYSIS PULSE RADIOLYSIS I NMR LINE BROADENING I... 

Ultrasonics Flash or laser photolysis Pulse radiolysis nmr ... 

Photolysis - Pulsed Laser Induced Fluorescence (PLP-PLIF), a technique which allows us to directly measure OH reaction rates under atmospheric conditions and also permits the sub-microsecond time resolution necessary to observe fast equilibration processes. 

Many optical studies have employed a quasi-static cell, through which the photolytic precursor of one of the reagents and the stable molecular reagent are slowly flowed. The reaction is then initiated by laser photolysis of the precursor, and the products are detected a short time after the photolysis event. To avoid collisional relaxation of the internal degrees of freedom of the product, the products must be detected in a shorter time when compared to the time between gas-kinetic collisions, that depends inversely upon the total pressure in the cell. In some cases, for example in case of the stable NO product from the H + NO2 reaction discussed in section B2.3.3.2. the products are not removed by collisions with the walls and may have long residence times in the apparatus. Study of such reactions are better carried out with pulsed introduction of the reagents into the cell or under crossed-beam conditions. 

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]. 

As an example, we mention the detection of iodine atoms in their P3/2 ground state with a 3 + 2 multiphoton ionization process at a laser wavelength of 474.3 run. Excited iodine atoms ( Pi/2) can also be detected selectively as the resonance condition is reached at a different laser wavelength of 477.7 run. As an example, figure B2.5.17 hows REMPI iodine atom detection after IR laser photolysis of CF I. This pump-probe experiment involves two, delayed, laser pulses, with a 200 ns IR photolysis pulse and a 10 ns probe pulse, which detects iodine atoms at different times during and after the photolysis pulse. This experiment illustrates a frindamental problem of product detection by multiphoton ionization with its high intensity, the short-wavelength probe laser radiation alone can photolyse the... 

Figure B2.5.17. (a) Time-dependent intensity / and redueed fluenee F/Fq for a single-mode CO2 laser pulse used in the IR laser photolysis of CF I. Fq is the total fluenee of the laser pulse, (b) VIS-REMPI iodine atom signals obtained with CO2 laser pulses of different fluenee (after [113]).

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. 

Pulsed lasers (Chapter 9) may be used both for photolysis and as a source. Since the pulses can be extremely short, of the order of a few picoseconds or less, species with comparably short lifetimes, such as an atom or molecule in a short-lived excited electronic state, may be investigated. 

Morishima et al. [75, 76] have shown a remarkable effect of the polyelectrolyte surface potential on photoinduced ET in the laser photolysis of APh-x (8) and QPh-x (12) with viologens as electron acceptors. Decay profiles for the SPV (14) radical anion (SPV- ) generated by the photoinduced ET following a 347.1-nm laser excitation were monitored at 602 nm (Fig. 13) [75], For APh-9, the SPV- transient absorption persisted for several hundred microseconds after the laser pulse. The second-order rate constant (kb) for the back ET from SPV- to the oxidized Phen residue (Phen+) was estimated to be 8.7 x 107 M 1 s-1 for the APh-9-SPV system. For the monomer model system (AM(15)-SPV), on the other hand, kb was 2.8 x 109 M-1 s-1. This marked retardation of the back ET in the APh-9-SPV system is attributed to the electrostatic repulsion of SPV- by the electric field on the molecular surface of APh-9. The addition of NaCl decreases the electrostatic interaction. In fact, it increased the back ET rate. For example, at NaCl concentrations of 0.025 and 0.2 M, the value of kb increased to 2.5 x 108 and... 

Pulsed laser photolysis (PLP) has emerged as the most reliable method for extracting absolute rate constants for the propagation step of radical polymerizations,343 The method can be traced to the work of Aleksandrov el al.370 PLP in its present form owes its existence to the extensive work of Olaj and eoworkers 71 and the efforts of an 1UPAC working party/45"351 The method has now been successfully applied to establish rate constants, /rp(overall), for many polymerizations and copolymerizations. 

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

JOVANOVIC s V, HARA Y, STEENKEN s and SIMIC M G (1995) Autioxidant potential of gaUocatechins. A pulse radiolysis and laser photolysis study, J Am. Chem Soc, 117, 9881-88. 

Final resolution of these problems, particularly the complications from multiple matrix sites, came from investigations using spectroscopic methods with higher time resolution, viz. laser flash photolysis. Short laser pulse irradiation of diazofluorene (36) in cold organic glasses produced the corresponding fluorenylidene (37), which could be detected by UV/VIS spectroscopy. Now, in contrast to the results from EPR spectroscopy, single exponential decays of the carbene could be observed in matrices... 

Cr(CO)5 interacts with solvent molecules and in solution cannot be considered as naked. The interaction is much weaker with fluorocarbon solvents than hydrocarbon 33). Using a pulsed laser photolysis source (frequency tripled NdYAG) and C7F14 as a solvent, Kelly and Bonneau 33) measured the rate constants for the reaction of Cr(CO)5 with C6H12, CO, and other ligands [Eq. (3)]. 

Note added in typing In a very recent paper (81) Vaida and co-workers have used picosecond laser photolysis to show that, in cyclohexane solution, Cr(CO)5...cyclohexane (Amax 497 nm) is formed within 25 ps of the photolysis of Cr(C0)5 This suggests that, in solution, the primary photoproduct is Cr(C0)5 and that there is essentially no activation energy for the reaction of Cr(C0)5 with the solvent. Clearly, experiments with pulsed KrF lasers on carbonyls in solution and matrix may be very revealing. 

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. 

By using pulsed laser sources and fast measuring devices, direct observation of redox products in flash photolysis experiments provides evidence regarding oxidative and reductive quenching mechanisms in Ru(bpy)3+. 

The photoacoustic calorimetry technique employs photolysis by laser pulses of a mixture containing di-im-butyl peroxide, an appropriate metal hydride, and solvent. Photolysis of the peroxide gives i-BuO radicals that abstract a hydrogen atom from the hydride, and the measured photoacoustic signal is proportional to the overall reaction enthalpy. After calibration,... 

Photolysis and pulse radiolysis are powerful methods for producing sizeable amounts of reactive transients whose physical and chemical properties may be examined. Structural information on the transient and the characterization of early (rapid) steps in an overall reaction can be very helpful for understanding the overall mechanism in a complex reaction. Chemical equilibria may be disturbed by photolysis or radiolysis since one of the components may be most affected by the beam and its concentration thereby changed. The original equilibrium will be reestablished on removing the disturbance and the associated change can be examined just as in the relaxation methods. The approach has been more effectively used in laser photolysis and since very short perturbations are possible the rates associated with very labile equilibria may be measured. 

The second category comprises the flash photolysis experiments using the short high power light pulses from Q-switched lasers, furthermore all investigations of time-dependent behavior of excited dye molecules, which play an important role as active material in dye lasers or as saturable absorbers in passive Q-switched giant pulse lasers. 

A description of a fast laser photolysis experimental arrangement has been given by Porter and Topp who used a 1.5 Joule, 20nsec ruby giant pulse, frequency doubled in ADP, to measure singlet lifetimes in phenantrene, pyrene and other organic molecules. 

Novak and Windsor developed a very elegant and effective method for laser photolysis experiments, using the same laser to produce both pump and analyzing pulses. Their experimental set-up is shown in Fig. 9... 

Fig. 9. Experimental arrangement for laser photolysis, using the frequency-doubled output from a giant-pulse ruby laser as pump pulse and the wavelength continuum from a laser-induced high-temperature gas plasma as analysing pulse. (From Novak, J.R., Windsor, M.W., ref. 15 ))...

From photolysis of methylene blue by ruby-laser giant pulses, Danzinger et al. found that a 0.5 Joule, 30 nsec laser pulse causes almost total conversion of the original molecules into transients, but that the photochemical change is completely reversible. The lifetimes of three transients have been measured as 2, 30 and 140 jusec resp. at a 5.5 x 10 M dye solution. 

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