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Photoemission electron microscopy (PEEM) excited by x-ray and UV sources is used to investigate epitaxial anatase thin films embedded with rutile nanocrystals, a model system for the study of heterocatalysis on mixed-phase TiO2. Both excitation sources show distinct contrast between the two TiO2 phases, however, the contrast is reversed. Rutile nanocrystals appear darker than the anatase film in X-ray PEEM images but brighter in UV-PEEM images. Topography-induced contrast is dominant X-ray PEEM imaging, whereas work function contrast, dominates for UV-PEEM. Work function contrast results from the differences in work function and surface defect state densities between the two phases near the Fermi level. This assertion is confirmed by UPS data that shows the rutile work function to be 0.2 eV lower and a greater occupied valence band density-of-states in rutile (100) than in anatase (001). Since the boundaries between rutile nanocrystals and the anatase film are clearly resolved, these results indicate that PEEM studies of excited state dynamics and heterocatalysis are possible at chemically intriguing mixed-phase TiO2 interfaces and grain boundaries.

The thermally-induced martensitic phase transformation in a polycrystalline NiTiCu thin film shape memory alloy was probed by photoelectron emission microscopy (PEEM). In situ PEEM images reveal distinct changes in microstructure and photoemission intensity at the phase transition temperatures. In particular, images of the low temperature, martensite phase are brighter than that of the high temperature, austenite phase, due to the relatively lower work function of the martensite. Ultra-violet photoelectron spectroscopy shows that the effective work function changes by about 0.16 eV during thermal cycling. In situ PEEM images also show that the network of trenches observed on the room temperature film disappear suddenly during heating and reappear suddenly during subsequent cooling. These trenches are also characterized by atomic force microscopy at selected temperatures. We describe implications of these observations with respect to the spatial distribution of phases during thermal cycling in this thin film shape memory alloy.

Photoemission electron microscopy is used to probe photon-induced oxygen vacancies generated on TiO2 (110)-(1×2) surfaces. An increased oxygen vacancy concentration within the irradiated region leads to an increase of local photoelectron emission. The local oxygen deficient region can be compensated by exposing the surface to molecular oxygen at 1×10-5 Torr, or via surface diffusion at 450 K in vacuum. The surface diffusion coefficient was estimated to be on the order of 10-12 m2/s. Photoemission electron microscopy allows in situ studies of surface electronic defect formation and removal.

Matrix-assisted laser desorption/ionization combined with time-of-flight mass spectrometry (MALDI/TOF-MS) was used for the analysis of low molecular mass compounds. Three classes of molecules were studied: organic acids, salts of oxyanions and amine-based chelating compounds. Mass spectra from samples of citric, propionic, butyric, oxalic and stearic acid; ethylenediaminetetraacetic acid (EDTA),N-(2-hydroxyethyl) ethylenediaminetriacetic acid (HEDTA), ethylenediamineN,N′-diacetic acid (EDDA) and nitrilotriacetic acid (NTA); and sulfate, nitrate, nitrite and phosphate salts were obtained. These species were analyzed alone and as mixtures in both the positive and negative ion modes. The organic acids and oxyanion salts displayed much stronger signals in the negative ion detection mode whereas chelating compounds, which contain basic amine functional groups, yielded stronger signals in the positive ion mode. This implies that detection sensitivity is often better for a particular ion mode in the analysis of small molecules containing limited classes of functional groups. In all analyses, the presence of high concentrations of sodium was found to quench the MALDI signals. To increase the detection sensitivity, some samples were processed through an ion-exchange column to remove sodium ions. This step was found to enhance the signal by two orders of magnitude over untreated samples. © 1997 by John Wiley & Sons, Ltd.

Absolute rate coefficients for the reactions of CN with ethane, propane, and five of their deuterated isotopes are determined over the temperature range 294-736 K by using the laser photolysis/CW laser-induced-fluorescence technique. The measured rate coefficients for the reactions of CN with C{sub 2}H{sub 6} (k{sub 1}) and C{sub 2}D{sub 6} (k{sub 2}) are well described by the three-parameter modified Arrhenius expressions k{sub 1}(T) = (7.40 {times} 10{sup {minus}15})T{sup 1.29} exp(+512 cal mol{sup {minus}1}/RT) cm{sup 3} molecule{sup {minus}1} s{sup {minus}1} and k{sub 2}(T) = (8.32 {times} 10{sup {minus}16})T{sup 1.57} exp(+460 cal mol{sup {minus}1}/RT) cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}. The rate coefficient for the reaction of CN with the half-deuterated ethane compound, CH{sub 3}CD{sub 3} (k{sub 3}), is well described ty (1/2)(k{sub 1} + k{sub 2}). We measure rate coefficients for CN reactions with C{sub 3}H{sub 8} (k{sub 4}), C{sub 3}D{sub 8} (k{sub 5}), C{sub 3}H{sub 6}D{sub 2} (k{sub 6}), and C{sub 3}D{sub 6}H{sub 2} (k{sub 7}) and derive site-specific rate constants for abstraction of primary and secondary hydrogens. These rate coefficients, on a per hydrogen atom basis, are, in the units cm{sup 3} molecule{sup {minus}1} s{sup {minus}1}, k{sub H}{sup P}(T) = (3.58 {times} 10{sup {minus}15})T{sup 1.14} exp(+566 cal mol{supmore » {minus}1}/RT), k{sub D}{sup P}(T) = (2.63 {times} 10{sup {minus}16})T{sup 1.47} exp(+568 cal mol{sup {minus}1}/RT), k{sub H}{sup S}(T) = (3.30 {times} 10{sup {minus}13})T{sup 0.56} exp(+649 cal mol{sup {minus}1}/RT), and k{sub D}{sup S}(T) = (2.96 {times} 10{sup {minus}13})T{sup 0.52} exp(+656 cal mol{sup {minus}1}/RT). The k{sub 4} through k{sub 7} are well represented by the sum of the appropriate three-parameter expressions for H- and D-atom abstraction from primary and secondary sites.« less

Initial measurements on I-asterisk yields of alkyl iodides at 266 nm are reported using gain vs. absorption spectroscopy with an InGaAsP diode probe laser. The results are 102 percent + or - 4 percent, 102 percent + or - 7 percent, and 73 percent + or - 4 percent for n-C3F7I, i-C3F7I, and CH3I respectively. Future prospects for the development of diode laser systems and for their use in dynamical studies are discussed.