Photothermal Methods

The simplest sensing device is a thermistor that can directly measure the temperature changes (97). Thetemperaturechangeswere correlated with the quantum efficiency. A serious deficiency of this method is the slow equilibration time of the thermistor ( 1 sec.). The equilibration time can be considerably reduced by shifting to photoacoustic detection either in the form of a microphone (98) or a piezoelectric transducer (99). These developments follow the success that photoacoustic spectroscopy had in determining absorption spectra of solids (100) and efficiency of photovoltaic devices (101). 

Variants of these techniques are the Photothermal Deflection Spectroscopy (PDS or Mirage effect) and Photothermal Displacement Spectroscopy (102). These techniques are based on deflection of a light beam due to refractive index gradients either In a fluid (or air) in contact with a light absorbing solid or in the solid itself. If the fluid is inert the technique can be used to measure absorption spectra of solid materials and transport properties. A version of these techniques was applied to electrochemical and photoelectrochemical systems (103). The authors describe the experimental conditions needed to separate the contributions from the temperature and concentration gradients. Once this is done the results can be correlated with the kinetics and mechanism of the electrochemical reactions. 

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Although other fluorescence (e.g., Rapsomanikis et al., 1988 Genfa et al., 1989), chemiluminescence (Maeda and Takenaka, 1992), and photoacoustic or photothermal methods (e.g., De Vries et al., 1995) have been proposed, they have not found widespread use. 

In current versions of the photothermal method adapted to single-object microscopy [12, 13], a first pump beam, modulated at high frequency, is absorbed by the object to be detected. As schematically shown in Fig. 3.1,... 

The measurement of triplet quantum yields triplet states are transient intermediates. Phosphorescence quantum yields ph represent a lower limit for ph =1i isc3, 7ph but the measurement of [Pg.127]

Gensch, T., Viappiani, C., Time resolved Photothermal Methods Accessing Time resolved Thermodynamics of Photoinduced Processes in Chemistry and Biology, Photochem. Photobiol. Sci. 2003, 2, 699 721. 

The overall diagram of evolution of the excited states and reactive intermediates of a photoinitiating system working through its triplet state can be depicted in Scheme 10.2 [249]. Various time resolved laser techniques (absorption spectroscopy in the nanosecond and picosecond timescales), photothermal methods (thermal lens spectrometry and laser-induced photocalorimetry), photoconductivity, laser-induced step scan FTIR vibrational spectroscopy, CIDEP-ESR and CIDNP-NMR) as well as quantum mechanical calculations (performed at high level of theory) provide unique kinetic and thermodynamical data on the processes that govern the overall efficiency of PIS. 

Dazzi, A., Prazeres, R., Glotin, R, Ortega, J. M., Al-Sawafrah, M., and de Frutos, M. 2008. Chemical mapping of the distribution of viruses into infected bacteria with a photothermal method. Ultramicroscopy 108, 635-641. 

Photothermal methods may be used for depth profiling spectroscopic and/or thermal properties of condensed-phase samples because the sampling distance of the thermal waves may be controlled by varying the modulation frequency (as in eqn [1]) or the observation time (as in eqn [2]). 

For photothermal methods that monitor surface temperature changes, heat generation must occur approximately within the thermal wave damping distance of the sample s surface, in order to contribute to the measured signal. As /t increases in an optically transmitting sample, the measured absorption spectrum of the sample may change, as more deeply buried layers contribute to the measured signal. 

It is possible to perform depth-resolved spectroscopy using these FTIR photothermal methods. Two commonly available instrumentation designs are used to perform FTIR photothermal spectroscopy. In constant scan rate FTIR photothermal instruments, the uniform motion of the movable interferometer mirror... 

The depth profiling capabilities available with photothermal methods have found important applications in biology and biophysics. Photoacoustic and... 

Depth profile analysis using photothermal methods has contributed a number of nondestructive methods for studying the spectroscopy of plant tissues. The phase rotation method of signal recovery has been used to examine a number of specimens of botanical interest, including leaves, tissues, and lichens. 

Tissues such as skin have also been studied using photothermal methods, especially IR radiometry, which is capable of noncontact measurement, and is robust to alignment instabilities. In addition to the thermal and optical depth profiling of the various layers of the skin, photothermal spectroscopy has been used to study the time-dependent penetration of topically applied cosmetics and sunscreens below the skin surface. Photothermal studies have been used to monitor the time for which a topically applied film exists as a discrete phase on the outer surface of the skin. 

Other examples of tissues that have been investigated with photothermal methods include ocular and arterial tissues. IR photothermal radiometry, in particular, shows promise as a general diagnostic tool in biomedical studies, in the nondestructive imaging of very thin layers. 

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