Electron Spin Resonance Determination of the Radiolytic Yields of Trapped Electrons and Other Species in an Organic Glass

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Electron spin resonance (e.s.r.) methods have been used to search for the ?-ray-induced formation of trapped electrons and free radicals in 13 straight-chain and branched-chain terminal olefins a t 77 OK. When e.s.r. spectra were observed they were used to identify the species present. Changes induced by subsequent exposure to visible and ultraviolet (u.v.) light were investigated. Trapped electrons were observed in three olefins that exist as glasses a t 77 OK, hexene-1, hexene-2, and 2-methylbutene-1, but were not detected in two other glassy olefins, possibly because of rapid annealing. No trapped electrons were found in polycrystalline olefins a t 77 OK. The possibility that COz-ions have been misinterpreted as trapped electrons in these and other systems is discussed and eliminated. The formation and behavior of free radicals and trapped electrons in irradiated 2-methylbutene-1 are analogous to those previously reported for 2-methylpentene-1 under similar conditions. These observations support the hypothesis that in these two systems ions produced initially isomerize rapidly, leading to the formation of the free radicals detected by e.s.r. The results of a subsidiary experiment suggest that hydrogen abstraction from the matrix is induced by u.v. photolysis of the radicals present in 2-methylpentene-I.

Section: Trapped Electronssupporting

confidence: 92%

“…absorption exhibits similar microwave saturation properties to the e.s.r. line which we attribute to trapped electrons in other systems (2,8,9). These similarities in line width and microwave saturation properties are not surprising because in each case they arise from relatively 1% eak nuclear hyperfine interaction.…”

Section: Comparison With Coz-supporting

confidence: 50%

Electron spin resonance observations on trapped electrons and free radicals formed in olefins at 77 °K by γ-irradiation

Smith¹,

Okenka²,

Pieroni³

1967

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“…The total radical concentration was measured in a powdered sample of 1<1aNH2S03, using a previously described e.s.r. technique (8). The relative radical concentrations were estimated using a single crystal of K16NHaS03.…”

Section: E X P E R I M E N T a Lmentioning

confidence: 99%

ELECTRON SPIN RESONANCE SPECTRA OF ¹⁵N-CENTERED RADICALS AT LOW TEMPERATURES: II. THE RADIOLYSIS OF POTASSIUM SULFAMATE

Morton¹,

Smith²

1966

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“…[6] shows that the quantum efficiency will vary as the probability of recapture by the positive ion (PN) varies and this provdes an explanation for the variation in the quantuin efficiency of bleaching. Suppose that the trapped electrons differed simply in their proximity to a positive ion.…”

Section: Tlze Bleaclzing Of the Electron Bandmentioning

confidence: 99%

“…Then (putting a p = 3aB) ZP = 6 X 1016cB. If the quantuin efficiency of release froin the traps is given by equation [6] and P, and PT are proportional to Zp and ZT If @ 4 = 1 the maximum value of would be 0.02. Referring to the observed quantum efficiencies (Fig.…”

Section: The Diffusion and Trapping Of Electrons I N M T H F Glasses mentioning

confidence: 99%

Photochemistry of Electrons Trapped in Glasses of Methyltetrahydrofuran at 77°k

Dyne¹,

Miller²

1965

I n the radiolysis of methyltetrahydrofuran glass a t 77 "I< electrons are produced with a high yield. These are immobilized in traps in the glass and do not recombine with positive ions. They can be released from their traps by illumination with visible and infrared light and may then diffuse to positive ions and be captured. The quantum efficiencies for bleaching have been measured as a function of wavelength. This quantum efficiency falls as bleaching proceeds and depends on the fraction of the electrons bleached and not on their concentrations. About half the electrons are trapped close to positive ions and, on release, have a high probability of diffusing to the positive ion. The remaining half are trapped hon~ogeneously. On release from their traps, few of these electrons reach positive ions although they diffuse over long distances before being retrapped in the glass. This behavior is compared to that discussed by Samuel and Magee for the thermalization and capture of electrons in liquid water.The rate of recombination of an electron and positive ion in a condensed phase is an alinost unltnown quantity and the uncertainties which remain about this process lie a t the root of inany fundainental problems of radiation chemistry. Important advances, permitting a direct study of the process under special conditions, have, however, recently been inade by W. H. Hamill who has extended to radiolytic reactions the techniques of studying the photocheinistry of organic glasses which were pioneered by G. N. In an organic glass a t low teinperature reactive interinediates forined in radiolytic or photolytic decoinposition are held iininobile in a rigid solvent cage. They have long lifetimes and can be observed, a t leisure, by conventional optical techniques. Ronayne, Guarino, and Hamill (2) (henceforth R. G. and H.) found that, on radiolysis of glasses of 2-methyltetrahydrofuran (i\lITHF), a strong optical absorption is observed in the near infrared kvhich is due to electrons trapped in the glass. In glasses containing biphenyl as a solute the intensity of the electron band is reduced and a new absorption due to the biphenyl anion 4~ appears.'The electron band is photosensitive, being bleached by visible and near infrared light. If biphenyl is present in a bleaching experinlent the intensity of the biphenyl anion band increases as the intensity of the electron band is diminished. Electrons released froin their traps in the the glass inay be captured by positive ions or by biphenyl molecules.In a sense this technique allows one to observe, in slow motion, the recombination of electrons and positive ions. This paper describes a study of the photocheinistry of the bleaching of trapped electrons in i\lITHF glasses which shows that thermal electrons have a high mobility in the glass. While few electrons recoinbine with the positive ion before being trapped, they prefer trapping locations in the vicinity of positive ions. E X P E R I M E N T A LThe experimental techniques for purifying M T H F , making optical cell...