Radiolysis, optical pulse

The spectral and kinetic properties of the ascorbate free radical have been studied extensively by optical pulse radiolysis experiments (21-24,26). When dilute aqueous solutions are exposed to ionizing radiation, essentially all of the energy is absorbed by the water yielding (28) ... 

Early work (21,22) on the absorption spectrum of the ascorbate radical failed to take into account the complex nature of the reaction between ascorbate and the OH radical, which was later shown by ESR studies (18,19) and by optical pulse radiolysis using very short pulses (23). 

Figure 5. Normalised ESR signal (o) and [NzHi ] at equilibrium as obtained fi om optical pulse radiolysis (A). [Reproduced fi om reference [52] with permission.]...

Asmus et al. unambiguously identified a variety of [R2S.. SR2] radical cations in solution and measured their optical absorption spectra using pulse radiolysis techniques [133]. They proposed that the spectrum of [H2S. .SH2] arises from the transition in the three-electron S.. S... 

It is pertinent that S20g accepts an electron generated by pulse radiolysis of water to give optically detectable S04 within 1.5 x 10 sec . ... 

The optical absorption spectra of sulfonyl radicals have been measured by using modulation spectroscopy s, flash photolysis and pulse radiolysis s techniques. These spectra show broad absorption bands in the 280-600 nm region, with well-defined maxima at ca. 340 nm. All the available data are summarized in Table 3. Multiple Scattering X, calculations s successfully reproduce the experimental UV-visible spectra of MeSO 2 and PhSO 2 radicals, indicating that the most important transition observed in this region is due to transfer of electrons from the lone pair orbitals of the oxygen atoms to... 

Flash Photolysis and Pulse Radiolysis Studies Non-linear Optical Effects 142... 

The radical cations of urazole-annelated azoalkanes 65 were generated by pulse radiolysis and the transients characterized spectrally and kinetically by time-resolved optical monitoring. The initial distonic 1,3 radical cations 66 were detected, and the methyl-substituted 66 further deprotonates to radical 67 (Scheme 1) <1997JA10673>. 

X = 0, CH2, CHCOOH, C(COOH)2, NH, NCH3 N(CH2CH=CH2), N(CHs)2 Cl Bobrowski and Das published a series of papers on the transients in the pulse radiolysis of retinyl polyenes31-37, due to their importance in a variety of biomolecular processes. They studied32 the kinetics and mechanisms of protonation reaction. The protons were released by pulse radiolysis, on a nanosecond time scale, of 2-propanol air-saturated solutions containing, in addition to the retinyl polyenes, also 0.5 M acetone and 0.2 M CCI4. Within less than 300 ns, the electron beam pulse results in formation of HC1. The protonated products of retinyl polyenes were found to absorb optically with Xmax at the range of 475-585 nm and were measured by this absorption. They found that the protonation rate constants of polyene s Schiff bases depend on the polyene chain... 

Pulse radiolysis, using as time-resolved detection methods optical absorption, luminescence, electrical conductivity or electron spin resonance can be expected to give information on the formation of transient or permanent radiation products and on their movement. 

Henglein A, Tausch-Treml R (1981) Optical absorption and catalytic activity of subcolloidal and colloidal silver in aqueous solution a pulse radiolysis study. J Colloid Interface Sci 80 84-93... 

V. Jagannadham and S. Steenken, J. Amer. Chem. Soc. 106, 6542 (1984). The reaction of RCHOH, generated by pulse radiolysis, was studied with p-substituted nitrobenzenes using time-resolved optical and conductance detection. The radical anion of the nitrobenzene is produced directly and indirectly. 

On the basis of results obtained from (in-situ-radiolysis [19]) electron spin resonance [16, 20] and pulse radiolysis with optical and conductance detection [19], a-alkoxyalkyl radicals react in aqueous solution exclusively via addition to give alkoxynitroxyl radicals (cf. Eq. 10). This is in contrast to the reactions of CHaCH OH (see Sect. 2.1.1) and 5,6-dihydropyrimidine-6-methyl-6-yl radicals (see Sect. 2.1.3), where addition and redox products are formed. 

The solute benzene radical cation was formed on pulse radiolysis of an acidic aqueous solution of benzene. The transient optical absorption bands (A-max = 310, 350-500 nm) were assigned to the solute benzene radical cation which is formed on acid-catalysed dehydration of the OH adduct. The radical cation is able to undergo an electron-transfer reaction with Br and was found to be a strong electron oxidant. Pulse radiolysis has been used to study the complex reaction that follows electron addition to hydroxybenzophenones (HOBPs). The various radical species involved have been characterized spectrally and their p/fa values evaluated. The differences... 

Figure 8 The time-dependent behavior of the hydrated electron obtained in the subpicosecond pulse radiolysis of neat water using 2-mm optical path sample cell, monitored at the wavelength of 780 nm.

The subpicosecond pulse radiolysis [74,77] detects the optical absorption of short-lived intermediates in the time region of subpicoseconds by using a so-called stroboscopic technique as described in Sec. 10.2.2 ( History of Picosecond and Subpicosecosecond Pulse Radiolysis ). The short-lived intermediates produced in a sample by an electron pulse are detected by measuring the optical absorption using a very short probe light (a femtosecond laser in our system). The time profile of the optical absorption can be obtained by changing the delay between the electron pulse and the probe light. 

The experiments were carried out using the subpicosecond pulse radiolysis system [77] described in Sec. 10.2.2 ( Subpicosecond Pulse Radiolysis ). Considering the signal intensity and the degradation of the time resolution, a sample cell with the optical length of 2 mm was mostly used. The sample was saturated by Ar gas to eliminate the scavenging effect by the remaining O2 gas. 

In a pulse-radiolysis optical-absorption method, the value of kje, where 8 is a molar absorption coefficient, is measured by the time-resolved measurement of the optical absorption of solvated electrons, and then the kr value is determined by the observed value of kr/s and the value of s known separately. 

Shortly after the discovery of the hydrated electron. Hart and Boag [7] developed the method of pulse radiolysis, which enabled them to make the first direct observation of this species by optical spectroscopy. In the 1960s, pulse radiolysis facilities became quite widely available and attention was focussed on the measurement of the rate constants of reactions that were expected to take place in the spurs. Armed with this information, Schwarz [8] reported in 1969 the first detailed spur-diffusion model for water to make the link between the yields of the products in reaction (7) at ca. 10 sec and those present initially in the spurs at ca. 10 sec. This time scale was then only partially accessible experimentally, down to ca. 10 ° sec, by using high concentrations of scavengers (up to ca. 1 mol dm ) to capture the radicals in the spurs. From then on, advancements were made in the time resolution of pulse radiolysis equipment from microseconds (10 sec) to picoseconds (10 sec), which permitted spur processes to be measured by direct observation. Simultaneously, the increase in computational power has enabled more sophisticated models of the radiation chemistry of water to be developed and tested against the experimental data. 

The first pulse radiolysis experiments to measure G°(e ) directly were made in the 1970s, with reported values of 4.0 0.2 molecules (100 eV) at 30 psec [45] and 4.1 0.1 molecules (100 eV) at > 200 psec [46]. The latter value was subsequently revised to 4.6 0.2 molecules (100 eV) at 100 psec [47], and later a yield of 4.8 0.3 molecules (100 eV) at 30 psec was reported by Sumiyoshi et al. [48]. The method of evaluating G(e ) at these short times is either to use dosimetry [45,46] and the molar absorption coefficient of e, or to compare the optical absorbance at short times with that observed at 10 -10 sec and take G(e q) = 2.7 molecules (100 eV) at this time [48]. The causes of the discrepancies between these pulse radiolysis values have been reviewed recently by Bartels et al. [49], who have also made new measurements of the spur decay of e. ... 

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