Sample-radiation interaction

As the reflected radiation is emitted from the sample in a random direction, diffusely reflected radiation can be separated from, potentially sensor-blinding, specular reflections. Common techniques are off-angle positioning of the sensor with respect to the position(s) of the illumination source(s) and the use of polarisation filters. Application restrictions apply to optically clear samples with little to no scattering centres, thin samples on an absorbing background and dark samples. In either of these cases, the intensity of radiation diffusely reflected off such samples is frequently insufficient for spectral analysis. While dark objectives remain a problem, thin and/or transparent samples can be measured in transmission or in transflectance. 

Transmission arrangements for SI applications follow the classical transmission layout, with a radiation source on one side of a sample and the SI analyser on the other side. The information depth is either constant or can easily be measured, which is an advantage in particular for (semi-)quantitative analysis. Although occasionally applied, such transmission systems suffer from the restriction that the sample must either be freely suspended or rest on a support that is transparent in the investigated spectral range. 

Transflection is essentially a cross between transmission and reflection. When light is shined onto a reflective surface covered by an optically clear sample, two processes occur - reflection off the top surface of the sample and transmission and reflection off the mirror, followed by a second transmission. As for most materials the surface reflection is low in comparison to the reflection off the mirror, the total collected radiation corresponds to a transmission measurement with double pathlength. Instead of a transparent support, transflection systems require only a spectrally neutral, or at least constant, broadband reflector under the sample. 

The optical layout depends on the reflector. With diffusely reflecting reflectors, for example dense polytetrafluoroethylene (PTFE) or compacted titanium dioxide, 

For the analysis of clear organic coatings, in particular on metal, placing the SI analyser in a position where it records the specularly reflected radiation - attenuated by two transmissions through the material to be analysed - is preferable. As the metal under the coating acts as mirror reflector, such systems have high reliability and low maintenance requirements, provided the reflection properties of the coated metal remain reasonably constant. 

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In practice, surface modifications are restricted to sensors of the ATR- or FEWS-type. For other transducer layouts, the sample - radiation interaction is less localised, making a modification difficult to impossible. Depending on the analytes and the environment of the sensor, two basic surface modification strategies can be used to enhance the function of vibrational spectroscopic optical chemical sensors. The functional layers can either be... 

The radiation that results from nuclear de-excitation has to be distinguished from the prompt radiation, i.e., from the photons that pass through the sample without interaction and from those that are scattered by the electrons. This is achieved if three characteristic times (Ati, At2, and t) are properly related ... 

Radiation incident on a sample results in radiation interacting with the sample to be transmitted, reflected, refracted, absorbed or emitted. Almost any of these modes19 may be used for examining the absorbance characteristics in a microscopic configuration. Since microscopy places unique sampling demands upon an... 

Attenuated total reflection FTIR is a well-established technique for obtaining absorbance spectra of opaque samples. The mode of interaction is unique because the probing radiation is propagated in a high index-of-refraction internal reflection element (IRE). The radiation interacts with the material of interest, which is in close contact with the IRE, forming an interface across which a nonpropagating evanescent field penetrates the surface of the material of interest to a depth in the order of one wavelength of the radiation. The electric field at the interface penetrates the rarer medium in the form of an evanescent field whose amplitude decays exponentially with distance into the rarer medium. 

The sample absorbs microwave radiation and converts the absorbed electromagnetic radiation to heat that the sample retains. Hence, the sample can be brought to a very high temperature without needing to heat the surroundings. Not all materials absorb microwave radiation. As with any matter-electromagnetic radiation interaction, three possibilities exist. The material can (1) reflect the radiation, (2) transmit the radiation with minimal attenuation, or (3) absorb the radiation. 

In y-irradiated samples, radiation damage is predominantly caused by the cascade of electrons that result from the interaction of the y-ray... 

Concentrating on metalloenzymes, we have developed a strategy based on stopped flow X-ray absorption spectroscopy (XAS) to elucidate in detail the molecular mechanisms at work during substrate turnover (Fig. 4). Importantly, XAS provides local stmctural and electronic information about the nearest coordination environment surrounding the catalytic metal ion within the active site of a metalloprotein in solution. When the X-rays hit a sample, the electromagnetic radiation interacts with the electrons bound in the metal atom. The radiation can be scattered by these electrons, or it can be absorbed, thereby exciting... 

This operates by detecting the scintillations (fluorescence flashes ) produced when radiation interacts with certain chemicals called fluors. In solid (or external) scintillation counters (often referred to as gamma counters ) the radioactivity causes scintillations in a crystal of fluorescent material held close to the sample. This method is only suitable for radioisotopes producing penetrating radiation. 

When radiation interacts with a sample scattering or diffraction occurs due to spatial and temporal correlations in the sample. In this section, the basic quantities and correlation functions will be introduced. In elastic and quasi-elastic scattering experiments the most important quantity is the magnitude of the so-called scattering vector given... 

Even when the wavelength of the electromagnetic radiation is small relative to the length of the bulk molecular sample, it is often necessary to think of the matter ->radiation interaction in terms of a bulk polarization induced by the radiation and detected via one or more coherence terms of the sample polarization (Mukamel, 1995). When the wavelength is small relative to sample size,... 

Diffuse reflectance or DRIFTS (diffuse reflectance infrared Fourier-lransform spectroscopy) allows the sain)le to be analysed neat, ot diluted in a non-absorbing matrix (e.g. KCl or KBr at 1-5% w/w analyte). DRIFTS also may be used to obtain the spectrum of a solute in a volatile solvent by evaporating the solution onto KBr. When the IR radiation interacts with the powdered sample it will be absorbed, reflected and diffracted. The radiation which has been diffusely reflected contains vibrational information on the molecule. This technique allows non-destructive testing of neat materials and is suited to quantitative analysis, although care must be taken to ensure that a consistent particle size is used. 

The principle of optical spectroscopy involves the measurement of the amount of light (radiation) that is absorbed by the sample when the radiation interacts with the sample. The most basic method involves the determination of the fraction of the radiation that is actually transmitted through a sample. The aspects of the measurement, and their relationship to the actual absorption of radiation are illustrated in Fig. 56. In this example, 7o is the power of the incident radiation from the infrared light source, and I is the actual amount of radiation transmitted through the sample. The fundamental relationships are provided with Fig. 56, and these form the basis of a fundamental expression that is used to correlate the analytical spectrum with the amount(s) of material(s) present in a sample. This fundamental expression is a simple rendering of the Beer-Lambert-Bouguer law, which is used in one form or another in the quantitative determination of material composition. 

Lik qther types of electromagnetic radiation, when X-radiation passes through a sample of matter, the electric vector of the radiation interacts with the electrons... 

The formed dose-response functions were incorporated into a Computational Fluid Dynamics (CFD) software program, simulating the environment of a sample within a Phoenix-exposed IP/DP (Instrument Panel/Door Panel box) box, based on sun position and weather conditions, including radiation interactions. Observed local effects as well as the general ageing advance of PP hats are compared with respect to simulation and experiment. 

High-powered lasers have proved to be useful sources for the direct ablation of solids. In atomic emission spectrometry, ruby and Nd YAG lasers have been used since the 1970s for solids ablation. When laser radiation interacts with a solid, a laser plume is formed. This is a dense plasma containing both atomized material and small solid particles that have evaporated and or have been ejected from the sample due to atom and ion bombardment. The processes occurring and the figures of merit in terms of ablation rate, crater diameter (around 10 pm), and depth... 

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