Atom Concentrations Atomic Resonance

The copper system appears to behave similarly to the silver system, and it may be used here in order to illustrate the idea of "selective, naked-cluster cryophotochemistry 150,151). A typical series of optical-spectral traces that illustrate these effects for Cu atoms is given in Fig. 15, which shows the absorptions of isolated Cu atoms in the presence of small proportions of Cu2, and traces of Cus molecules. Under these concentration conditions, the outcome of 300-nm, narrow-band photoexcitation of atomic Cu is photoaggregation up to the Cus stage. The growth-decay behavior of the various cluster-absorptions allows unequivocal pinpointing of UV-visible, electronic transitions associated with Cuj and Cus 150). With the distribution of Cui,2,3 shown in Fig. 15, 370-nm, narrow-band excitation of Cu2 can be considered. Immediately apparent from these optical spectra is the growth (—10%) of the Cu atomic-resonance lines. Noticeable also is the concomitant... 

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. 

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 photo-oxidation of n-butane has been modelled by ab initio and DFT computational methods, in which the key role of 1- and 2-butoxyl radicals was confirmed.52 These radicals, formed from the reaction of the corresponding butyl radicals with molecular oxygen, account for the formation of the major oxidation products including hydrocarbons, peroxides, aldehydes, and peroxyaldehydes. The differing behaviour of n-pentane and cyclopentane towards autoignition at 873 K has been found to depend on the relative concentrations of resonance-stabilized radicals in the reaction medium.53 The manganese-mediated oxidation of dihydroanthracene to anthracene has been reported via hydrogen atom abstraction.54 The oxidation reactions of hydrocarbon radicals and their OH adducts are reported.55... 

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. 

The related method of atomic resonance fluorescence—the measurement of intensity of fluorescence excited by absorption of resonance radiation—has several advantages over resonance absorption for kinetic studies of reactions of ground state atoms. When the usual strongly self-reversed microwave discharge lamps are used as the sources of resonance radiation, resonance fluorescence is much more sensitive than resonance absorption. The following lower limits of concentration detectable by resonance fluorescence have been found in this laboratory for particular instrumental conditions [Cl] 5 x 10 g atomcm ... 

Alternative techniques for producing Cl Pj atoms have been described. The reaction of excess O atoms with OCIO has been used to produce known concentrations of Cl Pj atoms for the calibration of C-atom resonance fluorescence intensity as a function of the concentration of C atom. The following reaction stoicheiometry was used ... 

An alternative approach is to produce known concentrations of O, Br, or I atoms, using rapid reactions of established stoicheimnetry. This ai Hoadi is useful, for instance in calibrating Br-atom resonance fluorescence intensity, where the concentration of Br atoms in the system may be varied simply by altering the flow rate of Bt2 added to an excess concentration ( 10 cm ) of O i atoms. The reactions occurring are both fast and give the overall stoicheiometry ... 

Although weak fluorescence in the P— P lines was observed, from a practical standpoint only the P— P lines were found to be intense enough for resonance fluorescence work at low atom concentrations. The fluorescence count rates using the whole of the fully allowed P— P transition of F were typically 2 counts s" at [F] = 1 X 10 cm . These data set a lower concentration limit of 1 X 10 cm" for the smallest detectable concentration of F P atoms. Similar lower limits for O, Br, and I atoms are appreciably less, and are continually being improved by better attention to collimation and detection. Because of the low count rates observed in the F-atom resonance fluorescence studies, it is a better approach to use resonance absorption with a non-reversed line source (see above). 

A series of related reactions of chlorine and bromine atoms with halogen and interhalogen molecules has been investigated by Clyne and Cruse, using atomic resonance fluorescence spectrometry. Use of QNO as a titrant for determination of Cl and Br atom concentrations has been described. The results of these measurements are summarized in Table 8. 

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... 

The production of NF(a) with a branching ratio >0.9 was demonstrated by two independent methods (A) Atomic resonance-fluorescence was used in a discharge flow system to study the concentration profiles of ground state NC S°) and excited state N(2p°) atoms which had... 

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]. 

The method of atomic resonance spectrometry in the vacuum ultraviolet, with detection of either absorption or fluorescence, has become one of the most useful direct methods for the measurements of reaction rates of ground (18)(and metastable excited (19,20)) state atoms. The sensitivity cuid scope of atomic resonance in this respect rivals, and possibly surpasses, that of other methods such as epr and mass spectrometry. Recently, it has been shown that atomic resoneince may be used to measure fluorine atom concentrations in a flow system. The sensitivity for... 

---