Pulsed photolysis/laser-induced fluorescence

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. 

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... 

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. 

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.

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)...

Rate constants for reaction of the CH radical with a number of atomic and molecular collision partners have been reported, with multiple-photon dissociation of suitable precursor molecules using either infrared or ultraviolet " laser radiation used as the pulsed photolysis source, and laser-induced fluorescence near 431 nm employed as a sensitive time-resolved detection method. A similar technique has been used to measure removal rates of CH2 and CDj with... 

Laser-induced fluorescence measurements of atmospheric OH have been carried out now for several years, and shown to be capable of detecting extremely low concentrations of the radical. It has been pointed out however that interference from laser-generated OH could affect the results considerably " the wavelength used for OH excitation, 282 nm, generates 0( Z)) from O3 photolysis, and this reacts with H2O to form OH in the troposphere in a time ( 1 ns) which is shorter than the laser pulse width. Calculations and experimental assessments of the importance of this effect have been described. The reaction of OH with CS2... 

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). 

PHa + Oj- Products. The rate constants k were measured at 300 K by flash photolysis (FP) of PH3/O2/N2 mixtures (0.35 3.5 to 17 250 to 270 Torr) and PH3/O2 mixtures (0.35 1.5 Torr), above the upper and below the lower explosion limit, followed by absorption of radiation near 455 nm (from a pulsed dye laser) by PH2. k = (1.2 0.3)x10" at a total pressure of -270 Torr (i.e., above the upper explosion limit) was derived from the dependence of the effective rate constant k ff=d ln([PH2]o/[PH2])/dt on the O2 partial pressure, k = (0.8 0.2) x10" at 1.85 Torr (i.e., below the lower explosion limit) was obtained from a computer calculation of keff with known rate constants for PH3 + H PH2 + H2 (see p. 233) and 2PH2+M P2H4+M (see below). The agreement of both k values points to a pressure-independent reaction PH2 + 02 products. The step PH2 + 02 P02 + H2 is probably responsible for the observed rapid decay of PH2 [3, 4]. A later measurement of the removal of rotationally thermalized PH2 (X Bi, v = 0) in the presence of O2 by laser-induced fluorescence (LIF) under pseudo-first-order conditions gave k = 2.7x10 . The removal of vibrationally excited PH2 by O2 was also investigated [5]. 

No attempt will be made here to provide a detailed description of the kinetic experiments at low and ultra-low temperatures. In both cases, pulsed laser photolysis (PLP) is used to generate free radicals (CN, OH, CH) from a suitable precursor (e.g. NCNO or ICN, HNO3 or H2O2, CHBr3) and laser-induced fluorescence (LIF) is used to observe them. The LIF signal decays as the time delay between the pulses from the photolysis and probe lasers is increased, providing information about the rate of loss of radicals by reaction. 

In most experiments, ultraviolet or infrared absorption, resonance fluorescence, or laser-induced fluorescence (LIF) is used to follow how transient concentrations change after the photolysis pulse. These optical techniques vary considerably in their sensitivity and hence to the extent to which they isolate the primary reaction. LIF is extremely sensitive, enabling one to follow decays of concentrations from an initial value of 10 ° cm , but its use is restricted to species with a bound-bound electronic transition within the range of tunable dye lasers. LIF has been used to follow the kinetics of reactions of, inter alia, the radicals OH [12-14], CN [15] and CH3O [16,17]. It is more difficult to apply to radical atoms vihich usually have allowed electronic transitions only in the vacuum ultraviolet. Some LIF measurements utilising two-photon excitation of atoms have been reported [18]. 

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