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Studies on the hydrated electron

University of British Columbia

This describes a study of some unusual features of the hydrated electron, [symbol omitted] in particular the kinetics of its decay during a period of non-homogeneity lasting tens of nanoseconds, the formation and photodissociation of a hydrated dielectron species [symbol omitted], and the photoexcitation of [symbol omitted]. Nanosecond pulse radiolysis (p.r.) studies on the kinetic behaviour of [symbol omitted](> 10⁻⁴ M) in pure deaerated water revealed a complicated interplay of mechanisms for the first half life ∼110 nsec. This is partly attributable to an initial non-homogeneity in the distribution of reacting species within the system, because the spurs are essentially isolated for tens of nanoseconds. Calculations based on a qualitative model revealed that the times necessary for spur-overlap through diffusion (during which > 40% [symbol omitted] were lost to reaction) were in agreement with experimental observations. However the anomalous trends in κ, a rate parameter describing [formula omitted] within this period, led to the subsequent discovery of a process by which [symbol omitted] were formed after the electron pulse. The use of selective ion and radical scavengers strongly implied that the increase in [symbol omitted] occurred via another radiolytic product, XB. Three plausible mechanisms have been outlined in which XB is (e_ ) [formulae omitted]. XB undoubtedly affects the values of κ but it is not possible at this time to discard the notion of microscopic non-homogeneity within the spur itself as the trends in κ might suggest. Four conclusions are drawn; (i) in some p.r. studies we may not calculate meaningful second order rate constants with concentrations evaluated from optical density data, (ii) the "instantaneous" yield of [symbol omitted] seen through nsec p.r. is higher than that established through μsec p.r. or steady-state techniques because of the rapid initial loss of [symbol omitted] (iii) but the total [symbol omitted] yield will be less since the latter techniques cannot distinguish the source of [symbol omitted]. (iv) there is a critical need for a nanosecond p.r. yield of [symbol omitted] to establish the true primary yield, [symbol omitted]. Some microsecond flash photolysis (f.p.) experiments were performed on hydrogen saturated alkaline solutions. Hydrated electrons were produced following the ultra-violet photolysis of OH⁻ and reacted bimolecularly to give a species which on subsequent infra-red flash photolysis regenerated [symbol omitted]. This species is postulated to be a hydrated electron dimer [formula omitted] the spin state of which is unspecified The remaining purpose of this work was to photoexcite [symbol omitted]. The nature of the excited state of [symbol omitted] and the origin of the optical absorption band is still open to speculation although Jortner and others have performed calculations in which the transition at λmax is assigned to a 2p + 1s excitation. The photolysis of [symbol omitted] was attempted through both p.r. and f.p. techniques, neither of which yielded any conclusive information because of the presence of XB or [symbol omitted] in the system.

Text

2011-05-19

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http://hdl.handle.net/2429/34730

Chemistry

University of British Columbia

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