Recent studies have demonstrated that functionalized portions of long-chain hydrocarbons (CH3(CH2) n X, where X = SH, S, SS, NH2, Br, I) physisorbed on graphite at the liquid−solid interface exhibit enhanced contrast (increased tunneling current) when imaged by scanning tunneling microscopy (STM). The tunneling mechanism for such molecules can be investigated by examining the interplay between topographic and electronic structure/coupling factors. Utilizing a scanning tunneling spectroscopy methodology, changes in the STM images of two-dimensional thin films of amide-, bromide-, and sulfide-derivatized molecules have been observed as a function of applied bias voltage. For octadecanamide, the relative topographic height (increased tunneling probability) of the functional group compared to that of the hydrocarbon backbone peaks at |V| = 1.4 V; this differs greatly from the voltage dependence obtained for docosane bromide, where the measured relative topographic height of the bromide end group remains constant for all voltages sampled. These trends are discussed in terms of the geometry of the molecular adsorbate and its electronic coupling to the substrate.

Ambientdetection of NO2 by cavity ring-down spectroscopy is examined in the wavelength region near 405.23 nm, and possible interferences by particulates, water vapor, and carbon dioxide are characterized. Particulates can be efficiently removed by the use of a 0.45 microm fluoropolymer filter. Water vapor has a response of 2.8 ppb (NO2 equivalent) for 1.0% water vapor (80% relative humidity at 10 degrees C) in air at 405.23 nm in a broad continuous absorption feature. Carbon dioxide has a response of 0.8 ppb (NO2 equivalent) for 1.0% CO2 attributable to Rayleigh scattering and would not contribute significant interference in ambient measurements due to the lower ambient CO2 levels. Water vapor interference and in general broad background in the absorption spectrum can be accounted for by removing NO2 selectively in the ambient air stream with an annular denuder coated with sodium hydroxide and methoxyphenol (guiacol). Subtraction of the resulting background signal provides NO2 measurements with a limit of detection of 150 ppt/10 s (SIN = 3). Reliable NO2 measurements could be obtained by this method without the need for frequent calibration with calibration gas. Ambient NO2 measurements are carried out to demonstrate this method.

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A study of secondary ionization mechanism for LiF on Au by timeofflight secondary ion mass spectrometry and direct recoil measurements J. Vac. Sci. Technol. A 9, 190 (1991); 10.1116/1.577519 A molecular beam timeofflight mass spectrometer using lowenergyelectron impact ionization Rev. Sci. Instrum. 61, 775 (1990); Multiphoton ionization in a reflectron timeofflight mass spectrometer: Individual rates of competing dissociation channels in energyselected benzene cations Energy-selective electron impact ionization of laser-produced photofragments together with time-of-flight mass spectrometry is a general and sensitive means of studying primary photodissociation processes. Low-energy electrons ionize photofragments without the production of background fragment ions from dissociative ionization ofthe parent molecules, and the time-of-flight mass spectral peak shapes provide direct information on the photofragment recoil anisotropy. In the first application of this combination of techniques, we have studied the photodissociation of methyl iodide at 229.4 nm, the short-wavelength end of the A band, in order to assess the contribution of transitions to the IQI state to the absorption profile. The results presented here show that leP I / 2 ) is the primary iodine-atom product and that the transition is largely parallel [,8 = (1.6 ± 0.1) for leP I / 2 )] at 229.4 nm. These data together with previous photofragmentation results suggest that excitation to the 3Q state dominates the entire A band absorption profile. 0

The photochemistry of gas-phase 1,1,1-trifluoroacetylacetone (TFAA) excited with ultraviolet (UV) light involves a significant photoelimination channel that produces hydrogen fluoride and a fluorinated methylfuranone, 2,2-difluoro-5-methyl-3(2H)-furanone (2FMF). This pathway is remarkable because it is a gas-phase unimolecular reaction that forms a five-membered ring product. This report is the first of such a TFAA photoelimination channel, which is similar to one observed with 1,1,1,5,5,5-hexafluoroacetylacetone (HFAA), resulting in 2,2-difluoro-5-trifluoromethyl-3(2H)-furanone. We present infrared spectral observations of 2FMF produced by pulsed, UV-laser excitation of TFAA, along with analogous results from HFAA, supported by density functional theory (DFT) computational studies. DFT results for the infrared spectrum of 5-methyl-3(2H)-furanone, the expected comparable acetylacetone photoelimination product, help suggest that UV excitation of acetylacetone fails to follow a similar type of photoelimination. We use a weighted RMS approach as a figure of merit for comparing calculated infrared frequencies with experimental data. Results from the three acetylacetones reveal how the presence of fluorine atoms in acetylacetone influences the gas-phase molecular photochemistry.