The photovoltaic effect in a GaAs p-n junction exposed to short laser pulses of the 1.06–3.0 μm spectral range is investigated experimentally. At a low excitation level of 1.06 μm radiation, the intraband single photon absorption of light dominates, and the photoresponse is found to be caused mainly by the hot carriers. As the laser intensity is increased, the photoresponse signal across the junction consists of two components; the hot carrier photovoltage and the classical photovoltage due to electron-hole pair generation resulting from two-photon absorption. The generation-induced photovoltage decreases with the increase in the radiation wavelength following the reduction of the two-photon absorption coefficient, while the carriers are shown to be heated by the intraband light absorption as well as by residual photon energy left over during the electron-hole pair generation. It is established that carrier heating by light reduces conversion efficiency of a solar cell not only via the thermalization process but also due to the competition of the hot carrier and the classical photovoltages which are of opposite polarities.
We describe a mobile spectroscopic system for trace gas analysis based on the open path differential absorption spectrometer and the photoacoustic spectrometer. The first method allows long distance measurements (up to a few kilometers) while the second one provides local in situ detection of pollutants. The open path system is based on the nanosecond (f = 10 Hz, tau = 5 ns) lamp pumped Nd:YAG laser and a tunable two cascade optical parametric generator operating in the 5-12 microm spectral region. This source was mounted into the lidar setup based on the coaxial transmitter/receiver. The photoacoustic system was constructed using the same laser as well as a nonresonant photoacoustic cell.
Near-infrared absorption spectra of sulfite cellulose with a varied adsorbed water content were studied. Analysis of the spectra has been made on the basis of previously published relations and results of the cellulose surface hydroxyls' interaction with absorbed water. Spectra of the OH overtone region of 1.3–1.65 μm were deconvoluted to components of cellulose volume and surface hydroxyls as well as to those of absorbed water. Besides the 1.53–1.55 μm component of “bound” water, the 1.42–1.44 μm component of “free” water starts to grow as the surface coverage exceeds 1.5. Comparison of the peak positions of these water components with those of liquid water suggests that bound water is constituted of hydrogen-bonded molecules. The 1.42–1.44 μm component of free water implies nonhydrogen-bonded and partially hydrogen-bonded molecules. The whole 1.363 μm band and a part of the 1.424 μm band of surface hydroxyls turn into the 1.52–1.53 and 1.57–1.59 μm peaks, respectively, after attachment of H2O molecules. The red shift of the surface bands by approximately 780 cm−1 caused by the adsorbate is consistent with a shift of the fundamental OH band of silica gel. The possible chemical, morphological, and physical causes of the two 1.363 and 1.424 m surface hydroxyl bands are discussed.
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