Laser calorimetry

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... 

The measurement of very small absorption coefficients (down to lO-5 cm-1) of optical materials has been carried out by laser calorimetry. In this method, the temperature difference between a sample illuminated with a laser beam and a reference sample is measured and converted into an absorption coefficient at the laser energy by calibration [13]. Photoacoustic spectroscopy, where the thermal elastic waves generated in a gas-filled cell by the radiation absorbed by the sample are detected by a microphone, has also been performed at LHeT [34]. Photoacoustic detection using a laser source allows the detection of very small absorption coefficients [14]. Photoacoustic spectroscopy is also used at smaller absorption sensitivity with commercial FTSs for the study of powdered or opaque samples. Calorimetric absorption spectroscopy (CAS) has also been used at LHeT and at mK temperatures in measurement using a tunable monochromatic source. In this method, the temperature rise of the sample due to the non-radiative relaxation of the excited state after photon absorption by a specific transition is measured by a thermometer in good thermal contact with the sample [34,36]. 

Awaji, 2007, Development of modulated laser calorimetry using solid platinum sphere as a reference ,... 

More recently, Scaiano et al. (1991) observed (Zs)->(Z)-isomerization of 1,3-di-phenyltriazene also in methanol by using flash photolysis, transient spectroscopy, and laser-induced optoacoustic calorimetry (LIOAC). The interpretation of the data is consistent with the mechanism shown in Scheme 13-4, involving two solvent molecules. 

The photoacoustic calorimetry technique employs photolysis by laser pulses of a mixture containing di-im-butyl peroxide, an appropriate metal hydride, and solvent. Photolysis of the peroxide gives i-BuO radicals that abstract a hydrogen atom from the hydride, and the measured photoacoustic signal is proportional to the overall reaction enthalpy. After calibration,... 

Figure 19.1. Schematic diagram of a general pump-probe-detect laser spectrometer suitable for picosecond electronic absorption, infrared (IR) absorption, Raman, optical calorimetry, and dichroism measurements. For picosecond fluorescence—a pump-detect method, no probe pulse needs to be generated.

Radical-anion complexes Scope of this review 91 Thermodynamic and kinetic methodologies Voltammetric methods 92 Homogeneous redox catalysis 94 Convolution analysis 98 Laser flash photolysis 102 Photoacoustic calorimetry 103 Thermochemical estimates 105 Fleduction of C—O and O O bonds 106 Reduction of ethers 107 Reduction of peroxides and endoperoxides Reduction of S—S and C—S bonds 136 Reduction of disulfides 137 Reduction of sulfides 150 Concluding remarks 157 Fleferences 160... 

It is useful to briefly discuss some of the common and, perhaps, less common experimental approaches to determine the kinetics and thermodynamics of radical anion reactions. While electrochemical methods tend to be most often employed, other complementary techniques are increasingly valuable. In particular, laser flash photolysis and photoacoustic calorimetry provide independent measures of kinetics and thermodynamics of molecules and ion radicals. As most readers will not be familiar with all of these techniques, they will be briefly reviewed. In addition, the use of convolution voltammetry for the determination of electrode kinetics is discussed in more detail as this technique is not routinely used even by most electrochemists. Throughout this chapter we will reference all electrode potentials to the saturated calomel electrode and energies are reported in kcal mol. ... 

A fascinating category of experiments can be found in Table IV. These are the use of lasers to determine thermodynamic parameters. These include calorimetry (43), enthalpies of vaporization and vaporization rates (44, 45), and heat capacities (46). Other laser experiments that can be found in Table IV include the use of CW laser spectroscopy to determine the iodine binding-energy curve (47), the study of vibrational line profiles to determine intermolecular interactions (48), two photon ionization spectrometry (49), a study of optical activity and optical rotatory dispersion (50) and the development of several experiments using blue diode lasers (57). 

Next, the thermal properties of the dye must be such that absorption of the laser energy will result in dye diffusion but not in decomposition. The melting temperature Tm, the latent heat of fusion, AH, and the specific heat for these dyes were determined by differential scanning calorimetry using a DuPont 990 Thermal Analyzer. The data are given in Table II. No thermal decomposition products for these dyes were detected upon heating to 600 °C for 20 msec. 

Since enthalpy changes can be obtained directly from measurement of heat absorption at constant pressure, even small values of AH for chemical and biochemical reactions can be measured using a micro-calorimeter.1112 Using the technique of pulsed acoustic calorimetry, changes during biochemical processes can be followed on a timescale of fractions of a millisecond. An example is the laser-induced dissociation of a carbon monoxide-myoglobin complex.13... 

Solvent effects including 2-methyl-l,3-dioxepane (MDOP), as a solvent, on the propagation kinetics of methyl acrylate (MMA) have been investigated using the PLP-SEC technique (PLP = pulse laser polymerization) <2005MI267>, and the composition of dioxolane-dioxepane copolymers has been studied by IR and differential scanning calorimetry (DSC) <2004PB349>. 

The seminal work of Marcus and Hush has had a significant impact on the development of PET. Pioneering efforts by Sutin, Hopfield, Jortner, and others established the connection between thermal electron transfer and photoelectron transfer [6]. This work set the stage for a notable series of experiments where laser flash spectroscopy [7], chemically induced nuclear polarization [8], resonance Raman spectroscopy [9], time-resolved microwave conductivity [10], and time-resolved photoacoustic calorimetry [11], to site only a few examples, have been successfully employed to chart the dynamics of PET in homogeneous solution, the solid-state, and organized assemblies. 

In this section, some case studies will be presented on the characterization of CMP pad and slurry [17-20] using such advanced analytical techniques as dynamic mechanical analysis (DMA), modulated differential scanning calorimetry (MDSC), thermal gravimetric analysis (TGA), thermal mechanical analysis (TMA), dynamic rheometry, dual emission laser induced fluorescence (DELIF), and the dynamic nuclear magnetic resonance (DNMR). More specifically, these techniques were used to characterize (a) the effect of heat... 

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