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We report in situ scanning tunneling microscopy at atomic-layer resolution of the topographic changes accompanying electrochemical reactions. In dilute perchloric acid, Au(l 11) terraces roughen during the electrochemical formation and reduction of more than a monolayer of gold oxide, but anneal in minutes at moderate potential. Trace chloride enhances step motion, prevents the observation of roughening, and promotes the dissolution of gold during oxidation and rereduction. These observations illustrate the role adsorbates have in determining the surface mobility of substrate atoms.PACS numbers: 61.16.Di, 66.30.Lw, 81.60.Bn, 82.45.+ZThe microscopic details underlying important interfacial electrochemical processes such as electron transfer, plating, passivation, and corrosion are traditionally inferred from in situ measurements of macroscopic quantities: charge, potential, concentrations, surface tension, etc. Recently, various groups have demonstrated the feasibility of using ex situ electron spectroscopies, microscopies, and diffraction techniques 1 " 5 and in situ techniques such as glancing angle x-ray absorption and diffraction 6 " 8 and infrared reflection-absorption spectroscopy 9 to obtain structural information about the electrochemical interface at the atomic level. Scanning tunneling microscopy (STM) could offer a microscopic, realspace view of the electrochemical interface to complement these other tools. 10 One immediate target for investigation with the STM is the growth, dissolution, or rearrangement of crystalline surfaces due to electrochemical reactions. A prototypical example of electrochemically induced change at a crystalline surface is the electrochemical oxidation and rereduction of gold. 11 " 16 Recently, Wiechers et al. have demonstrated resolution of monatomic gold layers at the interface of Au(lll) and aqueous solutions while controlling the electrochemical potential of the gold surface. 17 Here, we report the first in situ STM observations with atomic-layer resolution of the consequences of an electrochemical surface reaction: the roughening, annealing, and dissolution accompanying the oxidation and rereduction of the Au(lll) surface.Gold (111) films on mica were prepared by evaporation of gold onto mica at 225 °C as reported earlier. 18 The samples were clamped to the base plate of a Nanoscope I STM (Digital Electronics) with a Teflon ring, which also formed the electrochemical cell. The counter electrode was a Pt wire, and the reference electrode was an Ag wire in an AgClC>4 solution separated from the cell by a length of Teflon tubing. All electrochemical potentials are reported with respect to the normal hydrogen electrode (NHE). HCIO4 was Ultra grade (Hopkin and Williams Co.), HC1 was reagent grade, water was from a Milli-Q purification train (Millipore Corp.), and oxygen was not removed from the cell. The sample potential was controlled relative to the reference electrode by driving the potential of the counter electrode, and the tip-tosample bias was fixed at -10 mV.The tunnelin...

New experiments in a fast-flow reactor have uncovered a strong correlation between the reactivity of free iron clusters and cluster ionization thresholds: Clusters with low thresholds efficiently add molecular hydrogen, and the relative rates of this reaction closely follow variations in clusterelectron binding energy. This correspondence can be understood in terms of a requirement for metal-to-hydrogen charge transfer in the activation of the H 2 bond.

Using the pulsed cluster beam flow reactor technique, we have measured the reactivity of aluminum clusters toward several molecules under thermal conditions. For each different molecule we observe that the reactivity exhibits a unique dependence on the number of aluminum atoms in the cluster. The overall reactivity of aluminum clusters toward different molecules is ordered roughly as 02 > CH3OH > CO > D20 > D2 > CH4 with CH4 showing no reaction under these experimental conditions. In addition, we are able to place upper and lower bounds on the ionization thresholds for the smaller aluminum clusters.

We report the first measurement of the magnetic moments of gas phase aluminum clusters ranging in size from 2 to 25 atoms. Aluminum clusters are produced by pulsed laser vaporization of an aluminum rod inside the throat of a high pressure pulsed nozzle. The highly collimated cluster beam is passed through a Stern–Gerlach magnet and the deflected beam is analyzed by spatially resolved photoionization time-of-flight mass spectrometry. Aluminum clusters less than nine atoms in size are found to have magnetic moments generally consistent with those predicted from spin and orbital moments of the ground electronic states. As expected, a general trend toward reduced magnetic moment per atom with increasing cluster size is observed.

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The 584 Å photoelectron spectra of rotationally cold C2H2 and C2D2 were obtained with improved resolution, permitting the first three electronic states of the ions to be characterized in greater detail. Temperature-dependent studies led to a definitive assignment of the low intensity features in the X̃ 2∏u state, yielding ν4=837±12 cm−1 for C2H+2 and ν4=702±12 cm−1 for C2D+2. The ν5 origin of the Renner–Teller multiplet was identified. In the case of C2D+2, a Fermi resonance with this multiplet contributed intensity to the ν1 mode, facilitating its evaluation at 2572±16 cm−1. The C2h geometry of the Ã 2Ag state was determined from the two previously unobserved bending progressions, assigned to ν4 and ν5B, and evaluated at 492±12 and 605±12 cm−1 for C2H+2 and 339±12 and 516±12 cm−1 for C2D+2, respectively. A more extensive vibrational progression than previously evident, comprised of irregular spectral features indicative of nonadiabatic effects, was observed for the B 2∑+u state. Autocorrelation functions were derived from the spectra for all three electronic states, and the two electronically excited states exhibit an ultrafast decay on a 10−14 s time scale. The Ã 2Ag state decays within one period of bending vibration, while the B̃ 2∑+u state survives only 14 fs, corresponding to a single period of symmetric stretching motion.

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