Transparent interface method

The philosophy behind the transparent interface method is to optimize the description of the interatomic forces across the interface, instead of focusing on the accurate reproduction of the total energy of the system. The method utilizes a single energy functional to describe the whole system and pseudoatoms to connect the quantum and classical regions. More in detail, a pseudoatom is placed in between each quantum-described oxygen and classically described silicon to correctly terminate the quantum wave functions, and the Hamiltonian for the whole system is given by... 

In the transparent interface method, the forces on each atom are derived from the gradient of the total potential energy in Eq. [71]. However, the constraint forces acting on quantum atoms at the boundary, due to the constaints imposed on the pseudoatoms, are neglected in this approach, leading to an approximation of the Hamiltonian equations that provides correct forces and dynamics (within the limit of a classical force field) but violates energy conservation. 

Application The transparent interface method has been tested on the SiiOyHg system and then applied to the study of SiO and SiOi molecules and their interaction with water. 

In addition to the surface/interface selectivity, IR-Visible SFG spectroscopy provides a number of attractive features since it is a coherent process (i) Detection efficiency is very high because the angle of emission of SFG light is strictly determined by the momentum conservation of the two incident beams, together with the fact that SFG can be detected by a photomultiplier (PMT) or CCD, which are the most efficient light detectors, because the SFG beam is in the visible region, (ii) The polarization feature that NLO intrinsically provides enables us to obtain information about a conformational and lateral order of adsorbed molecules on a flat surface, which cannot be obtained by traditional vibrational spectroscopy [29-32]. (iii) A pump and SFG probe measurement can be used for an ultra-fast dynamics study with a time-resolution determined by the incident laser pulses [33-37]. (iv) As a photon-in/photon-out method, SFG is applicable to essentially any system as long as one side of the interface is optically transparent. 

Light-pipe interface is the easiest method for connecting a GC and an FUR spectrometer. The lightpipe is a long flow cell with reflective inner coating and IR transparent windows at both ends of the cell. It is very easy to operate and to maintain. The problem of light-pipe interface is that it is not very sensitive compared to a normal bench-top mass spectrometer, which makes the analysis of the same samples with both instruments difficult. 

The technique can also be used for multilayered structures. The corresponding equations are then more complicated and are usually applied to inorganic semiconductors [36-38] due to their better defined interfaces and geometry compared with organic semiconductors. In the case of transparent media (k = 0), the ellipsometric equations can be used to determine both n and the thickness of the film with sensitivity below 1 A. This is much better than can be achieved by methods based on 7Z and T, thus reducing the uncertainty in the n determination. Several examples of ellipsometry applied to CPs are reported in the literature [32,43,44],... 

Up to date, besides the SFA, several non-interferometric techniques have been developed for direct measurements of surface forces between solid surfaces. The most popular and widespread is atomic force microscopy, AFM [14]. This technique has been refined for surface forces measurements by introducing the colloidal probe technique [15,16], The AFM colloidal probe method is, compared to the SFA, rapid and allows for considerable flexibility with respect to the used substrates, taken into account that there is no requirement for the surfaces to be neither transparent, nor atomically smooth over macroscopic areas. However, it suffers an inherent drawback as compared to the SFA It is not possible to determine the absolute distance between the surfaces, which is a serious limitation, especially in studies of soft interfaces, such as, e.g., polymer adsorption layers. Another interesting surface forces technique that deserves attention is measurement and analysis of surface and interaction forces (MASIF), developed by Parker [17]. This technique allows measurement of interaction between two macroscopic surfaces and uses a bimorph as a force sensor. In analogy to the AFM, this technique allows for rapid measurements and expands flexibility with respect to substrate choice however, it fails if the absolute distance resolution is required. 

These are not necessarily the catalysts and reactions that are most suitable for analysis by UV-vis spectroscopy. Consequently, the power of the method has not been fully exploited, and one would hope for a broader range of applications in the future. A significant advantage of UV-vis spectroscopy is that many solvents are transparent in the respective wavelength ranges, and the method is suitable for investigating the preparation of catalysts and catalysis at solid-liquid interfaces. 

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