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
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