Afterglow Kinetics

In the next section of this article we illustrate the complex processes involving excited states formed in atom combination. In the following section the uses of lasers in providing the relevant kinetic and spectroscopic data are outlined. The concluding section contains suggestions for the future development of this field. 

Afterglow Kinetics.—Introduction, Combination of atoms in their ground electronic states can lead to the formation of the corresponding diatomic molecule in an electronically excited state. Radiative transitions to a lower-lying state may then lead to chemiluminescent ranission, known as an after ow. Examples include the combination of two nitrogen atoms, a nitrogen atom with an oxygen atom, and two chlorine or bromine atoms  

A general review of afterglows is available here, we concentrate on halogen afterglows. Information can be deduced from the study of afterglow kinetics concerning radiative, non-radiative, and quenching processes in excited states. 

These excited states of the halogens are 11. and II., the latter ll. state being split into substates 2., 1., Of, and 0 . Transitions to the X L ground state from the. 11(1.) or state, are partially allowed for intermediate 

Hund s case (a)—(c) coupling, have been assigned to the emission bands 

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After this brief introduction to instrumental aspects of laser-induced fluorescence, in the next sections laser excitation studies of the halogens and interhalogens are described to illustrate information available regarding the kinetics of quantum-resolved electronically excited states. From laser studies of this type, particularly lifetime measurements, rate constants for elementary steps of importance in modelling halogen afterglow kinetics are expected to become available. 

Figure A3.5.5. Rate constants for the reaction of Ar with O2 as a fiinction of temperature. CRESU stands for the French translation of reaction kinetics at supersonic conditions, SIFT is selected ion flow tube, FA is flowing afterglow and HTFA is high temperature flowing afterglow.

The associative reaction of oxygen atoms with nitric oxide produces the yellow-green chemiluminescence in the air afterglow, easily seen by the naked eye. The reaction has long been used to measure the concentrations of O atoms in kinetics experiments [49-51] and is so bright that it has been used to visualize... 

A modification of the conventional flowing afterglow apparatus, in which a drift section is incorporated, is shown schematically in Fig. 6.46i-141 In the so-called flow-drift apparatus reactant ions are produced in the upstream section just as in the conventional afterglow system, but the downstream section, where reactions with neutrals occur, is a drift tube, in which a uniform electric drift field is applied. In the latter section ions can be accelerated from thermal kinetic energies to several electron volts. The two sections of the apparatus are separated by an electronic ion shutter, which makes it possible to admit narrow pulses of ions into the drift region at specified times. This permits measurements of ion-drift velocity and, in... 

Gas-phase acid-base studies are usually performed by using one of the following techniques high-pressure mass spectrometry (HPMS), chemical ionization mass spectroscopy (CIMS) with mass-analysed ion kinetic energy spectroscopy/collision induced dissociation (MIKES/CID), flowing afterglow (FA) or ion cyclotron resonance (ICR) spectrometry. For a brief description of all methods, Reference 8 should be consulted. 

A decade ago, while considerable data had been compiled on the kinetic measurement of dissociative recombination (DR) reactions of small polyatomic ions, laboratory information on the product distributions of such reactions was restricted to the results of a few merged-beam and stationary-afterglow studies on DR of C02 and of H( [157,158], and the first explorations of combined flow tube/Langmuir probe/spectroscopic detector techniques, independently pursued by Rowe and co-workers (at Rennes) [159,160] and by Adams and co-work-ers (at Birmingham, and subsequently Atlanta) [161, 162]. Considerable advances have since been made, both in measurement of recombination coefficients (particularly for larger ions) and in the elucidation of product distributions for a still small but growing sample of important IS ions. 

The spectrum of the red luminescence has a complex stmcture. We were able to separate three components of red luminescence that each have their own kinetics and that differ by the character of excitation. The radiation band at 7300-7800 A is predominantly excited by light with A,i = 3700-4800 A and has a three - exponential time dependence of afterglow with time constants of 3 msec, 15 msec, and 75 msec. The relatively strong emission line at 7175 A has maxima in the excitation spectrum at 3700 and 5200 A. It is characterized by an afterglow time constant of x = 33 psec. Several narrow lines in the 6900-7200 A region are well excited with A,=4000-4800 A and are characterized by Xi = 3 and %2 = 22 msec. 

A further observation of Linnett et al. [511], which has also since become of interest in connection with kinetic modelling of the system, is that afterglows following explosions occurred in the alumina coated vessel, lasting sometimes as long as twenty seconds. 

Core-collapse supemovae with kinetic energy of the ejecta 10 — 30 times as high as the standard 1 foe (lfoe = 1051 erg) are now collectively called hypemovae . The term was introduced by B. Paczynski shortly after the discovery of first GRB afterglows in 1997 by the Beppo-SAX satellite [114] based on qualitative analysis of possible evolutionary ways leading to cosmic GRB explosions. 

The complementary techniques for determining rate constants for thermal electron attachment, detachment, and dissociation are the flowing afterglow, the microwave technique, the ion cyclotron resonance procedures, the swarm and beam procedures, the shock tube techniques, the detailed balancing procedures, the measurement of ion formation and decay, and the high-pressure mass spectrometer procedures. In all cases the measurement of an ion or electron concentration is made as a function of time so that kinetic information is obtained. In the determination of lifetimes for ions, a limiting value of the ion decay rate or k is obtained. 

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