Transition photoacoustic detection

Gas-phase photoacoustic detection of propane in N2 was attempted on a Si chip. In this method, a modulated light beam is incident on the sample. If the wavelength of the modulated light couples to an energy transition in a gas, the gas absorbs the modulated light resulting in periodic gas expansions and contractions, which are manifested as an acoustic wave. This wave can be detected by a microphone. In contrast to conventional absorption spectroscopy, the sensitivity of photoacoustic spectroscopy scales inversely with dimension, and hence this method is favored in the microscale. This is because photoacoustic spectroscopy is a differential technique in which the absorption is measured as the intensity per unit surface area [796],... 

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]. 

Photoacoustic Detection of Natural Circular Dichroism in Crystalline Transition Metal Complexes... 

The application of photoacoustic detection to the visible region has been reported by Stella et al. [76]. They placed the spectraphone inside the cavity of a cw dye laser and scanned the laser across the absorption bands of the CH4 and NH3 molecules. The high-quaUty spectra with resolving power of over 2 x 10 proved to be adequate to resolve single rotational features of the very weak vibrational ovatone transitions in these molecules. The experimental results are very useM for the investigation of the planetary atmospheres, where such weak overtone transitions are induced by the sun light. 

Figure 3-43 Schematic representation of the photoacoustic Raman scattering (PARS) process, (a) A simple energy level diagram illustrating the Raman interaction that occurs in the PARS process, (b) Basic elements of the PARS experimental arrangement. The pump beam is attenuated and the Stokes beam is amplified by the stimulated Raman process that takes place where the beams overlap in the gas sample cell. For each Stokes photon created by the Raman process, one molecule is transferred from the lower state to the upper state of the transition. Collisional relaxation of these excited molecules produces a pressure change that is detected by a microphone. (Reproduced with permission from Ref. 107.)...

Further recent developments in DSC and IR techniques, with respect to the study of SC barrier properties, include step-scan FT-IR photoacoustic spectroscopy [195] and combined microscopic differential calorimetry-Four-ier transform infrared (DSC-FTIR) spectroscopy [196]. The former allows depth profiling of the membrane the latter enables the simultaneous detection of calorimetric and structural modifications during a thermal transition. Technological advances in DSC and IR will, no doubt, continue to expand the application of these techniques to the study of skin barrier function. 

The theory of rotation effects on prolate luminescent molecules in solution and its experimental verification have been developed and compared. Generalized diffusion equations for the rotational motion of an asymmetric rigid motor have been used to given an expression for steady-state fluorescence depolarization. " The radiationless transition from the first excited singlet state of Eosin has been measured by optoacoustic relaxation, and the absolute fluorescence quantum yields of organic dyes in poly(vinyl alcohol) have also been measured by the photoacoustic method. The accuracy of the method has been discussed in the latter paper. Actinometry in flash photolysis experiments has been assisted by new measurements on the extinction coefficient of triplet benzophenone. Matrix-isolation fluorescence spectrometry has been used to detect polycyclic aromatic hydrocarbons from gas chromatography. ... 

Eu(tta)3L2] and [Eu(bta)3L2] have been determined by photoacoustic methods.Although glass transitions of the vitrified mesophase could not be detected by luminescence measurements, photoacoustic spectra clearly showed this transition. 

LIF (Ezekiel and Weiss, 1968 Cruse, et al., 1973 Zare and Dagdigian, 1974 Kinsey, 1977) is an example of an indirect technique for the detection of a one-photon resonant upward transition. There are many other indirect detection techniques (optogalvanic, optothermal, photoacoustic, cavity ringdown), but Multi-Photon Ionization (MPI) is a special type of indirect technique uniquely well suited for combining absorption detection with other useful functionalities (see Section 1.2.1.1). In MPI, photo-ion detection replaces photon detection. The one-color, singly-resonant-enhanced (n + m) REMPI f process consists of an n-photon resonant e, v, J <— e",v",J" excitation, followed by a further nonresonant m-photon excitation into the ionization continuum... 

The resulting spectrum differs from both of the equivalent transmittance or reflectance spectra since the technique detects non- radiative transitions in the sample. Photoacoustic spectroscopy is useful because the detected signal is proportional to the sample concentration and can be used with very black or highly absorbing samples. This technique probes mainly material on the surface or a few microns below the surface of the sample, and thus is very useful for surface studies. 

A high resolution study of the Vj band of HCN at 3 )un has been recently reported using a standard small volume cell of non-resonant design . The 25 lowest rotational levels of the V3 band were excited with a tunable < lor center laser at an HCN pressure of 20 m Torr. The pressure of the N2 buffer gas was varied between 20 and 400 Torr to investigate pressure broadening and to detect possible frequency shifts induced by the buffer gas. In intracavity experiments performed at an HCN pressure of only 0,02 mTorr no shift of the transition frequency could be detected, to the accuracy of the wavemeter readout. The results of this photoacoustic analysis of the V3 band of HCN are ... 

There are some less widely studied types of inorganic photochemical systems. There is the phenomenon of photochromism, in which a system is driven one way by light and returns either thermally or photochemically. An example for us was the family of transition metal dithizonate complexes [51]. Photochemistry and calorimetry may be combined in photocalorimetry, to obtain enthalpies of reactions which are not clean or are not obtainable at all thermally [52], Similar information can be obtained from the photoacoustic effect [53]. Another interesting phenomenon is that of chemiluminescence, that is, the chemical production of an excited state reaction product, and the related process of electrogenerated chemiluminescence. This type of emission can be detected even when... 

Although most LD studies are performed by recording the intensity of transmitted light, other methods are also used. One of them detects the LD via fluorescence, the other by photoacoustic spectroscopy. Both methods are very sensitive and avoid some of the errors inherent in the transition mode, such as light scattering. 

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