Electron pulse irradiation

The pulse radiolysis studies of liquid alkanes have relevance to the radiolysis of polyethylene and related polymers. In liquid alkanes at ambient temperature, the reaction intermediates such as alkane radical-cations, olefin radical-cations, olefine dimer-cations, excited states, and alkyl radicals have been observed after the electron-pulse irradiation [90-93]. According to the nanosecond and subnanosecond studies by Tagawa et al., the observed species were alkane radical cations, excited states, and alkyl radicals in n-dodecane excited states and cyclohexyl radical were observed in cyclohexane, and only radicals in neopentane [91, 93]. Olefin radical-cations were also detected in cyclohexane containing carbon tetrachloride [92],... 

The alkane radical-cations generated in electron-pulse irradiated n-dodecane show an absorption band in the visible with its maximum at 800 nm (Fig. 15) [93], The position of the absorption maximum changed from 600 nm to 900 nm depending on the carbon number of the alkane. It was noted that the lifetime of alkane-radical cations was shorter than that of the solvated electrons observed in the near infrared region. These phenomena were interpreted in terms of the following ion-molecular reaction. 

A relatively sharp absorption in the UV region due to alkyl radicals is observed in electron-pulse irradiated alkanes [93]. It has an absorption maximum at 240 nm in n-dodecane and cyclohexane. (Mehnart et al. did not see this absorption maximum but found another short-lived absorption band peaking at 270 nm in n-hexane, n-heptane, and n-hexadecane containing 10 mmol dm-3 CC14. This absorption band was assigned to olefin monomer radical-cation... 

Ethylene-propylene copolymer films gave a very broad absorption in the visible region upon electron-pulse irradiation (Fig. 16) [93], It was comprised of at least three species, electrons, excited states, and alkane radical cations. At about 700 nm and 800 nm the contributions from excited states and radical cations, respectively, were largest. The lifetime of the radical cation determined... 

Mehnert R, Brede O, Naumann W. (1984) spectral properties and kinetics of cationic transients generated in electron pulse irradiated C7- to Cj -alkanes. Ber Buns enges Phys Chem 88 71-78. 

Bensasson R, Land EJ and Maudinas B (1976) Triplet states of carotenoids from photosynthetic bacteria studied by nanosecond ultraviolet and electron pulse irradiation. Photochem Photobiol 23 189-193... 

FIGURE 16.4 STM images of a 6 ML nanocrystal of Cso molecules subjected to an electron pulse irradiation (30 V and 60 nC) (a) 5 min and (b) 16 min after the irradiation. Mobile Cgo agglomerates form an ordered island in the irradiated region. (From Bolotov, L. and Kanayama, T., Low-energy electron irradiation of fullerene films formed on Si(lll)-(7x7) surfaces, Appl. Phys. Lett., 81, 1684-1686, Copyright 2002, with permission from American Institute of Physics.)... 

As a heavy metal azide, it is considerably endothermic (A// +279.5 kJ/mol, 1.86 kJ/g). While pine silver azide explodes at 340°C [1], the presence of impurities may cause explosion at 270° C. It is also impact-sensitive and explosions are usually violent [2], Its use as a detonator has been proposed. Application of an electric field to crystals of the azide will detonate them, at down to — 100°C [3], and it may be initiated by irradiation with electron pulses of nanosecond duration [4], See other catalytic impurity incidents, irradiation decomposition... 

The hydrated electron reacts with H202 with a diffusion-controlled rate (see Table 6.6), giving OH and OH-. An intermediate product of this reaction, H202, may be responsible for prolonged conductivity in pulse-irradiated water. The rate of this reaction is consistent with rates of similar one-electron reduction reactions of H202. 

Rzad et al.( 1970) compared the consequences of the lifetime distribution obtained by ILT method (Eq. 7.27) with the experiment of Thomas et al. (1968) for the decay of biphenylide ion (10-800 ns) after a 10-ns pulse-irradiation of 0.1 M biphenyl solution of cyclohexane. It was necessary to correct for the finite pulse width also, a factor rwas introduced to account for the increase of lifetime on converting the electron to a negative ion. Taking r = 17 and Gfi = 0.12 in consistence with free-ion yield measurement, they obtained rather good agreement between calculated and experimental results. The agreement actually depends on A /r, rather than separately on A or r. 

Shimamori and Hatano (1976) describe a Febetron-injected microwave cavity apparatus for measuring electron concentration following pulse irradiation. Its application to thermalization in Ar and CH4 is similar to the method of Warman and Sauer (1975). In a related experiment, Hatano et al. (private communication) measure the electron collision frequency directly. 

Tembe and Mozumder (1984) applied the DPM method to calculate the time-dependent electron mobility in pulse-irradiated gaseous Ar. They used the gas kinetic formula for mobility (Huxley and Crompton, 1974),... 

Electrons have not been detected by optical absorption in alkanes in which the mobility is greater than 10 cm /Vs. For example, Gillis et al. [82] report seeing no infrared absorption in pulse-irradiated liquid methane at 93 K. This is not surprising since the electron mobility in methane is 500 cm /Vs [81] and trapping does not occur. Geminately recombining electrons have, however, been detected by IR absorption in 2,2,4-trimethyl-pentane in a subpicosecond laser pulse experiment [83]. The drift mobility in this alkane is 6.5 cm /Vs, and the quasi-free mobility, as measured by the Hall mobility, is 22 cm /Vs (see Sec. 6). Thus the electron is trapped two-thirds of the time. 

Fig. 33. Decay of the absorption at 1525 nm attributed to the solvated electron in liquid propane at 88 K irradiated with a pulse of 35 MeV electrons of duration 70 ns. The data were normalised to the initial absorption during the electron pulse and were obtained at electron doses of O, 7 X 1016 eV cm 3 n, 3.0 X 1016 eV cm 3 , 1.92 X1016 eV cm 3. After Gillis et al. [394g].

Bakale et al. [397] pulse irradiated the hydrocarbons cyclopentane, cyclohexane and n-hexane with 0.9 MeV electrons of duration 10 or 100 ns. The transient conductivity decreased approximately exponentially with time for low doses of radiation. The first-order decay of the conductance is probably due to electrons reacting with impurities. With higher doses, the conductance decays approximately as inverse time, characteristic of a second-order recombination of free ions. No evidence for time-dependent geminate ion-pair recombination effects was observed. 

Recent pulse radiolysis experiments (20, 30) have fully established the presence of the negative polarons (hydrated electrons) in irradiated water, and also have demonstrated the occurrence of Reaction 39 (15). 

The conversion of H atoms into electrons was directly demonstrated by Matheson and Rabani (39). They pulse irradiated solutions of 0.1 M H2, which converts all the OH radicals into H atoms. At pH = 11.6, the optical density (due to e aq absorption) showed an initial increase with time, before it decayed to zero. This was explained as being caused by Reaction 7, which under the appropriate conditions produces more electrons than the amount initially decaying. From (39) Figure 5, kn +oh- = 1.8 X 107 M l sec.-1 has been calculated. Fielden and Hart find recently h+oh- = 2.3 X 107 Af-1 sec.-1 by direct observation of the formation of e aq in alkaline H2 solutions. Since the decay of e aq, under the conditions of the experiments, was mainly by second order reactions with other species produced by the radiation, the initial increase in optical absorption showed dependency on both the pH and... 

The discovery of the hydrated electron in irradiated aqueous solutions has made it necessary to re-examine the mechanisms proposed for the irradiation of aqueous solutions of substances which are biologically important. The new technique of pulse radiolysis has provided a breakthrough in many ways, particularly in determining absolute rate constants. These advances have made it possible to begin working out the reactivity of solvated electrons in vivo, although it is not yet possible to specify the precise role of the reactions in radiation biology. 

As has already been mentioned, picosecond pulsed radiolysis offers great possibilities for studying the short-lived transient processes. In Ref. 326 the solutions of 2,5-diphenyloxazol (DPO) in different solvents were irradiated by picosecond electron pulses obtained from an accelerator. The authors have found two types of excitations of DPO, which they have named the fast and the slow excitations. With fast excitation the luminescence appears during the electron pulse and stops growing at the end of the pulse, after 10 ps. With slow excitation the luminescence is formed within 1 ns. At small DPO concentrations the observed intensity of fast luminescence cannot be explained by direct excitation by electrons (cf. data of Ref. 325). Analyzing the results of experiments with different solvents and different types of additives, Katsumura et al.326 conclude that the main part of the fast luminescence of DPO is due to VCR absorption. 

In pulse radiolysis studies of Urd and its derivatives (but not with dUrd), spectral changes are observed after the completion of the S04, reaction [k = 3 x 10s s 1 Bothe et al. 1990] that are not typical for S04 reactions with pyrimidines. On the basis of EPR experiments (Hildenbrand 1990 Catterall et al. 1992), these observations can be interpreted by an (overall) intramolecular H-transfer giving rise to a radical at the sugar moiety. This requires that considerable amounts of Ura are released which is indeed observed (Fujita et al. 1988 Aravindakumar et al. 2003 Table 10.4). Chain reactions occur as with the other pyrimidine/peroxodisulfate systems. This increases the Ura yield beyond that expected for a non-chain process, but when corrections are made for this by carrying out experiments at the very high dose rates of electron-beam irradiation, a... 

Table 10.4. G (base release) (unit 10-7 mol J-1) from some pyrimidine nucleosides and 2 -deoxynucleosides induced by the S04 radical [G(S04 ) = 3.3 x 10-7 mol J"1) at different dose rates pulsed electron-beam irradiation ( 6 Gy per 2 ps pulse, high dose rate) and y-irradiation (0.013 Gy s-1, low dose rate Aravindakumar et al. 2003) ...

Lewis FD, Letsinger RL, Wasielewski MR (2001) Dynamics of photoinduced charge transfer and hole transport in synthetic DNA hairpins. Acc Chem Res 34 159-170 Li Z, Cai Z, Sevilla MD (2001) Investigation of proton transfer within DNA base pair anion and cation radicals by density functional theory (DFT).J Phys Chem B 105 10115-10123 Li Z, Cai Z, Sevilla MD (2002) DFT calculations on the electron affinities of nucleic acid bases dealing with negative electron affinities. J Phys Chem A 106 1596-1603 Lillicrap SC, Fielden EM (1969) Luminescence kinetics following pulse irradiation. II. DNA. J Chem Phys 51 3503-3511... 

Chemical evidence for the existence of electrons in irradiated cyclohexane was obtained from pulse radiolysis studies of solutions containing aromatic solutes27. Because of the lifetime of the pulse these experiments only allowed the determination of ions still surviving after 10 6 sec. With benzophenone and anthracene as scavengers transient absorption peaks at 700 nm and 730 nm respectively, were obtained. These were consistent with the known spectra of the benzophenone and anthracene radical ions and are most simply accounted for by assuming direct electron capture by these solutes. Positively charged ion radicals may also be produced since these are likely to have similar spectra. Ion yields can be calculated since the absorption coefficients are known, but these yields necessarily represent the sum of the positive and negative ion yields. Some results are shown in Fig. 3. 

Rate constants for the radical reactions were carried out using pulse radiolysis. Briefly, a N20 saturated solution is pulse irradiated, with high energy electrons, producing OH radicals. 

Pulse radiolysis is a method that determines the reactivity of the unstable radical species produced by radiation using an electron pulse. When oxygenated water is irradiated with an electron pulse of 10 8 10 6-s duration in the presence of formate, 05 is produced by the following equations. 

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