The technique of optical second harmonic generation (SHG) is applied to the measurement of molecular adsorption at the interface between two immiscible electrolyte solutions (ITIES). The resonant second harmonic response from 2-(N-octadecyl)aminonaphthalene-6-sulfonate (ONS) is used in conjunction with interfacial tension measurements to optically determine the relatively surface coverage of the anionic surfactant molecule at a charged water- dichloroethane interface. At a pH of 9, ONS adsorption occurs at all potentials positive of the potential of zero charge. The potential dependent adsorption of ONS can be described by a Frumkin isotherm with a free energy of adsorption that varies linearly with applied potential. The potential dependence of the SHG from the interface provides important information on the position of the adsorbed ONS molecules with respect to the ITIES. At a pH of 3, both the anionic form of ONS and the protonated zwitterionic form of ONS are present at the liquid- liquid interface.
The laser-induced desorption of CO molecules from a Cu(111) surface is probed with 100 fsec time-resolution. This is accomplished in a pump-probe scheme using surface second harmonic generation as the probe. We find that the desorption reaction is completed in -18 cm2) desorption is a novel one: the transient hot substrate electrons (Telectron,max approximately 3600 K, while Tlattice,max approximately 160 K) drive multiple electronic excitation-de-excitation cycles of the CO-Cu complex within the vibrational relaxation time. This vibrationally pumps the molecules into desorbing.
Optical second harmonic generation (SHG) has been widely employed as a surface probe. Applications of the technique to adsorption, desorption, surface diffusion, molecular orientation and surface chemistry of adsorbed molecules are described as examples. Surface SHG and sum frequency generation (SFG) for surface spectroscopic studies are also described.
I review some of the general aspects of using optical diffractions from laser-induced surface density gratings to probe diffusion of adsorbates. Among examples, the recent progress in the study of quantum tunneling diffusions is emphasized.
Laser desorption in an ion-trap mass spectrometer shows significant promise for both qualitative and trace analysis. Several aspects of this methodology are discussed in this work. We previously demonstrated the generation of both negative and positive ions by laser desorption directly within a quadrupole ion trap. In the present work, we explore various combinations of dc, rf, and time-varying fields in order to optimize laser generated signals. In addition, we report on the application of this method to analyze samples containing compounds such as amines, metal complexes, carbon clusters, and polyaromatic hydrocarbons (PAHs). In some cases the ability to rapidly switch between positive and negative ion modes provides sufficient specificity to distinguish different compounds of a mixture with a single stage of mass spectrometry. In other experiments, we combined intensity variation studies with tandem mass spectrometry experiments and positive and negative ion detection to further enhance specificity.
The extended ion trapping time intrinsic to the Fourier transform mass spectrometry (FTMS) experiment allows detection of the extensive fragmentation that can take place during the matrix-assisted laser desorption/ionization process. Metastable ion fragmentation undoubtedly decreases mass resolution for molecular ions in time-of-flight mass spectrometry and, because of the much longer time scale involved, does so even more dramatically in FTMS analysis. However, in this work reported here, it is shown that choice of correct experimental conditions can minimize this problem. For example, for compounds with masses lower than 2000 daltons, mass spectral resolution > 100,000 can be obtained. Spectra of a porphyrin, several proteins and an organic polymer obtained with such resolution are presented. Addition of a sugar co-matrix produced sufficiently stable bovine insulin molecular ions, m/z 5734, to obtain resolution > 25,000.
This article reviews the evolution of the UCLA fullerene research over the past two years from the perspective of laser desorption mass spectrometry (LDMS). As a fast and sensitive analytical method, it has critically contributed to a wide range of fullerene related research projects. Immediately after the discovery of a method for making C60, C70 and selected higher fullerenes in macroscopic amounts, a concerted effort in different laboratories led to the isolation and structure determination of C76, C78, C82 and tentatively C84. Simultaneously with the work on pure fullerene, important progress was achieved in the synthesis of metallofullerenes and derivatized fullerenes. LDMS continually documented progress or failure in extraction, isolation and synthesis and helped to plan future research. Recently, synthesis of C60 and larger fullerenes from precursors, e.g., C18(CO)6, and coalescence reactions of fullerenes, induced by high fluence of the desorption laser, introduced a new aspect of this otherwise exclusively analytical method.
CO2-laser evaporation/ KrF- laser ionization technique combined with reflectron time-of- flight mass-spectrometry have been used for analysis of drinking water polluted with phenol. The minimum phenol concentration level of 10-7% was detected. A considerable temperature decreasing down to 150 K of the laser evaporated molecules from the frozen water surface has been obtained.
Laser ablation followed by resonant-enhanced multiphoton ionization combined with RETOF mass-spectrometry was used for trace element analysis of industry-made semiconductor samples and organic compounds. Ablated neutral species entered an ion extraction system where they were resonantly excited via intermediate atomic levels and ionized by ionization lasers, accelerated to 1.7 keV energy, reflected by a gridless ion mirror and detected by a double-microchannel plate assembly. Detection limit down to 1 ppb was achieved for the number of trace elements (Fe, B, Cr, Al).
Results of a systematic investigation of the Raman, hardness, and resistivity properties of sputtered amorphous carbon films on silicon substrates for hydrogen stoichiometries between 14% and 39% are presented. Frequency shift and line broadening effects of the Raman active graphite-like vibrations are consistent with a model of both harness and strain present in the amorphous network being independently controlled by the unprotonated sp3 carbon-carbon bonding fraction and the hydrogen concentration. Comparison to constraint counting models for covalent random networks indicates that the hardness of hydrogenated amorphous carbon follows the degree of network strain. We find no evidence for phonon confinement effects in micro crystals consistent with a random network structure without predominating microcrystals. Resistivity data indicate that the vibrational properties are largely independent of strong variations in the electronic properties with stoichiometry.
With the enhanced sensitivity of Photothermal Deflection Spectroscopy (PDS), vibrational overtone spectra have been obtained in organic thin films for the first time, covering the range from the near IR to the visible (0.4 - 2 eV). The absorption spectra of polycarbonate, for instance, exhibits C-H stretch modes for (Delta) n > 1 where n is the vibrational quantum level. Previous measurements were constrained to either long fibers, which pose the problem of various scattering losses not related to absorption, or to a restricted wavelength region in the visible accessible by dye laser sources. The unparalleled sensitivity of PDS allows precise determination of frequency, lineshape and intensity of the various modes, even for films approximately 10 (mu) /m thick over a broad energy range. The overtone spectra can be used as a probe of various basic molecular properties. In a manner similar to NMR spectroscopy, it is possible to study specific atomic bonds.
Current state-of-the-art long-length fiber-optic remote Raman probes are limited to about 100 m in length for weak scatterers. Not only is this due to probe design, but also to the wavelengths and hardware chosen. By using the portability and versatility of diode lasers and their wavelength match to the transmission windows of telecommunication fiber-optics, previously designed systems may now be extended to at least a factor of five over previously designed systems. Such a system is described, along with a review of currently available Raman probe designs.
Resonance Raman techniques, together with lattice-dynamics and Peierls-Hubband modelling, are used to explore the electronic and vibrational dynamics of the quasi-one-dimensional metal-halogen chain solids [Pt(en)2][Pt(en)2X2](ClO4)4, (en equals C2H8N2 and X equals Cl, Br), abbreviated 'PtX.' The mixed-halide materials PtCl1-xBrx and PtCl1-xIx consist of long mixed chains with heterojunctions between segments of the two constituent materials. Thus, in addition to providing mesoscale modulation of the chain electronic states, they serve as prototypes for elucidating the properties to be expected for macroscopic heterojunctions of these highly non-linear materials. Once a detailed understanding of the various local vibrational modes occurring in these disordered solids is developed, the electronic structure of the chain segments and junctions can be probed by tuning the Raman excitation through their various electronic resonances.
The Hadamard transform technique has been extended to include imaging with a two- dimensional stationary Hadamard encoding mask. Images have been obtained by measuring laser induced fluorescence, Raman scattering, and surface enhanced Raman scattering. Spots on thin-layer chromatography plates have been imaged using laser induced fluorescence and surface enhanced Raman scattering.