Static spectra

Memorandum, OTSG, SGPS-PSP, 13 January 1994, subject The Sub-Radiofrequency Spectrum (Static to 3 kHz Band). 

Figure 11 Time-of-flight mass spectrum of multiphoton-ion-ized fullerenes. Upper spectrum static extraction, delayed ions give rise to asymmetric peaks. Lower spectrum pulsed extraction, delayed ions formed within a chosen time window are bunched into narrow peaks.

In one instrument, ions produced from an atmospheric-pressure ion source can be measured. If these are molecular ions, their relative molecular mass is obtained and often their elemental compositions. Fragment ions can be produced by suitable operation of an APCI inlet to obtain a full mass spectrum for each eluting substrate. The system can be used with the effluent from an LC column or with a solution from a static solution supply. When used with an LC column, any detectors generally used with the LC instrument itself can still be included, as with a UV/visible diode array detector sited in front of the mass spectrometer inlet. 

This compound has two crystallographically distinct vanadium sites. While the static spectrum is a superposition of two powder patterns of the kind shown in Figure 3, MAS leads to well-resolved sharp resonances. Weak peaks denoted by asterisks are spinning sidebands due to the quadrupolar interaction. 

One of the most common modes of characterization involves the determination of a material s surface chemistry. This is accomplished via interpretation of the fiag-mentation pattern in the static SIMS mass spectrum. This fingerprint yields a great deal of information about a sample s outer chemical nature, including the relative degree of unsaturation, the presence or absence of aromatic groups, and branching. In addition to the chemical information, the mass spectrum also provides data about any surface impurities or contaminants. 

Figure 1 shows a positive static SIMS spectrum (obtained using a quadrupole) for polyethylene over the mass range 0—200 amu. The data are plotted as secondary ion intensity on a linear y-axis as a function of their chaige-to-mass ratios (amu). This spectrum can be compared to a similar analysis from polystyrene seen in Figure 2. One can note easily the differences in fragmentation patterns between the...

As discussed in [91], the shape of a static spectrum determines significantly the spectral transformation as frequency exchange increases. In particular, spectral narrowing will take place only if the second moment of the spectrum is finite. In our case... 

The intensity at the periphery of the line ( Ageneral rule (2.62) [20, 104]. However, the most valuable advantage of general formula (3.34) is its ability to describe continuously the spectral transformation from a static contour to that narrowed by motion (Fig. 3.1). In the process of the spectrum s transformation its maximum is gradually shifted, the asymmetry disappears and it takes the form established by perturbation theory. 

It should be noted that the same method for calculation of isotropic scattering spectra is applied to spherical molecules as well. The only difference between linear and spherical molecules is the shape of the static spectrum, while its collapse proceeds in a qualitatively similar way. 

The quasi-classical description of the Q-branch becomes valid as soon as its rotational structure is washed out. There is no doubt that at this point its contour is close to a static one, and, consequently, asymmetric to a large extent. It is also established [136] that after narrowing of the contour its shape in the liquid is Lorentzian even in the far wings where the intensity is four orders less than in the centre (see Fig. 3.3). In this case it is more convenient to compare observed contours with calculated ones by their characteristic parameters. These are the half width at half height Aa)i/2 and the shift of the spectrum maximum ftW—< > = 5a>+A, which is usually assumed to be a sum of the rotational shift 5larger scale A determined by vibrational dephasing. 

It should be noted that there is a considerable difference between rotational structure narrowing caused by pressure and that caused by motional averaging of an adiabatically broadened spectrum [158, 159]. In the limiting case of fast motion, both of them are described by perturbation theory, thus, both widths in Eq. (3.16) and Eq (3.17) are expressed as a product of the frequency dispersion and the correlation time. However, the dispersion of the rotational structure (3.7) defined by intramolecular interaction is independent of the medium density, while the dispersion of the vibrational frequency shift (5 12) in (3.21) is linear in gas density. In principle, correlation times of the frequency modulation are also different. In the first case, it is the free rotation time te that is reduced as the medium density increases, and in the second case, it is the time of collision tc p/ v) that remains unchanged. As the density increases, the rotational contribution to the width decreases due to the reduction of t , while the vibrational contribution increases due to the dispersion growth. In nitrogen, they are of comparable magnitude after the initial (static) spectrum has become ten times narrower. At 77 K the rotational relaxation contribution is no less than 20% of the observed Q-branch width. If the rest of the contribution is entirely determined by... 

Firstly, we are going to demonstrate how branch interference may be taken into account within the quasi-classical impact theory. Then we shall analyse a quasi-static case, when the exchange frequency between branches is relatively small. An alternative case, when exchange is intensive and the spectrum collapses, has been already considered in Chapter 2. Now it will be shown how the quasi-static spectrum narrows with intensification of exchange. The models of weak and strong collisions will be compared with each other and with experimental data. Finally, the mutual agreement of various theoretical approaches to the problem will be considered. 

Pressure induced broadening and narrowing of a whole spectrum are described by the quasi-static approximation and the perturbation theory, correspondingly. Comparing inequalities (6.13) and (2.53) one can see that the border between the stages is determined by the criterion coij 1,... 

The pressure difference between the source of the mass spectrometer and the laboratory environment may be used to draw a solution, containing analyte and matrix material, through the probe via a piece of capillary tubing. When an adequate spectrum of the first analyte has been obtained, the capillary is simply placed in a reservoir containing another analyte (and matrix material) and the process repeated. This may therefore be used as a more convenient alternative to the conventional static FAB probe and this mode of operation may also benefit from the reduction in suppression effects if the analyte is one component of a mixture. 

Most Mossbauer spectra are split because of the hyperfine interaction of the absorber (or source) nuclei with their electron shell and chemical environment which lifts the degeneracy of the nuclear states. If the hyperfine interaction is static with respect to the nuclear lifetime, the Mossbauer spectrum is a superposition of separate lines (i), according to the number of possible transitions. Each line has its own effective thickness t i), which is a fraction of the total thickness, determined by the relative intensity W of the lines, such that t i) = Wit. 

One Mossbauer spectrum consists of 512 velocity channels (3 bytes per channel). One temperature interval consists of five Mossbauer spectra (one for each detector). There are 13 temperature intervals with selectable width. Thus, MIMOS II can accumulate simultaneously up to 65 Mossbauer spectra during one experiment session on Mars. All Mossbauer, energy, engineering, and temperature data taken during this session are stored in a volatile SRAM (Static Random Access... 

However, despite the rather dramatic change in coordination geometry that is observed upon comparing [TpBut Me]CuCl and [TpBut]CuCl (41), only rather minor perturbations are observed in comparing the structures of the Cud) dimers [TpBut]Cu 2 (37) and [TpBut,Me]Cu 2 (22). Thus, both the average Cu-N bond lengths and also the Cu - Cu separations in [TpBut Me]Cu 2 and [TpBut]Cu 2 are very similar. Nevertheless, although the coordination environment about each copper center is similar, the 5-methyl substituent does influence the fluxional nature of the molecule in solution. Thus, whereas [TpBut]Cu 2 is fluxional on the NMR time scale at room temperature, with a static structure that is only observed at -56°C, [TpBut Me]Cu 2 exhibits a static H NMR spectrum at room temperature. Furthermore, a static spectrum for... 

Static SIMS is appropriate for obtaining information on the lateral distribution of surface chemical species. A broad, defocussed ion beam is often used in order to minimise surface damage. In dynamic SIMS sample erosion takes place quite rapidly, and depth profiles are obtained by monitoring peak intensities in the mass spectrum of sputtered ions as bombardment proceeds. 

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