In transient infrared (IR) experiments, a molecular system may be photoexcited in a nonstationary conformational state, whose time evolution is monitored via IR spectroscopy with high temporal and structural resolution. As a theoretical formulation of these experiments, this work derives explicit expressions for transient one- and two-dimensional IR spectra and discusses various levels of approximation and sampling strategies. Adopting a photoswitchable octapeptide in water as a representative example, nonequilibrium molecular dynamics simulations are performed and the photoinduced conformational dynamics and associated IR spectra are discussed in detail. Interestingly, it is found that the time scales of dynamics and spectra may differ from residue to residue by up to an order of magnitude. Considering merely the cumulative spectrum of all residues, the contributions of the individual residues largely compensate each other, which may explain the surprisingly small frequency shifts and short photoproduct rise times found in experiment. Even when a localized amide I mode is probed (e.g., via isotope labeling), the vibrational frequency shift is shown to depend in a complicated way on the conformation of the entire peptide as well as on the interaction with the solvent. In this context, various issues concerning the interpretation of transient IR spectra and conformational dynamics in terms of a few exponential time scales are discussed.
This work describes a method to measure the metallized area on the front side of silicon wafer solar cells. The method is especially applicable to detect and quantify background plating, which can occur in the production of solar cells with plated front-side metallization. The metallized area of plated solar cells is determined by an image processing algorithm (\u93MetDetect\u94) using images of the solar cell front side obtained by a simple commercially available flatbed scanner. The algorithm is verified by the comparison of scanned images from test samples with microscope images. Furthermore a correlation of the metallized area with the measured short-circuit current density of the samples justifies the proposed method. With \u93MetDetect\u94 a precise quantification of the background plating on the plated solar cell front side can be realized. It is suitable for inline inspection in solar cell production or quantification of pinholes and cracks in the surface dielectric. In this work \u93MetDetect\u94 is applied to corroborate the observation that background plating of solar cells with a nickel copper front side metallization can surprisingly be removed during plating by a thin Sn-capping
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