First-order decay constant

Table B2.5.5. The photochemical decomposition of methyl radicals (UV excitation at 216 nm). ris tire wavenumber linewidth of the methyl radical absorption and /ris the effective first-order decay constant [54].

Radioactivity is the spontaneous emission of radiation from an unstable nucleus. Alpha (a) radiation consists of helium nuclei, small particles containing two protons and two neutrons (fHe). Beta (p) radiation consists of electrons ( e), and gamma (y) radiation consists of high-energy photons that have no mass. Positron emission is the conversion of a proton in the nucleus into a neutron plus an ejected positron, e or /3+, a particle that has the same mass as an electron but an opposite charge. Electron capture is the capture of an inner-shell electron by a proton in the nucleus. The process is accompanied by the emission of y rays and results in the conversion of a proton in the nucleus into a neutron. Every element in the periodic table has at least one radioactive isotope, or radioisotope. Radioactive decay is characterized kinetically by a first-order decay constant and by a half-life, h/2, the time required for the... 

Berner (1980) first introduced the following concept of a one-G model for determining first-order decay constants (k) of organic matter decomposition ... 

While the unimolecular chemiluminescence of dioxetanones appears to fall easily within the framework of conventional dioxetane chemiluminescence, the chemiluminescence of dioxetanones in the presence of certain fluorescers falls resoundingly outside that framework. Adam et al. (1974) noted that the addition of rubrene to solutions of dimethyldioxetanone gave a yield of light twenty times that obtained when an equivalent concentration of 9,10-diphenylanthracene was added. Importantly, the apparent dissimilarity between rubrene and diphenylanthracene is inexplicable by any conventional mechanism of dioxetane decomposition. Also, significantly, Adam et al. (1974) observed an increase in the first-order decay constant of the dioxetanone with the addition of rubrene, an observation for which they did not offer an explanation. Sawaki and Ogata (1977) also observed an unusual dependence of the chemiluminescence yield on the identity of added fluorescer in the base-catalyzed decomposition of or-hydroperoxyesters, for which a dioxetanone intermediate was proposed (25). 

Pig. 4.— Variation of the observed first order decay constants with partial pressure of NOj. The gas mixtures contained 2.5 Torr Ha and wwe made up to a total pressure of 30 Torr by adding He. 

Fig. 7.—Variation of the observed first order decay constants partial pressure of NO. The gas mixtures contained O, —0.125 Torr NO2+2.5 Torr H2 , —0.20 Ton NO2+2.5 Torr H2 , —0.125 Torr NO2 + I5 Torr H2. In all cases the total pressure was made up to 30 Torr by...

Next, estimate the first-order decay constant. From Fig. 2-11, the particle diffusion coefficient in water for a 1 -/xm particle is approximately 5 X 10-9... 

From Table 2-11, an approximate half-life for B[a]p due to direct photodegradation is 1 hr. From Eq. [1-20], the corresponding first-order decay constant is 0.69/hr. Then using Eq. [1-19] ... 

Calculate the first-order decay constant and the half-life of the reaction. 

First order decay constants for the longest lived species are given in Tables I and II. Rate constants for the first component of the two component curves cannot be calculated from available data. 

If weakly-absorbing transients not detectable by optical filter tests are formed and react in our system, they should be detectable by their subsequent reactions with Br2. However, in C02-free seawater Br2 showed a clean self-decay reaction and negligible first-order decay constants (Table VI) as summarized for the second-order component in Figure 2. 

Fig. 2.3. Upper panel First-order decay of the LIF signal from C2(a n.u) in the presence of 1.6 X 10 molecules cm of NO at 145 K in N2, fitted to a single-exponential decay, with residual shown below. The abscissa corresponds to the deiay time between the photolysis and probe laser pulses. Lower panel First-order decay constants for C2(a II ) at 145K in N2 plotted against the concentration of NO.

We are, of course, familiar with the division of the pseudo-first order decay constant k into individual contributions, based upon a simple scheme such as that below,... 

In the case of washout of sulfur dioxide (SO2), a precursor of acid rain, the high solubility and the chemical reactivity of aqueous SO2 result in nonattainment of equilibrium. Thus, semiempirical models have been proposed for the SO2 concentration in air beneath the rain-forming cloud. These models lump together all processes affecting SO2 removal from air (i.e., dissolution into water droplets, hydration, oxidation, and ionization). A first-order decay constant for the SO2 concentration. A, varies with the rainfall characteristics (rainfall rate and size of raindrops). Boubel et al. (1994) suggest a value of A equal to... 

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