Absorption hydrated electron

A surprising observation was made in the first experiments on the flash photolysis of CdS and CdS/ZnS co-colloids Immediately after the flash from, a frequency doubled ruby laser (X = 347.2 nm photon energy, = 3.57 eV) the absorption spectrum of the hydrated electron was recorded. This species disappeared within 5 to 10 microseconds. More recent studies showed that the quantum yield increased... 

The hydrated electrons then react according to e + Cd Cd, and the Cd ions which have a strong absorption at 300 nm react with the colloidal particles after the pulse. It was observed that the same bleaching took place during this reaction as in the reaction of e with CkiS particles, and it was concluded from this result that Cd" transfers an electron to a CdS particle Cd" + (CdS), - Cd + (CdS) . These observations also are of interest for our understanding of the formation of Cd atoms in the photocathodic dissolution of CdS (see Sect. 3.4). Cd" cannot be the intermediate of the overall reaction 2e + Cd - Cd° as already pointed out in discussing the mechanism of Eqs. (35) and (36)... 

Laser flash experiments were also carried out with Q-CdS sols, in which the emission of hydrated electrons was observed The quantum yield was significantly greater than in similar experiments with larger particles of yellow CdS (Sect. 3.7). The electron emission was attributed to the interaction of two excitonic states in a particle produced during the flash CdS(e — h >2 CdS(h" ) + e q. The emitted electrons disappeared after the laser flash within 10 ps. After this time a long-lived absorption remained which was identical with the above-mentioned absorption of holes produced by OH radicals in the pulse radiolysis experiment. 

FIGURE 6.2 Absorption spectrum of the hydrated electron. The spectrum is structureless, broad (half-width 0.84 eV), intense (oscillator strength 0.75), and has a single peak at 1.725 eV. (See text for details.)... 

The formation of hydrated electrons by the photolysis of halide ions in solution may be envisaged in two steps. The first step is the CTTS absorption leading to (X -). The second step is a slow, thermal process releasing the electron in competition with degradation and recapture. In the presence of acid and alcohol, photolysis of halide solutions generates H2 with a yield that increases both with acid and alcohol concentrations (seejortner et al., 1962, 1963, 1964). At 25°, the limiting quantum yields are 0.98 for Cl- at 185 nm, 0.6 and 0.5 for Brat 185 and 229 nm, respectively, and 0.3 and 0.25 for I- at 254 and 229 nm, respectively. Since most of these yields are less than 1, the direct reaction of HsO and (Xaq-) is ruled out. Instead, it is proposed that eh is produced from the... 

The pulse radiolysis technique gives a direct way for measuring the hydrated electron yield. To get the stationary yield, one can simply follow the electron absorption signal as a function of time and, from the known value of the extinction coefficient (Table 6.2), evaluate g(eh). Alternatively, the electron can be converted into a stable anion with a known extinction coefficient. An example of such an ion is the nitroform anion produced by reaction of eh with tetrani-tromethane (TNM) in aqueous solution ... 

The absorption spectrum of ehis intense, has a broad peak at about 720 nm (halfwidth -1 eV), and is structureless (see Sect. 6.1 and Figure 6.2). It covers at least 220 to 1000 nm and possibly extends on either side. There is some evidence that the absorption rises somewhat in the UV, which has been interpreted as the water absorption perturbed by the hydrated electron (Nielsen et al, 1969,1976). 

The intensity of absorption gives the product G , where G is the observed yield and is the molar extinction coefficient. The absolute value of was determined by Fielden and Hart (1967) using an H2-saturated alkaline solution and an alkaline permanganate-formate solution, where all radicals are converted into Mn042. They thus obtained = 1.09 x 104 M- cm1 at 578 nm, which is almost identical with that obtained by Rabani et al. (1965), who converted the hydrated electron into the nitroform anion in a neutral solution of tetrani-tromethane. From the shape of the absorption spectrum and the absolute value of at 578 nm, one can then find the absolute extinction coefficient at all wavelengths. In particular, at the peak of absorption, (720)/ (578) = 1.7 gives at 720 nm as 1.85 X 104 M 1cm 1. 

The first subnanosecond experiments on the eh yield were performed at Toronto (Hunt et al., 1973 Wolff et al., 1973). These were followed by the subnanosecond work of Jonah et al. (1976) and the subpicosecond works of Migus et al. (1987) and of Lu et al. (1989). Summarizing, we may note the following (1) the initial (-100 ps) yield of the hydrated electron is 4.6 0.2, which, together with the yield of 0.8 for dry neutralization, gives the total ionization yield in liquid water as 5.4 (2) there is -17% decay of the eh yield at 3 ns, of which about half occurs at 700 ps and (3) there is a relatively fast decay of the yield between 1 and 10 ns. Of these, items (1) and (3) are consistent with the Schwarz form of the diffusion model, but item (2) is not. In the time scale of 0.1-10 ns, the experimental yield is consistently greater than the calculated value. The subpicosecond experiments corroborated this finding and determined the evolution of the absorption spectrum of the trapped electron as well. 

A large variety of aqueous and a few nonaqueous solutions have been used or proposed as chemical dosimeters with respective dose ranges for use (Spinks and Woods, 1990 Draganic and Draganic, 1971). Of these, a special mention may be made of the hydrated electron dosimeter for pulse radiolytic use (l(h2 to 10+2 Gy per pulse). It is composed of an aqueous solution of 10 mM ethanol (or 0.7 mM H2) with 0.1 to 10 mM NaOH. Concentration of hydrated electrons formed in the solution by the absorption of radiation is monitored by fast spectrophotometry, which is then used for dosimetry with the known G value of the hydrated electron. 

Although its precise structure has not yet been settled, the hydrated electron may be visualized as an excess electron surrounded by a small number of oriented water molecules and behaving in some ways like a singly charged anion of about the same size as the iodide ion. Its intense absorption band in the visible region of the spectrum makes it a simple matter to measure its reaction rate constants using pulse radiolysis combined with kinetic spectrophotometry. Rate constants for several hundred different reactions have been obtained in this way, making kinetically one of the most studied chemical entities. 

The hydrated electron is one of the main water decomposition products, and it is known that the absorption band of the hydrated electron shifts to longer wavelength with... 

Other transient radicals such as (SCN)2 [78], carbonate radical (COj ) [79], Ag and Ag " [80], and benzophenone ketyl and anion radicals [81] have been observed from room temperature to 400°C in supercritical water. The (SCN)2 radical formation in aqueous solution has been widely taken as a standard and useful dosimeter in pulse radiolysis study [82,83], The lifetime of the (SCN)2 radical is longer than 10 psec at room temperature and becomes shorter with increasing temperature. This dosimeter is not useful anymore at elevated temperatures. The absorption spectrum of the (SCN)2 radical again shows a red shift with increasing temperature, but the degree of the shift is not significant as compared with the case of the hydrated electron. It is known that the (SCN) radical is equilibrated with SCN , and precise dynamic equilibration as a function of temperature has been analyzed to reproduce the observation [78],... 

Two-photon absorption chemistry of 2AP, specifically photoionization processes, can be induced by intense nanosecond 308-nm XeCl excimer laser pulses [10]. Typical transient absorption spectra of 2AP in deoxygenated neutral aqueous solutions are shown in Fig. 1. The stronger (385 nm) and weaker (510 nm) absorption bands were assigned to 2AP radicals derived from the ionization of 2AP (bleaching near 310 nm) [10], whereas a structureless absorption band from -500 to 750 nm corresponds to the well-known spectrum of the hydrated electron (eh ) [41]. 

Fig. 1a,b Transient absorption spectra of 2AP (0.1 mM) in deoxygenated 20 mM phosphate buffer (pH 7) solutions recorded after 308-nm XeCl excimer laser pulse excitation (70 mj pulse" cm" ) [10]. The decay of hydrated electrons was recorded at 650 nm (a) and bleaching of the 2AP band at 310 nm (b). Reprinted with permission from the J Phys Chem, Copyright (1999) American Chemical Society... 

The transient absorption spectra of duplexes with [2AP]A4GGAs are depicted in Fig. 5. At a delay time of 100 ns, the transient absorption spectrum is attributed to the superposition of the spectra of the 2AP(-H) and G /G (-H) radical products and the hydrated electrons. The structureless tail of the eh absorption in the 350-600 nm region decays completely within At<500 ns. The formation of G VG(-H) radicals monitored by the rise of the 310-nm absorption band and associated with the decay of the 2AP V 2AP(-H) transient absorption bands at 365 and 510 nm (Fig. 5) occurs in at least three well-separated time domains (Fig. 6). The prompt (<100 ns) rise of the transient absorption at 312 nm due to guanine oxidation by 2AP was not resolved in our experiments. However, the ampHtude, A((=ioo), related to the prompt formation of the G /G(-H) radicals (Fig. 6a) can be estimated using the extinction coefficients of the radical species at 312 and 330 nm (isosbestic point) [11]. The kinetics of the G VG(-H) formation in the yits and ms time intervals were time-resolved and characterized by two well-defined components shown in Fig. 6a (0.5 /zs) and Fig. 6b (60 /zs). 

That the hydrated electron is a separate chemical entity has been demonstrated by the technique of pulse radi l sis This consists of subjecting a sample of pure water to a very short pulse of accelerated electrons. The energetic electrons have the same effect upon water as a beam of y-ray photons. Shortly after the pulse of electrons has interacted with the water, a short flash of radiation (ultraviolet and visible radiation from a discharge tube) is passed through the irradiated water sample at an angle of 90° to the direction of the pulse to detect the absorption spectra... 

The quantum energy is very similar to the enthalpy of dehydration of the hydrated electron, and the absorption of the 715 nm radiation by the hydrated electron probably causes its dehydration, i.e. its reversion to the gaseous state, e"(g). 

Fig. 98. Lower parr, difference spectra of Q-CdS after the reaction with hydrated electrons (full line) and after the reaction with OH radicals (dashed line, lOx magnified). Upper part absorption spectrum of the investigated sample [576]...

Fig. 37. The decay of the hydrated electron concentration (normalised to an electron yield of 1.07 after 140 ns) following radiolysis with 3MeV protons of pulse duration 1 ns. Significant decay of the hydrated electron optical absorption occurred during...

When an electron is injected into a polar solvent such as water or alcohols, the electron is solvated and forms so-called the solvated electron. This solvated electron is considered the most basic anionic species in solutions and it has been extensively studied by variety of experimental and theoretical methods. Especially, the solvated electron in water (the hydrated electron) has been attracting much interest in wide fields because of its fundamental importance. It is well-known that the solvated electron in water exhibits a very broad absorption band peaked around 720 nm. This broad absorption is mainly attributed to the s- p transition of the electron in a solvent cavity. Recently, we measured picosecond time-resolved Raman scattering from water under the resonance condition with the s- p transition of the solvated electron, and found that strong transient Raman bands appeared in accordance with the generation of the solvated electron [1]. It was concluded that the observed transient Raman scattering was due to the water molecules that directly interact with the electron in the first solvation shell. Similar results were also obtained by a nanosecond Raman study [2]. This finding implies that we are now able to study the solvated electron by using vibrational spectroscopy. In this paper, we describe new information about the ultrafast dynamics of the solvated electron in water, which are obtained by time-resolved resonance Raman spectroscopy. 

In order to study the influence of electron concentration on the observed dynamics, we performed experiments with different laser power densities. As an illustration, the transient absorption signals recorded at 715 nm in ethylene glycol upon photoionisation of the solvent at 263 nm with three different laser power densities are presented in Fig.3. As expected for a two-photon ionization process, the signal intensity increases roughly with the square of the power density. However, the recorded decay kinetics does not depend on the 263 nm laser power density since the normalised transient signals are identical (Cf. Fig.3 inset). That result indicates that the same phenomena occur whatever the power density and consequently that the solvation dynamics are independent of the electron concentration in our experimental conditions i.e. we are still within the independent pair approximation as opposed to our previous work on hydrated electron [8]. 

A further possibility is that the signals arise from hydrated electrons or base radical ions produced by monophotonic ionization of the polymers. However, the quantum yield for photoionization of adenosine is reported to be approximately the same as that of poly(A) and poly(dA) [25], It is unlikely that photoionization of the polymers can account for the signals seen here since there is no detectable signal contribution from the photoionization of single bases [4], The most compelling argument that our pump-probe experiments monitor excited-state absorption by singlet states is the fact that ps and ns decay components have been observed in previous time-resolved emission experiments on adenine multimers [23,26-28]. 

Figure 1.5 Absorption spectrum of the hydrated electron the ordinate is the molar decadic extinction coefficient (m 1 cmr1)...

The solvated electron has been studied in a number of organic liquids, among which are the aliphatic alcohols (27, 28, 3, 2d, 2, 27), some ethers (25, 5), and certain amines (9, 22, 2). Of these systems, it is only in the alcohols, to which this paper is principally but not exclusively directed, that both the chemical reactivity and the optical absorption spectrum of the solvated electron have been investigated in detail. The method used in these studies is that of pulse radiolysis (22, 22), developed some five years ago. The way was shown for such investigations of the solvated electron by the observation of the absorption spectrum of the hydrated electron (6, 28, 19) and by the subsequent kinetic studies (2d, 22, 20) which are being discussed in other papers in this symposium. 

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