Heat wave spectroscopy

TMDSC) and heat wave spectroscopy (HWS) are particularly suited to study a reversible process like surface crystallization and melting. They extract heat flows associated with reversible structure changes and separate them from latent heats of fusion and crystallization. Experiments probe the reaction of a sample onto an imposed oscillating temperature... 

Values for the heat of formation were based on calorimetric measurements for the reaction N2F2(g) + f NH3(g) - 2NH4F(s) + 3N2(g) using mixtures of the isomers in different proportions. The bond lengths and bond angles were determined by electron diffraction (23) and micro-wave spectroscopy (176). 

Vasbinder, A.J., van Mil, P.J.J.M., Bot, A., and de Kruif, K.G. (2001). Acid-induced gelation of heat-treated milk studied by diffusing wave spectroscopy. Colloids Surf. B. 2, 245-... 

THE PHOTOACOUSTIC effect is based on the conversion of absorbed light energy into heat by means of radiationless deexcitation processes. On irradiation of a solid or liquid sample by intensity modulated light, a heat wave of the same frequency is generated in the sample. The heat wave is transferred by diffusion to the surface of the sample, where oscillatory thermal effects are generated in the coupled gas. These can be detected as an acoustic signal. This technique is known as gas-coupled photoacoustic spectroscopy. 

Ultrasonic Spectroscopy. Information on size distribution maybe obtained from the attenuation of sound waves traveling through a particle dispersion. Two distinct approaches are being used to extract particle size data from the attenuation spectmm an empirical approach based on the Bouguer-Lambert-Beerlaw (63) and a more fundamental or first-principle approach (64—66). The first-principle approach implies that no caHbration is required, but certain physical constants of both phases, ie, speed of sound, density, thermal coefficient of expansion, heat capacity, thermal conductivity. 

A very common heating sensing technique used in condensed matter is photoacoustic (PA) spectroscopy, which is based on detection of the acoustic waves that are generated after a pulse of light is absorbed by a luminescent system. These acoustic waves are produced in the whole solid sample and in the coupling medium adjacent to the sample as a result of the heat delivered by multiphonon relaxation processes. 

Photoacoustic Spectroscopy.7 When modulated IR radiation is absorbed by a sample, the substance heats and cools in response to modulated IR energy impinging on it. This thermal hysteresis is converted into pressure waves that can be communicated to surrounding gases and detected by acoustic detectors (essentially a sensitive microphone in the enclosed sample chamber). In such measurements, the acoustic detector replaces the IR detector of the spectrometer. 

Problems can arise in the investigation of rapid reactions if the reactants are not heated sufficiently fast to the desired temperature, and if the samples from the reactor are not cooled rapidly to stop the reaction. A more sophisticated approach consists of monitoring the changes in concentration in an optical cell, in situ, by means of spectroscopy. Both infra-red and Raman spectroscopy can be used, depending on the sensitivity of characteristic bonds and the wave-number range of interest. 

Muchall et al. (98CC238) have recently investigated the gas-phase thermolysis of 2,5-dihydro-2,2-dimethoxy-2,5,5-trimethyl-l//-l,2,4-oxadiazole (75) by PE spectroscopy. Decomposition of 75 was induced by means of a continuous wave (CW) C02 laser as directed heat source at 26 W, which corresponds to a temperature of 500 50°C. When the PE spectra of acetone, tetramethoxyethene, and dimethyl oxalate were subtracted from the pyrolysis spectrum, a sim-ple spectrum remained that could be identified as that of dimethoxycarbene. Thermolysis in solution (94JA1161) had shown formation of tetramethoxyethene, and FVP experiments (92JA8751) gave dimethyl oxalate, both of which arise from the common precursor, dimethoxycarbene. Thermolysis of oxadiazolines similar to 75 in solution affords dialkoxycarbenes via an intermediate carbonyl ylide (94JOC5071). 

The energy released as heat in the course of the nonradiative decay of P to the ground state and detected as a pressure wave by laser-induced optoacoustic spectroscopy (LIOAS) exhibits positive deviations (i.e., a> 1 cf. Eq. (1)) from the values which were calculated on the basis of the absorption spectrum of Pr alone (Figure 15) [90,115]. This indicates that already within the 15-ns duration of the excitation flash, one or several intermediates must have been formed. These in turn, within the same interval, may again absorb light from an intense laser flash and (at least in part) dissipate heat upon their return to the ground state of the same species (internal conversion) and/or to Pr (photochemical back reaction). The formation of primary photoproducts within the nanosecond flash duration was of course to be expected in view of the much shorter lifetimes of the photochromic fluorescence decay compo-... 

Time-resolved Photoacoustic Spectroscopy. In photoacoustic spectroscopy (PAS) the heat evolved by the absorption of light in the sample is transformed into sound waves which are detected by a microphone. In steady-state spectroscopy the light is continuous, but it is also possible to use a pulsed laser and to observe the change in the intensity of the sound signal with time. In this respect time-resolved PAS is somewhat similar to thermal lensing, but both techniques have different limitations and advantages. 

Hanson, R. K., "High-Resolution Spectroscopy of Shock-Heated Gases Using a Tunable Infrared Diode Laser" in "Shock Tube and Wave Research" University of Washington Press, 1978,... 

Unfortunately, the beam heats up the plasma considerably and increases the momentum of the plasma column, influencing the rotational speed of the plasma as well. Therefore, charge exchange spectroscopy can be used only in discharges with additional heating. In ohmic plasmas and plasmas heated by electromagnetic waves X-ray spectroscopy is the only diagnostic to determine the ion temperature, as well as the plasma rotation. 

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