Atomic absorption resonance

The success of this method depended upon the construction of a very high vacuum shock tube capable of attaining ultimate pressures of 10 to 

10 Torr and outgassing rates of 10 to 5 x 10 Torr min . These conditions permitted the control of impurities to within a few parts per million. An absorption trace is depicted in Fig. 7 for a mixture of 1% in argon producing atoms at 2490°K. 

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Fig. 16. The UV-visible spectra of Ag,jj /Kr mixtures (Ag/Kr = l/10 )at 10-12K (A) After a 30-min irradiation centered at the atomic resonance absorption lines. (B ) The outcome of a 10-min, 423-nm Agj irradiation, showing major decay of the bands associated with Ag, (indicated by arrows) and the appearance of two new bands near 450 nm. (C) The result of a 5-min, 25K bulk thermal annealing period, showing regeneration of the original Ag3 spectrum eind loss of the new band near 445 nm USD.

A more selective method for aggregation of metal atoms in an inert matrix is cryophotoaggregation e.g., atomic Ag is irradiated in an Ar matrix with UV at the atomic resonance absorption of the entrapped Ag atoms, i.e., 315 nm. After ca. 1 h irradiation, the UV spectrum of the matrix shows that the concentration of atomic Ag has decreased and that new absorptions corresponding to Agj and Ag, have appeared. Aggregates up to Ag, can be prepared in this way, and clusters of CU2 and Cu, in a matrix of Ar may be obtained similarly. 

Bromine-atom atomic resonance absorption spectrometry (ARAS) has been applied to measure the thermal decomposition rate constants of CF3Br in Kr over the temperature range 1222-1624 K. The results were found to be consistent with recently published theory. The formation of cyclopent[a]indene and acenaphthylene from alkyl esters of biphenyl-mono- and -di-carboxylic acids has been observed in flash vacuum pyrolyses at 1000-1100 °C. The kinetics and mechanisms of free-radical generation in the ternary system containing styrene epoxide, / -TsOH, and i-PrOH have been examined in both the presence and absence of O2. ... 

Ideally the reactant gas is at rest and isothermal after passage of the reflected shock. This condition can be difficult to achieve in a practical device, especially at higher reactant concentrations and for exothermic reactions. However, measuring techniques, such as atomic resonance absorption spectroscopy (ARAS) allowing very low initial reactant levels, and improved shock tube devices have made it possible to approach the ideal situation. 

The time dependence of the Si atom concentration determined by atomic resonance absorption spectroscopy during shock-wave-induced thermal decomposition of SiH4 was in accord with a stepwise dissociation via SiH2 intermediates293. 

Collisional quenching of Cl(3p , Pi/2) using time-resolved atomic resonance absorption... 

Time-resolved atomic resonance absorption in the VUV used to determine quantum yields for C1(3 P /,) and (3 Py2) the photolysis of CF3CI, CF2CI2, C2F3CI3, and CCI4... 

A) After a 30-tnin irradiation centered at the atomic resonance absorption lines. 

Above 1000 K, the CgHs + O2 C6H5O2 equilibrium will favour CgHs radicals which will then disappear in radical-radical processes or decompose. Stock tube studies [118] using atomic resonance absorption spectrometry to measure [H] provided strong evidence that the phenyl radical... 

Position of the emitting atom Resonance-absorption cross section Mass attenuation coefficients Line width Isotope abundance... 

Ozone. The rate constant for the reaction (1) has been measured over the temperature range 221—629 K by atomic resonance absorption. The valence... 

An Ar + Btj lamp for Br atom resonance absorption should also be feasible. The appreciable excess kinetic energy possessed in this case by the Br 5s excited atoms does not lead to a msuor loss of sensitivity because the lines are already broadmed (>Ai>b at 298 K) by nudear hypcrfinc splitting. - ... 

In the atomic resonance absorption spectrometric (ARAS) adaptation of the methods, atomic species are spectroscopically monitored as a function of time. H [7,9], D- [7,10], O- [7,11], N- [12], Cl- [13] and I-atom [14] reactions have been studied. Beer s law holds if absorbance, (ABS), is kept low, and then (ABS) s -ln(I/Io) (I and Iq are transmitted and incident intensities of the resonance light, respectively) is proportional to the atomic concentration. If the decay of atom A is controlled by a bimolecular reaction, A + R, where R is the stable reactant molecule, then the decay rate is pseudo-first-order provided [R] [A]. Because (ABS) is proportional to [A], observation of (ABS)t is sufficient to determine the decay constant. Values for kbini for each experiment are then determined by dividing the decay constant by [R]. The results from many experiments are usually displayed as Arrhenius plots. If a reaction is pressure dependent, experiments can also be carried by varying total density. Termolecular reactions can therefore be studied. In certain cases, chemical isolation is not possible, and numerical chemical simulations of the... 

Chemiluminescence due to NF(b) and NF(a) was observed when NF2 was introduced into a flow tube through which a stream of ground state 0( P ) or N( S ) atoms in Ar carrier gas from a MW discharge was passed [22]. Kinetic investigations of the NF2 + O (cf. p. 348) and NF2+ N (cf. p. 349) reactions were carried out by monitoring the decrease of the 0 or N concentrations by atomic resonance absorption and atomic resonance fluorescence in a discharge-flow system at 298 K under pseudo-first-order conditions. At initial concentration ratios [NF2]o/[0]q 30, resonance fluorescence yielded a rate constant kf = (1.8 0.9) x 10" cm -molecule" s which was ascribed to the reaction channel... 

Mass spectrometric detection of atomic fluorine in the reaction zone of the N + NF2 reaction was assumed to be consistent with the removal of NF by reaction (1) [2]. The rapid reaction (1) constituted the predominant decay channel for NF radicals formed in the H + NF2 reaction. ki = (7.0 3.5) X10" cm -molecule" s" at 298 K was derived by a computer model of the reaction scheme for the overall H-1-NF2 system (containing excess H2) and by fitting of the H( S) atom and NC S ) atom profiles obtained from atomic resonance absorption in the vacuum UV. Reaction (1) was assumed to proceed by way of a weakly bound N-N-F species which collisionally dissociates into N2 and F the alternative reaction channel 2NF N-i-NF2 was ruled out [3]. No distinction was made between ground- and excited-state NFs. In view of the relatively high molecular concentrations used, fast electronic quenching of the initially (and predominantly) formed NF(a A) was assumed to occur. Reaction (1) was attributed to two ground-state NF(X 2") radicals no evidence for rapid NF(a) -H NF(a) or NF(a) + NF(X) reactions was found [3, 4]. 

A. Lifshitz and M. Frenklach, The reaction between H2 and D2 in a shock tube Study of the atomic VS. molecular mechanism by atomic resonance absorption spectrometry, J. Chem. Phys. 67 ... 

Myerson s investigation employed the ARAS (atomic resonance absorption spectroscopy) technique to monitor the formation of O atoms behind incident shock waves. Mixtures studied were 0.1, 1.0, and 10% NO in Ar temperatures varied from 2600 to 6300 K, although only a portion of this range was suitable for determining the dissociation rate. A five-reaction model was employed to match computed and measured rates of O-atom formation. 

Roth and Just (1977) utilized atomic resonance absorption spectroscopy to monitor O atom formation behind reflected shock waves in N2 0/CH4/Ar mixtures and were able to infer at somewhat lower temperatures (1500 < T < 2250 K) than other workers. Postshock concentrations were below about 1.6 x 10 mol cm and, as in most other studies, no corrections were applied to the inferred second-order rate constant. 

Yumura and Asaba (1981) monitored H-atom formation behind incident shocks using atomic resonance absorption (>l = 121.6 nm) to infer k over... 

Yumura and Asaba (1981) determined the rate constant by monitoring the steady-state level of H atoms behind incident shocks in NH3/Ar mixtures using atomic resonance absorption at 121.6 nm. For the dilute mixtures used (<2000 ppm NH3) the principal H-atom formation reaction was... 

In a third shock tube study of ammonia pyrolysis, Yumura and Asaba (1981) monitored H atoms in dilue NH3/Ar mixtures using atomic resonance absorption and were able to infer k through comparisons with computer simulations. The critical reactions in the simulation were reactions (1), (3), and (4) together with... 

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