Far infrared reflectivity

A simple application of the multiple-oscillator theory is to fit measured reflectance data for MgO in the Reststrahlen region. In Section 9.1 we considered the electronic excitations of MgO, whereas we now turn our attention to its lattice vibrations. A glance at the far-infrared reflectance spectrum of MgO in Fig. 9.7 shows that it does not completely exhibit one-oscillator behavior there is an additional shoulder on the high-frequency side of the main reflectance peak, which signals a weaker, but still appreciable, second oscillator. The solid curves in Fig. 9.7 show the results of a two-oscillator calculation using (9.25) the reflectance data were taken from Jasperse et al. (1966), who give the following parameters for MgO at 295°K ... 

FAB FEA FEB FET FFP FIB FIELO FIR FLAPW FP FP-LMTO FWHM fast atom beam free A exciton free B exciton field effect transistor far field pattern focused ion beam facet-initiated epitaxial lateral overgrowth far infrared reflectance full-potential linearised augmented plane wave Fabry-Perot full-potential linear muffin-tin orbital full wave at half maximum... 

Laser Raman studies have now been made for representative single-crystal metal azides. Far-infrared transmission studies have also been made for many of the azides in the form of thin films. Far-infrared reflection measurements have also been made and include a diverse sampling of azides. KN3 is the only metal azide for which single-crystal coherent neutron inelastic scattering (CNIS) studies have been made, whereas NaN3 has been studied with neutrons as a poly crystal... 

TABLE 15.2 Comparison of Room Temperature DC Transport Properties and Far-Infrared Reflectance... 

Smith, S. M. The far infrared reflectance of optical black coatings, No. 759. SPIE. 

Smith (1981) The Far Infrared Reflectance of Optical Black Coatings by S. M. Smith, Proceedings of the 6th International Conference on Infrared and Millimeter Waves, Institute of Electrical and Electronics Engineers, NY. 

Perry, C. H., 1964. Far infrared reflectance spectra and dielectric dispersion of a variety of materials having the perovskite and related structures. Japan J. Appl. Phys., Suppl. 4 564. 

We have determined the optical properties as a function of temperature by measuring the reflectivity / (co) of the oriented CNT films from the far infrared (FIR) up to the ultra-violet (UV) (i.e., from 20 cm" up to 3x10 cm" ), using three spectrometers with overlapping frequency ranges [12, 13],... 

Conversely, in the summer, it would still have a high transparency for the visible, but a high reflectivity for the near infrared and a high emissivity for the far infrared. The present state of the art of thin-films deposition still falls short of this goal which may have to wait for the development of suitable photochromic coating materials. 

Consequently, these charge effects are reflected in the carbonyl stretching frequencies (87, 88). It has recently been found from studies of the far infrared spectra that the metal-carbon stretching frequencies also support the theory (89). These charge-distribution effects are supported further by the observed dipole moments (90-92). Thus the dipole moments of the chromium tricarbonyl complexes of hexamethylbenzene, benzene, and methylbenzoate lie in the order 6.22, 4.92, and 4.47 /x, respectively. The relationship of charge effects to chemical reactivity is described below. 

For infrared measurements, cells are commonly constructed of NaCI or KBr. For the 400 to 50 cm 1 far-infrared region, polyethylene is a transparent window. Solid samples are commonly ground to a fine powder, which can be added to mineral oil (a viscous hydrocarbon also called Nujol) to give a dispersion that is called a mull and is pressed between two KBr plates. The analyte spectrum is obscured in a few regions in which the mineral oil absorbs infrared radiation. Alternatively, a 1 wt% mixture of solid sample with KBr can be ground to a fine powder and pressed into a translucent pellet at a pressure of —60 MPa (600 bar). Solids and powders can also be examined by diffuse reflectance, in which reflected infrared radiation, instead of transmitted infrared radiation, is observed. Wavelengths absorbed by the sample are not reflected as well as other wavelengths. This technique is sensitive only to the surface of the sample. 

The primary dimensional requirement on a polymer sample is that it be sufficiently thin. (It is possible to obtain reflection spectra of polymers [Robinson and Price (187, 188)], in which case thin specimens are not required, but the use of this technique has thus far not proven to be as fruitful as transmission spectra, and we will not consider it here.) In the NaCl prism region (roughly 650 to 3500 cm-1) specimens as thin as 0.002 mm may be required in order to avoid essentially 100% absorption at some band peaks. The average thickness required in this region for most bands is usually about 0.02 mm. Thicknesses about ten times larger are optimum for frequencies above 3500 cm 1 (the overtone and combination region) and below 650 cm-1 (the far infrared region). Samples areas down to 1 by 3 mm are usable [Wood (247)], and even smaller if a microspectrometer is employed [Blout (76)]. 

In this work we address this issue by looking at the far-infrared equilibrium and photo-induced reflectivity of a MgB2 thin film and at the Electron Spin Resonance (ESR) of powder samples. 

We looked at the far-infrared equilibrium and photo-induced reflectivity of a MgB2 thin film and at the ESR spectra of powder samples. The photo-induced response shows one gap at 3 meV and another at 7 meV. However, excess quasiparticles are only found at the edge of the smaller gap. ESR data on the normal state shows a break down, around room temperature, of the... 

Ferraro JR, Basile LJ (1978) Fourier transform infrared application to national problems In Ferraro JR, Basile U (eds) Fourier transform infrared spectroscopy - applications to chemical systems, Vol 4 Academic Press, New York, 275-302 Ferraro JR, Rein AJ (1985) Application of diffuse reflectance spectroscopy in the far-infrared region In Ferraro JR, Basile LJ (eds) Fourier transform infrared spectroscopy -applications to chemical systems, Vol 4 Academic Press, New York, 244-282 Frank IE, Feikema J, Constantine N, Kowalski BR (1984) Prediction of product quality from spectral data using the partial least squares method J Chem Inf Comput Sci 24 20-24 Fuller MP, Griffiths PR (1980) Infrared microsampling by diffuse reflectance Fourier transform spectrometry Appl Spectrosc 34 533-539... 

Figure 1 Polarized near-normal-incidence reflection spectra of TEA(TCNQ)2 at room temperature in the far-infrared and infrared region. The spectra show the details for three light polarizations 1, parallel to the TCNQ chains 2 and 3, perpendicular to the chains (2 being within and 3 perpendicular to the alternating layers of TCNQ and TEA molecules). (From Ref. 18.)...

In the vacuum windows there is no conduction except through the spacers between the panes. There is also no reason to use a thick vacuum layer the panes need to be only so far apart that they never touch due to bending. The transport due to radiation is the same as for gas-filled windows. However, it can be decreased by infrared-reflecting layers on the window. This can reduce heat transfer by an order of magnitude - at least in principle. Because the panes are cormected via the spacers, they can be chosen thinner, leading to a lighter window constmction. 

Diffuse-reflection spectroscopy is a widely used experimental technique which, different from the previously mentioned techniques, is not only based on reflection and refraction but additionally on diffraction. The exact description, e.g. assuming Mie scattering, and quantitative simulation of the spectra is at least difficult (Grosse, 1990). The most comprehensive overview on all related aspects was given by Kortiim (1969). Experimental examples refer mostly to the visible spectral range, more recent reviews deal with near infrared (Osborne and Feam, 1986), infrared (Korte, 1990b), and far infrared spectroscopy (Ferraro and Rein, 1985). 

---