Wavelength dependence sample heating

The spectral dependence of the light sensitivity (as indicated by yellowing) of free films of Parylene-C was determined. A Heraeus Sun-Test chamber, equipped with a xenon arc lamp filtered to yield a simulated solar spectrum, was used for the irradiation. An additional infrared filter minimized sample heating. The irradiance at the sample location was originally 0.83 W/m2 at 340 nm, but the output decreased approximately 20% after 1500 hours use. Long band-pass optical filters with nominal cut-offs of 305 nm, 345 nm, 385 nm and 400 nm were inserted between the xenon lamp and the Parylene-C film samples to determine the wavelength threshold for yellowing. The sample temperature was maintained at 30+ 2 °C with a water-cooled... 

Virgin cellulose pellets and cellulose chars produced in the simulated fire apparatus were both examined. Two different measurements were made. One involved measuring the reflected radiation in the mid-infrared from 2.5 to 25 pro (4000 to 400 cm l). These measurements were performed in a diffuse reflectance cell within an FTIR spectrometer. These experiments revealed some wavelength dependence of refleclivity. Reflectance was also measured in-situ in the simulated fire apparatus, by arranging the samples, a fluxmeter. and the heating lamps such that surface reflection of (he incident radiation... 

It appeairs that the Increased permeation during ensonlflca-tlon Is due to heating of the sample by the sound source. The differences seen In Table VII can be accounted for by the 2 C variation In temperature of the cell during ensonlflcatlon. Whether there Is a frequency (or wavelength) dependence beyond the range of frequencies tested remains to be determined. 

The heated sample emits thermal radiation, which is used for temperature determination. The spectrum collected was measured in the wavelength range 515-820 nm corresponding to the range of maximal quantum efficiency of our CCD detector. To determine the temperature we fitted the Planck formula with a wavelength independent emissivity to the measured spectrum. The Planck formula [10] contains the temperature and the wavelength dependence of the thermal radiation intensity /bb( j of the black body (BB) ... 

From the introduction of TLS, cw lasers with discrete wavelengths (e.g., He-Ne, He-Cd, Ar", Kr. CO2) are the most widely used. These lasers have the best power stability and beam profile. In most cases, the absorption bands of the trace species of interest do not coincide with a discrete wavelength of the heating laser. Chemical pretreatment is necessary for formation of an analyte derivative with an appropriate absorption band. As in the case of LOAS. TLS has poor selectivity because of spectral interference of sample components. Selectivity depends mainly on the appropriate choice of reagent for analyte derivative formation. 

The apparent fluence threshold is wavelength-dependent and is inversely correlated with the absorption coefficient of the matrix. This is reproduced by the model and is again a vaporization effect. A higher absorption coefficient heats the upper sample layers more efficiently, so they vaporize at lower fluence. 

In Fig. 1.13a the experimentally determined instability wavelength X (e.g. determined from Fig. 1.11) is plotted versus the total heat flux /q. The linear l//q dependence of Eq. (1.22) describes well the experimental data. A second verification of the experimental model stems from the value of Q that is determined by a fit to the data. Rather than a different value of 0 for each data-set, we find a universal value of 0 that depends only on the materials used (substrate, polymer), but not on any of the other experimental parameters (sample geometry, temperature difference). A value of Q = 6.2 described all data sets for PS on silicon in Fig. 1.13a, with a value of Q = 83 for PS on gold. This allows us, in similarity to the electric field experiments in the previous section to introduce dimensionless... 

Isooptoacoustic point A wavelength, wavenumber, or frequency at which the total energy emitted by a sample as heat does not change upon a chemical reaction or physical change of the sample. Its position depends on the experimental conditions. The spectral differences between the isosbestic points and the isooptoacoustic points are the result of the nonlinear relationship between the molar absorption coefficient and the photoacoustic signal. 

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