Thermal lensing

Fig. 3. Vertical section of the Calvet microcalorimeter (16) microcalorimetric element (A) the metal block (B) metallic cones (C and C ) thick metal cylinder (D) thermostat consisting of several metal canisters (E) electrical heater (F) switch (G) thermal insulation (I) and thermal lenses (J and J ). Reprinted from Calvet and Prat (S3) with permission of Dunod.

Deans [192] have proposed a method for the colorimetric determination of traces of phosphorus with molybdenum blue, making use of the laser-induced thermal lensing effect. The procedure is described, and the results obtained on samples of sea water and lake water are presented. 

This temperature rise can be detected directly (laser calorimetry and optical calorimetry), or indirectly by measuring the change in either the refractive index (thermal lensing, beam deflection or refraction and thermal grating) or the volume (photo- or optoacoustic methods). This review will focus primarily on photoacoustic methods because they have been the most widely used to obtain thermodynamic and kinetic information about reactive intermediates. Other calorimetric methods are discussed in more detail in a recent review.7... 

A method for determining the alteration of the refractive index of a medium as a result of the temperature rise in the path of a beam of coherent light absorbed by the medium. Thermal lensing can also occur with pigmented proteins, and this phenomenon can influence the accuracy of concentration gradient measurements in small aperture flow cuvettes as well as in ultracentrifugation. 

MULTISUBSTRATE MECHANISMS THERAPEUTIC RATIO THERMAL EQUILIBRATION THERMAL LENSING Thermal noise,... 

Fig. 3 IPA (dashed line) and 2PA (solid line) spectra of liquid benzene. The 2PA spectrum was obtained by the thermal lensing method. The units on the energy scale are kilokaysers (1 kK = 1000 cm ) and refer to the total energy of the transition. 0-0 is the origin of the Lb band, 18, 14, and 1 are indices for the vibrational modes (the superscript and subscript are the quantum numbers for the mode in the excited state and ground state, respectively). Reproduced with permission from [33]. 1986, American Chemical Society...

Thermal Lensing. This technique is used for the measurement of the heat released (or taken up) by the sample after the photolytic flash. It is in... 

Figure 7.38 shows the principle of a thermal lensing experiment which uses a liquid sample contained in a square cell, SC. The probe light is a narrow cw laser beam which falls on a pinhole after passage through the sample. 

Figure 7.38 (a) Principle of thermal lensing. L, lens S, slit SC, sample cell PD, photodetector, (b) Energy diagram of fast and slow processes in thermal lensing... 

When heat is produced in the sample after the photolytic flash, the refractive index of the liquid changes and the probe beam is deflected. The intensity of this probe beam measured by a photomultiplier tube placed behind the pinhole decreases as the temperature of the irradiated volume increases (then its density and its refractive index decrease). The total optical signal change is a measurement of all the heat produced in the sample, i.e. the sum of non-radiative transitions, chemical reactions and solvation energies. Luminescence does not contribute to this signal (nor does scattered light) and for this reason thermal lensing can be used to determine luminescence quantum yields. 

Figure 7.39 shows a thermal lensing oscillogram in which the fast and slow components can be well separated to give the relative heats evolved in these processes. 

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. 

Since the length scales associated with the thermal lens are on the order of 10 to 1000 times the grating constant, their characteristic time scale interferes with polymer diffusion within the grating. Such thermal lensing has been ignored in many FRS experiments with pulsed laser excitation [27,46] and requires a rather complicated treatment. A detailed discussion of transient heating and finite size effects for the measurement of thermal diffusivities of liquids can be found in Ref. [47]. 

The fact is that the reaction free energies are hardly ever determined experimentally, but are simply calculated from the Rehm-Weller equation which will be discussed in detail in the next section [26]. There are still considerable technical problems in direct experimental measurements, because standard methods of calorimetry cannot cope with reactions in time scales of ns or ps but this is slowly changing with the advent of fast calorimetric techniques such as time-resolved photoacoustic spectroscopy [27] and thermal lensing [28] these are considered in the following section. Nevertheless, it appears that all the data currently used in the rate constant-energy plots simply use the Rehm-Weller equation (sometimes with various corrections) and it is obviously important to consider the assumptions built into this equation, its limitations, and possible improvements. 

Thermal lensing [45] This method relies on the change of refractive index of a liquid which is heated suddenly. Those regions of the liquid which are at a higher temperature then act as a lens, therefore called the thermal lens , which can be detected by the refraction of a beam of probe light (Fig. 7). 

The temporal response of the thermal lensing technique is limited by the acoustic transit time the heated liquid must expand in volume for the lens to be formed and this depends in particular on the size of the heated region. In practice the observation time scale is of about 1 ps to a few ms. 

There are in general several processes which generate heat in a photophysical or photochemical reaction. Figure 8 shows two examples with an outline of the temporal evolution of the temperature of the sample — this would be a simplified form of the kind of oscillogram observed in a thermal lensing experiment. The... 

Fig. 8. Outline of the time-scale of the processes observed during an electron transfer reaction observed through thermal lensing. Processes which occur in times below ca. 0.5 ps are very fast , beyond the temporal resolution of the thermal lensing technique they would appear as a step function in the kinetics of heat release. The slowest processes which would be observed in this case are the second-order recombinations of free ions, which take place in time scales of ps to several ms.

Future work in this area should focus on further development of novel extraction schemes that exploit one or more of the cited advantages of the nonionic cloud point method. It is worth noting that certain ionic, zwitterionic, microemulsion, and polymeric solutions also have critical consolution points (425,441). There appear to be no examples of the utilization of such media in extractions to date. Consequently, the use of some of these other systems could lead to additional useful concentration methods especially in view of the fact that electrostatic interactions with analyte molecules is possible in such media whereas they are not in the nonionic surfactant systems. The use of the cloud point event should also be useful in that it allows for enhanced thermal lensing methods of detection. 

Pyridine is another molecule of enduring interest. The two photon spectrum of liquid pyridine has been obtained by thermal lensing techniques. The two lowest htt states are of benzene type and the next higher excited state is nir. The fluorescence of the trajTS - 2 - s tyr ylqu inoxaline conformer has been found to change markedly with solvent. The photophysics of 6-(2/-hydroxy-4 -methoxyphenyl)-s-triazine photostabilizer has been examined in... 

Thermal lensing contribution to the measured nonlinear optical properties. If the pulse duration is longer than the characteristic time of the heat diffusion in the medium, or if this time is itself longer than the delay between successive pulses, material heating may lead to an observable transient thermal lens phenomenon [120, 165, 212, 218, 219], This can show itself, in experiments, with characteristics similar to those of a pure (electronic) Kerr effect. There have been some attempts to extract the respective values of the thermal and electronic contributions to y from z-scan measurements [136, 160, 165, 166, 175, 220], However, de Nalda et al. proved later that this method was not reliable enough to get quantitative results [219],... 

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