Photothermal Spectroscopy Methods


Photothermal Spectroscopy Methods

Thermal lens spectroscopy (TLS) is one of a family of photothermal techniques that can be used for physical and chemical analysis and is well suited for the characterization of transparent materials. An improvement in the limits of optical absorption detection of three orders of magnitude or more than that obtained by conventional absorbance spectrophotometric techniques has been achieved, which is ideal for ultra-trace determination and absorption spectrum measurements. By using TLS, absolute fluorescent quantum yields can be deduced precisely without the requirement for luminescent standards. TLS has also been employed for the measurements of thermal properties such as thermal diffusivity of solid and liquid samples.
In the thermal lens measurement, a transparent sample is illuminated using radiation from a laser beam (excitation beam) Some of the radiation is absorbed by the sample or by chromophores within the sample. Excited-state species either lose energy radiatively; e.g.; fluorescence or phosphorescence or by nonradiative routes which results in the generation of heat. The flow of heat from the region illuminated by the laser results in a thermal gradient that is proportional to the beam intensity profile (heating is stronger at the center of the beam profile than in the wings) in the sample which may be a solution, solid or gas. Due to temperature gradient, the refractive index gradient is produced creating a lens like an optical element - the thermal lens (TL). The coefficient of refractive index with temperature varies for different materials and the lens formed may be converging or diverging. But normally the variation of coefficient of refractive index with temperature is negative for gas and almost all liquids.


Photothermal Deflection Spectroscopy (PDS) is a very sensitive technique that measures energy (E), based upon local heating of the sample (mK) by absorption of light of a certain (sub-gap) wavelength. In the technique, an intensity-modulated monochromatic light beam is focussed on the sample (pump beam). If the light of the pump beam is absorbed by the sample, its energy will be converted into heat by non-radiative recombination. As a result, periodic temperature fluctuation is generated, causing a thermal wave to propagate into the sample and its surrounding medium. Since the refractive index (n) of material is temperature-dependent, these thermal waves cause a periodic refractive index change in the sample and its surrounding medium (mirage effect). Therefore, in PDS, the sample is immersed in a liquid. These changes are probed by a deflecting HeNe laser (probe beam), directed parallel to the sample and perpendicular to the pump beam, grazing the surface and thus only probing the changes of the index of refraction of the liquid. This deflection occurs at the same frequency of the modulated pump beam and its magnitude is detected by a position-sensitive detector and a lock-in amplifier. In this way, absorption coefficients down to down to 1 cm-1 can be measured.

Photothermal deflection spectroscopy (PDS) measures optical absorption in an indirect way by making use of the periodic heating which results when (part of) an intensity-modulated light beam is absorbed by a sample (Amer and Jackson 1984). This heat spreads into the surroundings, where it modulates the index of refraction of a liquid, most frequently carbon tetrachloride, which adjoins the sample and through which a probing laser beam passes parallel to the sample surface. The probe beam will consequently be periodically deflected with an amplitude that indicates the amount of energy absorbed from the intensity-modulated pumping beam. Synchronous, phase-sensitive detection of the deflection as a function of the photon energy of the pump beam will hence provide a measure of the absorption spectrum of the material. PDS requires good control over the heat flows in the system (including the substrate of thin-film samples), and some overlap with the spectral range of transmission data to facilitate calibration.


photo-thermal infrared imaging spectroscopy (PT-IRIS). This method measures the difference in the materials infrared radiance that is caused by absorption. Materials emission follows Stefan's law of thermal emission. this technique is used to find the thermal properties of layered and solid materials. It involves photo-thermal heating of the sample with a tunable quantum cascade laser and measuring the resulting increase in thermal emission with an infrared detector. Photo-thermal emission spectra resemble FTIR absorbance spectra and can be acquired in both stand-off and microscopy configurations. Furthermore, PT-IRIS allows the acquisition of absorbance-like photo-thermal spectra in a reflected geometry, suitable for field applications and for in-situ study of samples on optically IR-opaque substrates (metals, fabrics, paint, glass etc.). Conventional FTIR microscopes in reflection mode measure the reflectance spectra which are different from absorbance spectra and are usually not catalogued in FTIR spectral libraries.

Photothermal optical microscopy / "photothermal single particle microscopy" is a technique that is based on the detection of non-fluorescent labels. It relies on the absorption properties of labels (gold nanoparticles, semiconductor nanocrystals, etc.), and can be realized on a conventional microscope using a resonant modulated heating beam, non-resonant probe beam and lock-in detection of photothermal signals from a single nanoparticle. It is the extension of the macroscopic photothermal spectroscopy to the nanoscopic domain. The high sensitivity and selectivity of photothermal microscopy allows even the detection of single molecules by their absorption. Similar to Fluorescence Correlation Spectroscopy (FCS), the photothermal signal may be recorded with respect to time to study the diffusion and advection characteristics of absorbing nanoparticles in a solution. This technique is called photothermal correlation spectroscopy (PhoCS).







Sources
photo-thermal infrared imaging micro spectroscopy
Photothermal Deflection Spectroscopy
PHOTOTHERMAL DEFLECTION SPECTROSCOPY
Photothermal Lens Technique
Photothermal Single-Particle Microscopy

Comments

Popular posts from this blog

glucose detector noninvasive, new devices

noninvasive glucose prediction