Hydrated electrons are formed in the illumination of a CdS colloid and of ZnS/CdS co-colloids with a 347.2-nm laser flash. The yield is increased by excess sulfide anions. In the presence of 2 X M Na2S, the quantum yield is 0.15 hydrated electrons per absorbed photon. The photoelectron emission from the colloids shows the intensity dependence of a monophotonic process. A mechanism is proposed in which the storage of a huge number of electrons for a short time after scavenging of positive holes by sulfide anions plays an important role. The accompanying cathodic potential changes enable part of the electrons to migrate from the colloidal particles into the aqueous phase.Photochemical reactions in colloidal solutions of CdS, such as the reduction or oxidation of adsorbed molecules, the degradation of the colloid, and the quenching of its fluorescence, are understood in terms of the formation of electron-hole pairs by light and subsequent reactions of these charge carriers.14 We wish to report on a laser flash experiment in which electrons were emitted with a substantial quantum yield from the illuminated colloid into the aqueous phase.5The preparation of colloidal CdS from Cd(C104)2 and NazS with a Ludox HS30 silicon sol (13 nm; 6 X M) from Dupont as stabilizer has previously been described.' The mean particle diameter of this colloid was 37 nm as determined by fractional filtration through micropore filters. The onset of the absorption was at 515 nm, Le., the wavelength corresponding to the energy of 2.4 eV of the band gap of crystalline CdS. At shorter wavelengths, the rise in absorption was less steep than for a CdS crystal which is an indication for a more amorphous structure of the colloidal particles. Co-colloids of ZnS and CdS were prepared by adding a solution of Zn(C104)2 and Cd(C104)2 to a Na,S solution, again with HS30 as stabilizer. In these co-colloids, the absorption begins at shorter wavelengths with increasing ZnS contents.The flash experiments were made with a frequency doubled ruby laser (pulse duration: 20 ns; X = 347.2 nm; hu = 3.57 eV). Actinometry was carried out with a deaerated 3 X M benzophenone/O.l M naphthalene solution in hexane. Absorption of the laser light occurred here by benzophenone (L: = 100 M-l cm-') and the 425-nm absorption of the naphthalene triplet ( E = 2.3 X lo4 M-' cm-') was observed. The detection system consisted of a Bausch and Lomb monochromator, a five-stage photomultiplier (PM R928), a back-off circuit for base line compensation,6 an amplifier (Tektronix 7A13), and a transient recorder (Biomation 8100). The data were processed by a PDP 11/40. Immediately after the laser flash, an absorption signal in the 400-800-nm range was observed. It decayed according to a first-order rate law with a half-life of 2-3 ps, depending on the solution. At the largest laser dose used (producing triplets at a concentration of 3 X M in the actinometer solution) the absorption signal decayed in two steps as shown by the inset of Figure 1. The first step occurred with a half-life of 0.3 ps...
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