Pulsed laser photolysis measurement

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)]. 

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. 

Icqjj = 18.0 X 10" cm molecule" S" by relative rate method Icqjj = 16.7 x lO cm molecule s by pulse laser photolysis-laser induced fluorescence and atmospheric lifetime calculated to be 16 h at 298 2 K measured range 263-372 K (Moriarty et al. 2003)... 

Although such a 2-MHz ESR apparatus was very sophisticated, its time resolution was not enough for measurement of CIDEP. In 1973, Fessenden [5] found that the direct ESR measurement without field modulation improved the time resolution, observing CIDEP signals in solution with pulse radiolysis. This method was applied for laser-photolysis measurements in solids [6] and in solution [7]. A spin-echo ESR technique was also found to be useful for CIDEP [8]. Since then, CIDEP experiments with cw-ESR and pulsed-ESR spectrometers without field modulation have become much more popular than before. Through such transient ESR measurements, CIDEP due to not only the radical pair mechanism but also several other mechanisms have been observed in many chemical reactions including biologically important ones such as photosynthesis reactions. In this chapter, we will show several mechanisms for CIDEP with several typical examples. 

The author s group [12] tried to find saturation behavior of the MFEs due to the AgM in fluid solutions with our pulsed magnet and found that the MFEs on the escape radical yield (1e(B)) observed for the photoreduction of 4-methoxybenzophenone with thiophenol (Reaction S-5 in Table 7-2) were almost saturated by the fields of -30 T. The isotropic g-values of the thiyl and ketyl radicals have been determined to 2.0082 and 2.0027 so Ag=0.0055 [12]. From ns-laser photolysis measurements with our electromagnet, superconducting magnet, and pulsed magnet, we observed the time profiles of the transient absorption (A(f) curves) of the ketyl radical and obtained the MFEs (A(B)=Ye(B)/1e(0 T)) on the )4eld. The R(B) values obtained at room temperature in 2-methyl-1-propanol are plotted... 

After the application of the microwave pulse at r = to. Curve a of Fig. 14-10 was found to be changed to Curve b of Fig. 14-10. The b-a difference is illustrated by Curve d in Fig. 14-12(a). This curve shows that a-b value gradually increases immediately after the microwave pulse irradiation and that the value slowly decreases after its maximum at t=/Mi- The initial increase corresponds to the disappearance of [M] with its rate constant (kp) given by Eq. (14-8a). In principle, this rate constant for radical pairs generated by the photoreduction of carbonyl and quinone molecules in micelles can be obtained from their A(t) profiles with ns-laser photolysis measurements. In the actual measurements, however, this rate constant has never been obtained from their A(t) profiles because there are many components due to other species such as triplet precursors and reaction products in the profiles. From the present ODESR measurement, Sakaguchi et al. could purely generate the mixed M state with the microwave pulse and succeeded in direct measurement of the kp value for the first time. From the decay of the a-b value, the kj value can be obtained. They, therefore, could determine the kp value for the first time from their ODESR method. 

Chagovetz and Grissom [10] also made laser-photolysis measurements of AdoCbl" in 75%(v/v) glycerol in water (77/770 = 30) and water (77/770 = 1) at 20°C with a ps-laser. Photolysis at 532 nm was accomplished with a frequency-doubled NdiYag mode-locked laser with a pulse width of 30 ps. Cob(II)alamin formation occurred within the 30-ps photolyzing... 

The T dependence of (ib) was unexpected as it implies that there are strong attractive forces acting between 02(A ) and I at relatively large internuclear separations. To explore this phenomena in more detail, and to test the reliability of Eqs. (20) and (21), we examined reaction (lb) at a temperature of 150 K. ° These measurements were carried out using a Laval nozzle to provide low temperature gas flows. Traces of I2 were entrained in He/02 mixtures, and pulsed laser photolysis was used to generate I by exciting just above the 12(B) dissociation limit. Near threshold photodissociation was used to ensure that I was produced with a translational temperature in equilibrium with the local conditions. I decay kinetics were... 

The rate eonstants of the reaetions of OH with the vinyl ethers have been measured in the temperature and pressure ranges 230-373 K and 30-300 Torr using the pulsed laser photolysis-laser indueed fluoreseenee (PLP - LIF) method and also the relative kinetie method at 760 Torr and 298 K. The measured rate eoeffieients in the temperature range 230-372 K are shown in the eonventional Arrhenius form (k = in Figure 1. The data... 

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... 

Recently, Houston and Moore (486) have measured the CO production rate following the pulsed laser photolysis of HjCO and DjCO at 3371 A. They found that at the low pressure limit, the CO rate of production is more than 100 times slower than the fluorescence decay rate. They suggest that CO is not produced from the initially formed fluorescing state S, by light absorption but rather from an intermediate state I. The intermediate state I, cither the or the vibrationally excited ground state, is formed from S, either by collisions or by a spontaneous decay process. The I state dissociates into Hj + CO to a small extent by a slow spontaneous process (>4 /jscc) but to a large extent by collisions with each other or with NO and O2 molecules. The quantum yield of CO production at 3371 A is independent of formaldehyde pressure in the range 0.1 to 10 torr. 

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). 

The reaction has been studied experimentally at temperatures up to 3500 K because of its importance in combustion. Sims et al. have measured the overall rate coefficient using conventional pulsed laser photolysis [61] and the CRESU technique [62, 63], both with LIF detectiOTi of CN, over the temperature range 13-761 K, obtaining k = 2.5 x 10 (77298 The channel branching ratio... 

Jimenez at al. (2005b) measured rate coefficients as a function of temperature at 263-354 K using pulsed laser photolysis combined with pulsed laser induced fluorescence. The results from Jimenez at al. (2005b) are presented in table H-B-31 and shown in figure ll-B-21. An Arrhenius fit to the data gives k = 2.0 x 10" exp(160/r)cm molecule" s". This expression gives k = 3.4 x 10" cm molecule" s" at 298 K with an uncertainty estimated to be 30%. 

The only measurement of the rate coefficient for reaction of Cl with ETBE is that of Notario et al. (2000b), obtained via pulsed laser photolysis-resonance fluorescence, see table III-B-23. Their value of k with an estimated uncertainty of 20%. 

The rate coefficient for reaction of OH with 2-isopropoxyethanol has been measured by Porter et al. (1997), using both a relative rate and pulsed laser photolysis-laser-induced fluorescence technique. An average value of 2.1 x 10 cm molecule s was reported at 298 K see table ni-C-14. In the absence of corroborative measurements, an uncertainty of 25% is estimated. 

Thiault and Mellouki (2006) have measured the rate coefficient for reaction of OH with these two species using both absolute and relative rate techniques, see table ni-D-12. Rate coefficients for both species are found to be 1.1 x 10 cm molecule" s at 298 K, consistent with the data shown in the previous sections for other vinyl ethers. The pulsed laser photolysis-LIF data from Thiault and Mellouki lead to k= l.6x 10 exp(567/T) cm molecule" s and k = IJ x 10 x exp(549/r) cm molecule" s", respectively, for isobutyl and tert-butyl vinyl ethers, respectively. The uncertainty on the 298 K rate coefficient is estimated to be 15%. 

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