Infrared reflection mode

Infrared microscopes can focus the beam down to a 20-pm spot size for microprobing in either the transmission or reflection mode. Trace analysis, microparticle analysis, and spatial profiling can be performed routinely. 

Transmission infrared spectroscopy is very popular for studying the adsorption of gases on supported catalysts and for studying the decomposition of infrared active catalyst precursors during catalyst preparation. Infrared spectroscopy is an in situ technique that is applicable in transmission or diffuse reflection mode on real catalysts. 

In the diffuse reflectance mode, samples can be measured as loose powders, with the advantages that not only is the tedious preparation of wafers unnecessary but also diffusion limitations associated with tightly pressed samples are avoided. Diffuse reflectance is also the indicated technique for strongly scattering or absorbing particles. The often-used acronyms DRIFT or DRIFTS stand for diffuse reflectance infrared Fourier transform spectroscopy. The diffusely scattered radiation is collected by an ellipsoidal mirror and focussed on the detector. The infrared absorption spectrum is described the Kubelka-Munk function ... 

These three main classes of process sample streams are in increasing order of difficulty for near-infrared process analysis. In general, liquid streams are best measured in a transmission sampling mode, solids (powders) in diffuse reflectance mode, and slurries in either diffuse reflectance or diffuse transmission according to whether the liquid phase or the suspended phase is of greater analytical signihcance. If the... 

P. Merckle and K.A. Kovar, Assay of effervescent tablets by near-infrared spectroscopy in transmittance and reflectance mode acetylsalicylic acid in mono and combination formulations, J. Pharm. Biomed. Anal, 17, 365-374 (1998). 

Besides the peaks of the local proton modes typical for hydrogen bond, a sharp peak at 28 meV was observed in KDP [34] and attracted much attention [34,38,39]. This peak exists in DKDP at somewhat higher frequency its intensity decreases in both crystals and its width decreases upon the transition from the FE to the PE phase, without any softening of its frequency [38]. Hence, it is concluded that this mode is connected with the phase transition dynamics, i.e., coupled to the polarization fluctuations. This mode is not the tunneling mode or any local mode of proton or deuteron, but rather some collective optical mode of the lattice that involves substantial proton or deuteron displacement. It has been suggested [38] that this mode corresponds to the mode that has a peak at about 200 cm (25 meV) in Raman scattering and infrared reflectivity spectra, and that it is coupled to the soft mode and usually... 

In contrast to the minimal activity in infrared reflection studies the technique of inelastic electron tunneling spectroscopy (IETS) recently has contributed a large amount of information on monolayer adsorption of organic molecules on smooth metal oxide surfaces,Q),aluminum oxide layers on evaporated aluminum. These results indicate that a variety of organic molecules with acidic hydrogens, such as carboxylic acids and phenols chemisorb on aluminum Oxide overlayers by proton dissociation - 1 — and that monolayer coverage can be attained quite repro-ducibly by solution doping techniques. - The IETS technique is sensitive to both infrared and Raman modes. — However, almost no examples exist in which Raman il and or infrared spectra have been taken for an adsorbate/substrate system for which IETS spectra have been observed. 

Fig. 8.9 Set-up for infrared measurements in attenuated total reflection mode on a planar model catalyst. The silicon ATR crystal has a top layer of SiC>2 which serves as the supported for the catalyst particles. In reality, the infrared beam undergoes 25 reflections with the surface in the crystal the evanescent waves protruding outside the crystal are used...

Infrared reflection and Raman spectroscopies have been employed to derive zone centre and some zone boundary phonon energies in wurtzite and in zincblende GaN (TABLE 1) [1-10]. Phonon and coupled modes have been employed to characterise stress conditions and carrier densities in thin films and device structures. 

FIGURE 1 Phonon, plasmon and phonon-plasmon coupled modes in Raman and infrared reflection spectroscopy as a function of free electron density (+ data from [25], see also [3]) [24],... 

Infrared absorption in typically highly n-type bulk GaN is mainly controlled by free carrier absorption [37,38], Infrared reflection of phonons and phonon-plasmon coupled modes in GaN in the range of the reststrahl band has been reported [1,8,9,37], Similarly to the interpretation of coupled modes in Raman... 

Indium nitride has twelve phonon modes at the zone centre (symmetry group Cev), three acoustic and nine optical with the acoustic branches essentially zero at k = 0. The infrared active modes are Ei(LO), Ei(TO), Ai(LO) and Ai(TO). A transverse optical mode has been identified at 478 cm 1 (59.3 meV) by reflectance [6] and 460 cm 1 (57.1 meV) by transmission [24], In both reports the location of a longitudinal optical mode is inferred from the Brout sum rule, giving respective values of 694 cm 1 (86.1 meV) and 719 cm 1 (89.2 meV). Raman scattering of single crystalline wurtzite InN reveals Ai(LO) and E22 peaks at 596 cm 1 and at 495 cm 1 respectively [25],... 

Infrared reflection and Raman spectroscopies have been used to derive the energies of the zone centre phonon modes in wurtzite AlxGai.xN (0 < x < 1) [1-5] (for GaN [6] and AIN [7] refer to the dedicated Datareviews). Selection rules in wurtzite allow a splitting of longitudinal and transversal modes into Ai and Ei modes and the occurrence of additional Raman active modes E2. 

AIN buffer layer on (0001) sapphire. Composition was determined by help of X-ray diffraction. A continuous increase of the mode energy with x was observed. An AlxGai.xN/GaN/sapphire heterostructure grown with the AIN buffer layer technique was studied in infrared reflection and Raman spectroscopy by Wetzel et al [2] (FIGURE 2). From an X-ray analysis of the c-axis an AlN-ftaction of x = 0.15 was derived. Recently, however, it was shown that AIN layers in heterostructures with GaN are coherently strained up to a thickness of at least 350 nm. This leads to misinterpretation of the AIN fraction [8], Including the deformation of the unit cell in the pseudomorphic structure above, a value 50% smaller is concluded (x = 0.08). In backscattering off the c-plane the Ai(LO) mode was determined at 752 cm 1 (square with cross symbol) in excellent agreement with the infrared reflection data [2],... 

Phonon modes in an AlxGai.xN/GaN/sapphire (x = 0.15 neglecting stress, x = 0.08 considering stress, as discussed above) heterostructure grown with the AIN buffer layer technique were studied in infrared reflection and Raman spectroscopy by Wetzel et al [2] (FIGURE 2). 

---