Photoacoustic spectroscopy of solids

The opto- or photo-acoustic effect used in gas analysis has been extended to the study of solids. This technique provides a simple method for obtaining information about optical absorptions and subsequent de-excitations in solids.     

Submillimeter BWO spectroscopy of solids

The potentialities of submillimeter spectroscopy (3–30 cm^–1) based on backward wave oscillators for studying solids are discussed. The scope of the research on this problem made at the Institute of General Physics of the USSR Academy of Sciences is outlined.     

Threshold photoemission spectroscopy in solids

Threshold photoemission spectroscopy (TPES) is used to measure the Fe 2p spectrum of a stainless steel sample. The obtained spectrum is compared with analogous spectra measured by X-ray photoemission and absorption spectroscopies. The results of this comparison suggest that resonant two-electron autoionization processes, rather than direct photoemission from the core level, are the main mechanisms contributing to the signal. Limits and applicability of this experimental approach to investigate bulk electronic properties in solids are discussed.     

Hadamard NMR spectroscopy in solids

The duration of 2D and 3D NMR experiments in solids can be reduced by several orders of magnitude by using frequency domain Hadamard encoding with long selective pulses. We demonstrate Hadamard encoded experiments in (13)C enriched solids samples. To avoid multiple quantum interferences, the Hadamard encoding pulses are applied sequentially rather than simultaneously in this study. Among other possible applications, dipolar assisted rotational resonance experiments and measurement of NOESY type build-up rates in proton driven spin diffusion are demonstrated.     

Photoacoustic infrared spectroscopy of some solids

Infrared spectra in the 100–15000 wavenumber region (0.67–100 m) of some ionic (as LiF) and semiconducting (as Ge, Si, GaAs) powders and crystals are reported. The experiment involves the use of a photoacoustic cell as an accessory to a multiple scan averaging Michelson interferometer. The results show the advantages of studying impurities in powders, and surface properties, particularly since the technique is immune to scattering induced errors.Work presented in part (by D.G.M.) at the 22nd Rocky Mountain Conference, Denver, CO, August 10–14, 1980.     

Electron energy-loss spectroscopy applied to solids

Several illustrating examples of recent electron energy-loss investigations of the electronic structure of solids are reviewed. In particular, studies on rare-gas bubbles in metals, on conducting polymers, and onL _2,3 edges of 3d transition metals are reported. Moreover, the electron energy-loss spectrometer, which was used for these investigations, is described briefly.     

Infrared diode laser spectroscopy

Three types of lasers (double-heterostructure 66 K InAsSb/InAsSbP laser diode, room temperature, multi quantum wells with distributed feedback (MQW with DFB) (GaInAsSb/AlGaAsSb based) diode laser and vertical cavity surface emitting lasers (VCSELs) (GaSb based) have been characterized using Fourier transform emission spectroscopy and compared. The photoacoustic technique was employed to determine the detection limit of formaldehyde (less than 1 ppmV) for the strongest absorption line of the v₃ + v₅ band in the emission region of the GaInAsSb/AlGaAsSb diode laser. The detection limit (less than 10 ppbV) of formaldehyde was achieved in the 2820 cm⁻¹ spectral range in case of InAsSb/InAsSbP laser (fundamental bands of v₁, v₅). Laser sensitive detection (laser absorption together with high resolution Fourier transform infrared technique including direct laser linewidth measurement, infrared photoacoustic detection of neutral molecules (methane, form-aldehyde) is discussed.     

Infrared laser modulation spectroscopy

The lack of sensitivity in far infrared of conventional modulation techniques can be overcome by the use of a laser beam as a powerful high resolution infrared source. As an illustration of these features we describe a thermoreflectance experiment performed on mercury telluride using a frequency stabilised CO₂ laser. In this experiment a thin slab of HgTe was illuminated by the beam of the laser. A continuous shift of the Γ_6v?Γ_8c energy interval was produced by a slow temperature variation while the sample was submitted to a slight temperature modulation obtained by low frequency Joule heating. Synchronous and direct detection of the reflected beam gave the relative variation of reflectivity as a function of the sample temperature. Several spectra obtained at different emission lines enable us to determine the energy difference (E_g) between Γ_6v and Γ_8c inverted states. As a first approach a qualificative fit has been obtained with a simple model of dielectric constant and its temperature derivative. These results give the first direct determination of E_g near room temperature E_g = ? 117.04 meV at T = 286 ± 2 K.     

Photoelectron spectroscopy of solids using gas-phase spectrometers

By using continuous deposition of metal surfaces or a continuous scrape technique the ultraviolet photoelectron spectra of clean polycrystalline Zn, Cd, Sn and Pb have been obtained using spectrometers originally designed for gas-phase work. Spectra are calibrated with respect to an internal gas-phase standard. A study of the oxidation of lead is presented to illustrate the applicability of these techniques to adsorption studies. A second oxide in addition to the established orthorhombic form of PbO is observed for low O₂ doses.     

Infrared spectroscopy of ICAO taggants

The IR absorption spectra of the ICAO taggants, ethylene glycol dinitrate (EGDN), o-mononitrotoluene (2-NT), p-mononitrotoluene (4-NT) and 2,3-dimethyl-2,3-dinitrobutane (DMDNB) in the vapor phase, are studied at room temperature over a wide frequency range (500–4000 cm^–1). The pre-assignment of the observed vibrational bands was performed. Modern quantum-chemical methods are applied to calculate the equilibrium geometries of these molecules and the frequencies of their fundamental vibrations. For the most intense bands of 2-NT and 4-NT, the absorption cross sections are estimated. Based on analysis of current laser technology, it has been concluded that their use in conjunction with the available spectroscopic data makes it possible to reliably perform local and remote detection and identification of ICAO taggants in an open atmosphere in real time.     

Infrared spectroscopy of nano-porous silicon

The first measurements of dielectric-permittivity and conductivity spectra of several samples of nano-porous silicon prepared by anodization of low-resistance monocrystalline silicon are performed at room temperature by means of subterahertz and IR spectroscopy in the frequency range 7–4000 cm^?1. The obtained spectra are analyzed in terms of the effective-medium theory with a size-dependent dielectric-response function of nanoinclusions and average dielectric parameters of the surronding medium. It is found that the dielectric properties of the inclusions are dependent on nano-size effects such as charge-carrier scattering at the boundaries of the nanocrystallites and increase in the band-gap energy due to the quantum size effect. The geometric and dielectric characteristics of silicon nano-size inclusions are determined. We consider the mechanisms of variation in the wide-band dielectric permittivity and conductivity spectra of monocrystalline silicon during transformation of its structure from monocrystalline to nano-porous.     

Nuclear reaction spectroscopy of vibrational modes of solids

The effects of target-atom vibrations on nuclear reaction kinetics have been examined. In particular, Doppler broadening of resonance-type and elastic-scattering reactions has been considered in detail. Depending on the specific process parameters, the resulting energy widths can be many orders of magnitude larger than the energies of the vibrational states of the target solid. Comparison of experimental results on a hydrogen-bearing thin target with the theoretical predictions shows excellent agreement.     

Infrared spectroscopy with synchrotron radiation

Up to now, storage rings have been used as sources of radiation on the X-ray and the u.v. part of the spectrum. It is shown that, with a specially designed part, a storage ring like ACO at Orsay is a very powerful far-infrared source, whose advantages over classical wide band sources are reviewed.     

Oxygen-17 cross-polarization nmr spectroscopy of inorganic solids

We have obtained ¹⁷O nuclear magnetic resonance spectra of a variety of ¹⁷O-labeled solids (Mg(OH)₂, Ca(OH)₂, boehmite (AlO(OH)), talc (Mg₃Si₄O₁₀(OH)₂), (C₆H₅)₃SiOH, and amorphous SiO₂) using high-field static and “magic-angle” sample spinning techniques, together with ¹H cross polarization and dipolar decoupling. Our results show that large cross-polarization enhancements can be obtained and that reliable second-order quadrupolar powder lineshapes can be observed under cross-polarization conditions. We have also investigated the dynamics of cross polarization for several samples, including measurements of cross-relaxation rates and ¹H and ¹⁷O rotating-frame spin-lattice relaxation times. We show that rapid ¹⁷O rotating-frame spin-lattice relaxation reduces the cross-polarization enhancement in some cases and that differences in cross-relaxation rates can be used to “edit” spectra by selectively enhancing protonated oxygen resonances (in general, hydroxide versus oxide ions, in inorganic solids). When applied to high surface area metal oxides such as amorphous silica, this selectivity enables the observation of resonances from surface hydroxyl groups that are difficult to detect by conventional ¹⁷O NMR. Overall, the cross-polarization approach appears to have considerable utility for aiding in the interpretation of ¹⁷O NMR spectra of complex inorganic solids.     

Photoacoustic spectroscopy of solids immersed in transparent liquids

A method is developed to obtain the photoacoustic spectra of solids by immersing them in a liquid cell with a submersed tranducer and detecting the acoustic pulse generated due to bulk absorption of pulsed laser radiation in the solids. Proper choice of liquids enables spectra to be obtained which are independent of the optical properties of the solid surfaces of the surrounding liquid. This technique is compared to other established PAS methods.     

Automatic spectroscopy of solids in the low-frequency range

An automatic low frequency spectrometer is described, which gives plots of impedance, admittance and dielectric data versus the logarithm of the frequency as well as complex-plane plots of these data in the frequency range 100 Hz to 10 MHz and in the temperature range from 220 to 470 K. The development of this spectrometer was started in 1975 using a processor-controlled synthesizer and gain/phase-meter. Some typical applications are summarized.