J. Phys. Chem. 1989, 93, 7701-7113
Another difference is that little if any evidence for NH4C1+ or H+(NH4Cl) exists in the mass spectra. This region of the mass spectrum is also shown in Figure 5 . Oddly, Cheung et aLzl report observing NH4C1+. Goldfinger and VerhagenI6 also failed to observe the molecular ion in a Knudsen cell experiment in which they vaporized solid ND4C1. They inferred the presence of the gas-phase ND4C1 from the appearance of ND4+. They suggest that a stable molecular ion would be observed only if the electron was removed from the ammonium rather than the chloride. In calculations by Clemer~ti,l*>~~ the highest occupied orbitals are largely centered on the chlorine atom. Thus, ionization should be expected to result in fragmentation. The observation of NH41+ ion and not the NH4C1+ ion may well be related to the higher polarizability of atomic I (4.3 A3) compared to atomic chlorine (2.1 A3) which could facilitate the retention of I compared to C1 in the cluster after ionization. Conclusions The experimental findings of the present study show that hydrogen halides easily become incorporated in ammonia clusters. (35) Clementi, E. J . Chem. Phys. 1967, 46, 3851.
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The mixed cluster ions observed by laser time-of-flight analysis are mainly of the form H+(NH3),(HX),, where n is greater than m and X is either I or C1. Although the initial formation step is believed to be a replacement reaction, the observed mixed cluster distributions are a reflection of the fragmentation upon ionization and the concomitant ion molecule clustering which occurs in these clusters. Examination of the distributions of the mixed product clusters did not reveal any particularly stable clusters, although a discontinuity in the cluster series containing one H I molecule is evident at n = 10. This is attributed to the formation of a solvated ion pair within the cluster and a subsequent increase in fragmentation upon ionization. Observation of the cluster ions NH4X+and H+(NH4X)is only clearly evident in the experiments with HI, and evidently arises due to fragmentation of clusters containing more than one H I molecule. This observation leads to the conclusion that the ionization of these mixed ammonia hydrogen halide clusters leads to essentially complete fragmentation in the case of the HC1-containing clusters. Acknowledgment. Financial support by the US.Department of Energy, Grant No. DE-AC02-82-ER60044 and DE-FG0288-ER60648, is gratefully acknowledged. Registry No. HCI, 7647-01-0; HI, 10034-85-2.
Sandwich Colloids of ZnO and ZnS in Aqueous Solutions Joseph Rabani’ Energy Research Center and The Department of Physical Chemistry, The Hebrew University of Jerusalem, Jerusalem 91 904, Israel (Received: July 8, 1988)
Colloid particles, composed of ZnO and ZnS particles in contact or ZnO colloid particles coated with ZnS, have k e n investigated. The effect of various amounts of ZnS on the ZnO visible emission was studied. The results show that ZnS quenches the ZnO visible emission by hole transfer; however, fast recombination between the electron and hole takes place at the ZnO-ZnS interface. This is reflected by a decrease in the yield of reduction of methyl viologen upon addition of ZnS to the ZnO-methyl viologen system.
Introduction In recent years, investigations of colloidal solutions of semiconducting materials have attracted much attention. A summary of the most recent results has been reported by Heng1ein.I The significance of semiconductor processes to the production of storable chemical fuels has been critically discussed by Memming.2 ZnO is of particular interest, since it is a common electrode material in conventional semiconductor electrochemistry and photoelectron Recently, the preparation of transparent ZnO colloidal solutions, ZnO,, became possible in the presence9 as well as in the absencelo of colloidal stabilizers. Fast kinetic methods of flash photolysis and pulse radiolysis have also been employed for the investigations of ZnO, as well as other colloids.” The recent observation that so-called “sandwich colloids” composed of two distinct semiconductor colloids are formed by mixing CdS or Cd3P, colloidal solutions with ZnO or TiO, colloids12opens a new scope for further investigations of such systems. ZnO colloid particles prepared according to the novel method of Bahnemann, Kormann, and HoffmannIO have an average diameter of 5 nm (2500 ZnO molecules/particle). It can be prepared and stored at pH 11-12 for several weeks. The zero point of charge is 9.3. When a sample is illuminated at the band-gap absorption (350 nm),fluorescence is observed at 365 nm with a lifetime
