Pulsed photolysis/laser-induced

Kinetic study of OH reactions with H2O2, C3H8 and CH4 using the pulsed photolysis laser induced fluorescence method,... 

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)... 

Fig. 2. Schematic description of the pulsed laser photolysis/laser-induced fluorescence (LP/LIF) pump-probe method for H-I-O2 chemical dynamics studies. TVanslationally energetic H atoms are generated by pulsed laser photolysis of appropriate precursor molecules HX and the nascent 0( Pj 2 i o) atoms produced in the reaction H -I- O2 are detected under collision free conditions via LIF.

In the present work, a new kinetics configuration utilizing a pulsed laser for photolysis and a quasi-cw, ultraviolet laser for fluorescence excitation has been developed. This technique combines the best features of the two kinetic methods mentioned above. Laser photolysis generally permits greater reactant formation specificity than does flashlamp photolysis. Laser-induced fluorescence detection outperforms resonance fluorescence detection because of its increased fluorescence excitation flux, decreased scattered light signal, and wavelength tunability. Cw fluorescence excitation is desirable over pulsed fluorescence excitation due to its freedom from pulse-to-pulse normalization constraints and, most importantly, because of its efficient duty cycle and the consequent increased density of points obtainable... 

Scheme of the combined technique laser photolysis/laser-induced fluorescence I, laser for photolysis 2, laser for excitation of fluorescence 3, measuring cell 4, detection system of fluorescence 5, mirror 6, controlling unit 7, unit for time delay between pulses of lasers I and 2. 

Pulsed electronic transition chemical laser operation appears to be possible in a much wider class of reaction systems than those defined by the foregoing considerations. Reaction initiation in premixed reagents by pulsed techniques can provide population inversions on time scales short compared with fluid mixing times and relaxation times for lower laser states. Such techniques include photolysis, laser-induced dissociation, electrical discharge production of atoms or metastables, and shock-induced dissociation and pyrolysis. Such lasers are subject to the pumping rate condition of Figure 3.21 which is much less restrictive than the condition for diffusive mixing... 

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. 

As noted above, O Donnell et al. (2004) studied the reaction of a series of alkoxy esters with OH, using both pulsed laser photolysis-laser-induced fluorescence (263-372 K) and a relative rate method (298 K only). Agreement between the relative and absolute rate data at 298 K is generally very good, around 10% on average and always better than 20%. The compounds possess 298 K rate coefficients that range from about 1.7 x 10 ... 

A number of higher diketones have been studied by Dagaut et al. (1988b) using flash photolysis coupled with resonance fluorescence, by Holloway et al. (2003), using both pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) and relative rate techniques, and by Zhou et al. (2008) using a relative rate method. 

Mellouki and Mu (2003) measured A OH + pyruvic acid) over a range of temperatures (273-371 K) using pulsed laser photolysis-laser-induced fluorescence. The data are summarized in table VI-E-1 and figure VI-E-1, and can be described by the simple Arrhenius expression k = 4.9 x 10 exp(276/7) cm molecule" s", with a rate coefficient of 1.2 x 10 cm molecule" s" at 298 K. Given the fact that only one measurement has been made, an uncertainty of a factor of 30% is assigned. 

The rate coefficients of Cox et al. (1976), Jenkin and Cox (1987) and Burkholder et al. (1992) are given in table VIII-I-1 and shown in figure VIII-I-1. Cox et al. (1976) measured their value relative to fc(OH + Hg). Jenkin and Cox (1987) and Burkholder et al. (1992) determined the rate coefficient as function of temperature using, respectively, molecular modulation combined with time-resolved absorption and pulsed laser photolysis-laser-induced fluorescence techniques. While the different ambient temperature measurements are in reasonable agreement, there are significant differences with respect to the temperature dependence between Jenkin and Cox (1987) and Burkholder et al. (1992). The preferred value at 298 K is an average of the three determinations. 

The occurrence of process (II) in the photodecomposition of CH3NO2 at 248 and 266 nm was demonstrated by Park et al. (2001) using pulsed laser photolysis-laser-induced fluorescence. Nascent 0( P) photofragments were detected via LIF under collision-free conditions. From Doppler profiles of the 0-atom, the fraction of the... 

Mellouki, A., S. Teton, G. Laverdet, A. QuUgars, and G. Le Bras (1994), Kinetic studies of OH reactions with H2O2, C3H8 and CH4 using the pulsed laser photolysis-laser induced fluorscence method, J. Chim. Phys., 91, 473-487. 

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 ))...

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)... 

Experimental Techniques A absorption CIMS = chemical ionization mass spectroscopy CK = competitive kinetics DF discharge flow EPR = electron paramagnetic resonance FP = flash photolysis FT = flow tube FTIR Fourier transform intra-red GC = gas chromatography, UF = laser induced fluorescence LMR = laser magnetic resonance MS = mass spectroscopy PLP = pulsed laser photolysis SC = smog chamber SP = steady (continuous) photolysis UVF = ultraviolet flourescence spectroscopy... 

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. 

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. 

While it is difficult to ensure the rehable observation of the radical processes during the mixing of reagents, these are easily detected in the photoinduced reactions of aryl-substituted compounds of Ge and Sn. Laser pulse photolysis experiments show that direct photoionization of Ph3Ge(Sn) anion results in the neutral radical . The apphcation of the Chemically Induced Dynamic Electron Polarization (CIDEP) method has allowed the detection of polarized emission signal of the radicals, thus leading to a conclusion that direct photoionization of Ph3Ge(Sn) anion occurs from the triplet state (equation 7). ... 

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... 

Gilles and coworkers [37], via pulsed laser photolysis followed by laser induced fluorescence, found the temperature-dependent rate constant for the OH reaction at 50 torr pressure to be 6.6 x cm molecule s for... 

Fig. 3. Doppler profiles for the three fine-structure states, j = 2, 1, 0, of 0( P) atoms formed in the reaction H-I-O2. TVanslationally excited H atoms with an average collision energy of Ec.m. = 10 eV were generated by pulsed laser photolysis of 50 mTorr H2S at 248 nm in the presence of 100 mTorr of O2. The 0( P) atom reaction products were detected 180 ns after the photolysis laser pulse by pulsed vacuum-UV laser-induced fluorescence.

In this chapter, we summarize recent progress in the photochemistry of shortlived species by use of multibeam excitation, including our recent achievements in this field. Our research group has employed various multibeam irradiation methods to reveal reaction processes of various excited intermediates, including basic molecules and biomolecules. We also achieved direct observation of shortlived species utilizing ultrashort pulse lasers. These results are interesting recent examples of reactions induced by multibeam irradiation. Based on the excitation method, this chapter is divided into the following sections Pulse Radiolysis-Laser Flash Photolysis (Section II), Two-Color Two-Laser Flash Photolysis (Section III), and Three-Color Three-Laser Flash Photolysis (Section IV). Each section is further divided into subsections based on the topics. 

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... 

Fig. 11 Laser-induced fluorescence spectra recorded from Napl-doped PMMA samples (1.2 wt%) after their irradiation with a single pump pulse at 248 nm at laser fluences below a, and above b the corresponding ablation thresholds. For comparison purposes, the spectra have been scaled. The figure also illustrates the approximate deconvolution of the probe spectrum into the emission bands of the suggested species (the Nap2 spectrum is recorded in the photolysis of high-concentration NapI solution, while the NapH/PMMA spectrum is recorded from PMMA doped with 0.08 wt% NapH)...

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