Temperature pulse decay technique

The temperature pulse decay technique has been used to measure both the in vivo and in vitro thermal conductivity and blood flow rate in various tissues (Xu et al., 1991 1998). The specimen does not need to be cut from the body, and this method minimizes the trauma by sensing the temperature with a very small thermistor bead. For the in vitro experimental measurement, the measurement of thermal conductivity is simple and relatively accurate. The infinitively large tissue area surrounding the probe implies that the area affected by the pulse heating is very small in comparison with the tissue region. This technique also requires that the temperature distribution before the pulse heating should reach steady state in the surrounding area of the probe. 

Temperature Pulse Decay Technique. As described in Sec. 2.4 under Temperature Pulse Decay (TPD) Technique, local blood perfusion rate can be derived from the comparison between the dieoretically predicted and experimentally measured temperature decay of a thermistor bead probe. The details of the measurement mechanism have been described in that section. The temperature pulse decay technique has been used to measure the in vivo blood perfusion rates of different physical or physiological conditions in varimis tissues (Xu et al., 1991 1998). The advantages of this technique are that it is fast and induces little trauma. Using the Pennes bioheat transfer equation, the intrinsic thermal conductivity and blood perfusion rate can be simultaneously measured. In some of the applications, a two-parameter least-square residual fit was first performed to obtain the intrinsic therm conductivity of the tissue. This calculated value of thermal conductivity was then used to perform a one-parameter curve fit for the TPD measurements to obtain the local blood perfusion... 

Temperature Pulse Decay (TPD) Technique. Temperature pulse decay (TPD) technique is based on the approach described and developed by Arkin, Chen, and Holmes [Arkins et al., 1986 1987], This method needs no insulation, in contrast to some of the methods described above, since testing times are short, usually on the order of seconds. However, the determination of the thermal conductivity or the blood flow rate requires the solution of the transient bioheat transfer equation. 

Quantitative investigations of the photoinduced electron transfer from excited Ru(II) (bpy)3 to MV2 + were made in Ref. [54], in which the effect of temperature has been studied by steady state and pulse photolysis techniques. The parameters ve and ae were found in Ref. [54] by fitting the experimental data on kinetics of the excited Ru(II) (bpy)3 decay with the kinetic equation of the Eq. (8) type. It was found that ae did not depend on temperature and was equal to 4.2 + 0.2 A. The frequency factor vc decreased about four orders of magnitude with decreasing the temperature down to 77 K, but the Arrhenius plot for W was not linear, as is shown in Fig. 9. 

The incident radiation is provided by a laser pulse (wavelength 360-760 nm, duration 1-15 ns, pulse rate 1-100 Hz, energy 15-30 pJ). The illuminated area is typically 0.5 mm in diameter and the final temperature decay curve is accumulated following up to 10 000 laser pulses. This technique is suitable for the in situ non-destructive study of surfaces, and has been used to measure the thermal diffusivity of polyester films [13] and pigments [14]. More recently OTTER has been extended to the in vivo study of water concentration gradients in the stratum comeum [15]. The principle drawback of OTTER is that the values of the estimated parameters are entirely model dependent and improving the quality of experimental models is the major focus of current research in this field. 

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

Fig. 9. Time decay of the occupied band tail density n Bx, measured by the voltage pulse charge sweepout technique, for various temperatures. The n-type doped a-Si.H sample was first annealed at 210°C for 10 min. and then cooled to the indicated temperatures (Street et al., 1988).

The laser heating technique can be applied to perform temperature jumps by irradiating short laser pulses at the sample container. Ernst et al. (54) used such a temperature jump protocol to perform stop-and-go experiments. After the start of the laser pulse, the temperature inside the sample volume is raised to the reaction temperature, the conversion of the adsorbed reactants proceeds, and the H MAS NMR measurement is performed. After the laser pulse is stopped, the temperature inside the sample volume decreases to ambient temperature, and the C MAS NMR measurement is made. Subsequently, the next laser pulse is started and, in this way, the reaction is recorded as a function of the reaction time. By use of the free-induction decay and the reaction time as time domains and respectively, a two-dimensional Fourier transformation leads to a two-dimensional spectrum, which contains the NMR spectrum in the Ej-dimension and the reaction rate information in the Ts-dimension (54,55). 

Vibrational dephasing provides us with a powerful method to probe the interaction of a chemical bond with the surrounding medium. Over the years, many experimental techniques have been developed to study dephasing of bonds in many molecular systems at various temperatures, pressures, and concentrations [122-124]. One popular experimental technique is the isotropic Raman lineshape. The other methods involve a coherent excitation of the vibration with a laser pulse and monitoring the decay of the phase coherence,... 

The term desorption is used in contrast to evaporation in cases in which a transition of a molecular or ion from the condensed into the gas phase is assumed to take place under non thermal equilibrium condition. The underlying idea is that at thermal equilibrium, temperatures for an evaporation would lead to a correspondingly high excitation of internal vibrational modes of excitation leading to fraigmentation of the molecule. As mentioned above, several characteristics of the ion spectra (2., 6.) cannot reasonably be fitted to an equilibrium temperature model. These properties seem to be the more pronounced, the higher the laser irradiance (i.e. usually the shorter the pulse) and are best documented for the LAMMA technique. Though metastable decay of ions is observed and will be discussed below, the decay rate for most of the ions is very small and decay... 

The technique of microwave-recovery provides crucial information about the substates involved in the ODMR transitions. For this experiment, Pd(2-thpy)2 is optically excited by a c. w. source. This leads to specific populations of the three triplet substates. At low temperature, they are thermally decoupled and thus emit according to their specific populations and their individual decay constants (e. g. see Sect. 3.1.3 and Table 2). In the microwave recovery experiment, the steady state conditions are perturbed by a microwave pulse being in resonance with the zero-field transition at 2886 MHz. Due to the microwave pulse, the populations of the two states involved are changed. Subsequently, one monitors the recovery of the emission intensity in time until the steady state situation is reached again. The microwave pulses have, for example, a duration of 20 ps and are applied repeatedly to enable a detection with signal averaging [61]. 

The SHG intensity from interfaces is determined by the second-order nonlinear susceptibility and the Fresnel coefficients. The SHG spectra of the probe pulses change depending on the transient electronic population and the orientation of the chromophores through these physical quantities. Hohlfeld and coworkers have studied hot electron dynamics in thin metal films by this technique [21]. From the transient response of the SHG intensity, electronic temperature decay due to the electron-phonon coupling in the metal substrate is extracted. Eisenthal and coworkers have studied ultrafast excited state dynamics of dye molecules at liquid interfaces [22]. Particularly, the isomerization dynamics of an organic dye at the interfaces was found to become significantly slower than in the bulk. 

The emission lifetime is often used to determine surface temperature with the advantage that the technique is insensitive to blackbody background. This technique requires excitation by a pulsed source, the persistence of the resulting fluorescence can be observed providing that the length of the source pulse is much shorter than the persistence time of the phosphor s fluorescence. For certain phosphors, the prompt fluorescence decay time (t) varies as a function of temperature and is deflned by ... 

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