Electron Energy Analyzer

The electron energy analyzer is the heart of a PE spectrometer. It is here that the electrons are discriminated with respect to their kinetic energy. The most important features of an electron kinetic energy analyzer (EKEA) are sensitivity and resolution. High resolution and high sensitivity contradict each other, and a given analyzer will always be a compromise between them. As chemical shifts are often small (< 0.5 eV) the spectral resolution, AE, of an analyzer used in XPS should not...

Anisotropic Hyperfine Interaction

The general spin Hamiltonian was given by Eq. (2), in which the interaction parameters were written in the general tensor form. The total hyperfine tensor A can be represented by a 3 x 3 matrix that connects the three components each of the electron spin angular momentum and the nuclear spin angular momentum. The hyperfine tensor is a real matrix and can always be diagonalized. Thus a general tensor referenced to an x-y-z axis system can be written as This general tensor can be transformed to...

Evaluation Of The Moments Of Inertia

The first step in the study of a rotational spectrum is to evaluate the moments of inertia, or rotational constants, from which the rigid rotor spectrum (discussed in the following section) can be predicted. The rotational problem is treated mathematically in terms of a molecule-fixed axis system with its origin at the center of mass of the molecule and its axes oriented along the principal inertial axes. With respect to these axes the moments of inertia are constant, and the intertia matrix is...

Vibrational Fine Structure

The low-energy PE spectra of small and medium size molecules often contain bands with well-resolved vibrational fine structure. Since the resolution in UPS is usually limited to about 150 cm-1 ( 15 meV), only excitations into well-separated vibrational states of the final ion state can be observed (see, however, Section I.I). As long as the temperature of the sample is not much higher than 300 K, vibrational excitations in the initial states do not perturb the observed spectra low-frequency...

Centrifugal Distortion Effects

The rigid-rotor treatment discussed in the previous section accounts for the general features of the rotational spectrum. These gross features are modified somewhat when the effects of nonrigidity, nuclear coupling, and so forth are taken into account. In this section, the effects of centrifugal distortion are considered. The centrifugal force produced by rotation distorts the molecule from its equilibrium configuration and the bond distance and angles change slightly. Hence, the rotational...

Derivatization Gas Chromatography

Conversion of sample compounds into volatile derivatives make it possible to separate and analyze by GC, groups of compounds for which GC analysis would otherwise be impossible, e.g., amino acids, sugars, prostaglandins, and related compounds. The presence of different polar groups in the molecules of such parent compounds is the most significant source of difficulty associated with their GC analysis. Carboxyl, hydroxyl, carbonyl, and amino groups, because of their polarity and tendency to form...

Linear and Symmetric Top Molecules

We now consider the Stark effect quantitatively for linear and symmetric-top molecules. The first-order effect possible for a symmetric top is given by standard first-order perturbation theory as for the level J, K, M, where the matrix element is evaluated in the symmetric rotor basis J, K, M). The rotational motion is like that of a top spinning about its symmetry axis and also precessing about the vertical Z axis. The frequency displacement is found by applying the selection rules J J + 1, K...

Michael B Eyring

Forensic Spectroscopy Techniques Chromatography A term that literally means graphing color or separating colors from a mixture. Today it refers to a wide variety of methods that are used to separate mixtures of compounds so that the separate components can be analyzed and identified. Dispersive system A component of a spectroscopy system that allows small portions of a spectrum to be separated, identified, and analyzed. A prism disperses white light into its colored components so that they...

The Photoelectric Effect and Secondary Processes

X-ray photoelectron spectroscopy (XPS) operates on the principle of the photoelectric effect, which occurs via a primary excitation process brought about by X-ray-irradiation producing electrons (photoelectrons) of discrete energy, containing chemical information regarding the surface analyte. It should be noted that X-rays are only one of many types of excitation sources that can be used to induce emission of electrons for analysis. X-ray photo-electron (XP) spectral peaks (generated by the...

Determination Of Carbon Hydrogen And Nitrogen

Oxygen Combution Flask

Determination of Carbon and Hydrogen With few exceptions, organic compounds always contain both carbon and hydrogen. When an organic compound is decomposed by heating at high temperatures (combustion) in the presence of oxygen, carbon dioxide and water are produced The resultant water vapor and carbon dioxide can be collected sequentially in separate receivers (absorption tubes). From the weights of carbon dioxide and water obtained, the percentages of carbon and hydrogen are calculated by...

Electrolysis And Voltammetry

Linear Sweep Voltammetry

For chemists, the second important application of electrochemistry (beyond potentimetry) is the measurement of species-specific e.g., iron(III) and iron(II) concentrations. This is accomplished by an experiment whereby the electrolysis current for a specific species is independent of applied potential (within narrow limits) and controlled by mass transfer across a concentration gradient, such that it is directly proportional to concentration (i kC). Although the contemporary methodology of...

Clinical Applications Of Magnetic Resonance Imaging

Once clinical MR scanners became available certain advantages and disadvantages of their use became established. Among the chief advantages of MR are the ability to image almost any region of the body, the very high contrast available between soft tissue structures, the ability to vary the plane of imaging at will, the ability to vary the tissue appearance by varying the scan parameters, the lack of the need for any invasive step such as the injection of contrast agents, and the absence of any...

Photoelectron Spectra of Solids

Escape Depth Photoelectrons

In the case of solids some special aspects, such as the very limited escape depth, must be considered. A photo-electron created inside a solid must escape into the vacuum to be measured. Since the probability of inelastic scattering is very high as long as the electron moves inside the solid, only electrons created close to the surface have a chance to escape without a secondary energy loss. Figure 4 shows the average escape depth as a function of the kinetic energy of the electron together...

Spin Relaxation

The energy between the magnetic energy levels at 3000 G, gjH, is only 10-3 of kT at 300 K. At thermal equilibrium the Boltzmann factor, exp(-gjH kt), gives the population ratio of the two levels, so the levels are almost equally populated. The application of microwave energy causes transitions between the magnetic levels. The microwave field stimulates transitions in both directions with a probability that depends on the microwave power and on the number of spins in each level. Transitions from...

Basic Experimental Techniques

Though normal Raman spectroscopy is a very selective technique for chemical analysis, there are some serious experimental disadvantages related to the sensitivity, large fluorescence interference, and lack of time resolution of the technique. These weaknesses have been addressed in the creation of new Raman-based techniques. The weak Raman signals due to inherently small Raman scattering efficiencies has been addressed by resonance Raman, surface-enhanced Raman and SPP-Raman techniques. Fourier...

Daniel Frisbie

Amplitude Setpoint Tapping Mode

Applications of Scanning Tunneling Microscopy (STM) III. Applications of Atomic Force Microscopy (AFM) IV. Other SPM Techniques and Applications Cantilever A projecting beam supported only at one end, the deflection of which is directly proportional to the force applied normal (perpendicular) to the beam axis. Constant-current feedback control Regulation of the piezoelectric scanner position based on the magnitude of the electron tunneling current in a scanning tunneling microscope....

Angle Resolved Photoelectron Spectra

A different situation occurs if a molecule is adsorbed at a surface and thereby fixed in space. For example, consider a rodlike molecule (e.g., CO) which, for a given final state M+, emits electrons preferentially in the direction of the molecular axis. In addition, assume that the emission probability is proportional to cos2 of the angle between molecular axis and electric vector E of the ionizing radiation. The system is fully described by three angles and the polarization of the radiation....

Magnetic Resonance As A Physical Phenomenon

Spin And Orbital Motion Electrons

Although the term nuclear magnetic resonance has been in use in the technical community since before World War II, some changes in terminology have accompanied the onset of the medical applications in the 1980s. Partly to avoid any fear on the part of patients that radioactive isotopes might be involved (they are not) and partly to forestall any confusion with the distinct discipline of nuclear medicine (which does utilize radioactive isotopes), the adjective nuclear is often dropped. The term...

Chloroform Overtone Spectra

Nacl Bond Ftir Spectrum

Absorbance Vertical coordinate used for infrared spectra that is equal to the log of the reciprocal of the sample transmittance. Absorptivity Constant characterizing the capacity of a sample to absorb radiation of a specific wavelength, independent of sample thickness or concentration. Dipole moment Magnitude of the positive or negative charge constituting a dipole, multiplied by the spacing between the charges. Fermi resonance Quantum mechanical interaction between close-lying energy states of...

Droplet Countercurrent Chromatography

The technique of countercurrent chromatography (CCC) has seen a rapid expansion following the introduction of new methods such as droplet countercurrent chromatogra-phy (DCCC), rotation locular countercurrent chromatog-raphy (RLCC), and coil planet centrifugation. These methods have the advantage of being more rapid and less solvent consuming than traditional CCC. Furthermore, the advent of commercially available, compact apparatus has led to a widespread acceptance of these new liquid-liquid...

Info

And is usually abbreviated to the shorthand notation H4Y, with H representing the carboxylic hydrogen atoms. This reagent is extremely important because it forms simple 1 1 complexes with a metal ion, that is, one metal atom to one EDTA molecule of very high stability. Note that the latter is associated with the relatively large number (6) of points of attachment (4 x COO + 2 x N) of the ligand to the metal. In the now familiar pattern discussed above, the titration involves the buret addition...