Photoacoustic spectroscopy with light scattering samples

Diffuse light scattering is shown, for some well-known examples such as powdered rare-earth oxides and semiconductors, to be a conditio sine qua non for obtaining any wavelength-dependent photoacoustic signal at all. However these photoacoustic spectra have not yet been used to get quantitative values of absorption coefficients obtained using well-known diffuse reflectance spectroscopy techniques. In view of these complications it is emphasized that photoacoustic spectroscopy should be concentrated on those applications which are unique for this method, such as depth profiling or special energy conversion such as photochemical oxygen production.

Radiative heating and cooling with spectrally selective surfaces

Matter continuously exchanges energy with its surroundings. This exchange can be dominated by radiation, conduction, or convection. In this brief review we discuss how proper design of radiative surface properties can be used for heating and cooling purposes. The desired properties can be understood once it is realized that solar and terrestrial radiation take place in different wavelength ranges and that only part of the solar spectrum is useful for vision and for photosynthesis in plants. These facts allow the possibility of tailoring the spectral absorptance, emittance, reflectance, and transmittance of a surface to meet different demands in different wavelength intervals, i.e., to take advantage of spectral selectivity. One example is the selective surface for efficient photothermal conversion of solar energy, which has high absorptance over the solar spectrum but low emittance for the longer wavelengths relevant to thermal reradiation. Below we discuss the pertinent spectral radiative properties of our ambience. These data are then used as background to the subsequent sections treating four examples of spectrally selective surfaces. The first example is the previously mentioned selective surface for converting solar radiation to useful heat. The second example considers surfaces capable of reaching low temperatures by benefiting from the spectral emittance of the clear night sky. The third example concerns two related types of transparent heat mirror. The fourth example, finally, treats radiative cooling of green leaves; this part is included since it gives a nice example of how nature solves a difficult problem in an elegant and efficient way. This example hence provides an interesting background to the other cruder types of artificial selective surfaces. Throughout our discussion we treat the ideal spectral properties, give an illustrative experimental example of how well this goal can be realized, and-where this is possible-show a corresponding theoretical curve indicating to what extent the measured results can be theoretically understood.

Composite material films: optical properties and applications

The optical properties of the composite systems Ag-SiO(2), Si-SiC, and Ge-Al(2)O(3) are compared with the predictions of the Maxwell-Garnett and the effective medium theories. Only the Maxwell-Garnett theory predicts the characteristic optical features of granular metals-a red shift relative to the pure metal in the plasma resonance and a dielectric anomaly. In the case of Si-SiC, the observed red shift in the transverse optical phonon frequency is too small to allow one to discriminate between the two theories. In the case of the Ge-AI(2)O(3) films, both theories are in good agreement with the experimental results for the optical constants near the absorption edge. The use of composite films for photothermal conversion and other optical applications is discussed.

Some Limitations on the Interpretation of Specular Reflection Spectra with an Application to Solid CO

A simple theoretical framework is used to calculate specular reflection spectra in the vicinity of an absorption band in a solid. The energy of the reflectivity peak is shown to differ from the energy of the absorption band even at normal incidence with thick samples and with thin film samples supported on a substrate. By considering the absorption band system of solid CO, synthetic reflection spectra of the Fourth Positive System of solid CO at 45 degrees incidence are obtained that successfully match the experimental results, including striking time-dependent changes.