Profile spectra

Figures (a) Scattering geometry for ERS (b) ERS spectrum from 200-A partially deuterated polystyrene on Si, E h 3.0 MeV (adapted from ref. 101 and (cl schematic ERS depth profile spectrum.

Fig. 1.3. Three representations of the molecular ion signal in the field desorption mass spectrum (Chap. 8) of tetrapentacontane, C54H110 (a) profile spectrum, (b) bar graph representation, and (c) tabular listing.

RJ Thrash, HLB Fang and GE Leroi (1977) The Raman excitation profile spectrum of -carotene in the... 

Because of the two frequencies, Wj and Wg, that enter into the Raman spectrum, Raman spectroscopy may be thought of as a two-dimensional fomi of spectroscopy. Nomially, one fixes oij and looks at the intensity as a frmction of tOj, however, one may vary tOj and probe the intensity as a frmction of tOj - tOg. This is called a Raman excitation profile. 

One group has successfiilly obtained infonnation about potential energy surfaces without measuring REPs. Instead, easily measured second derivative absorption profiles are obtained and linked to the fiill RRS spectrum taken at a single incident frequency. In this way, the painstaking task of measuring a REP is replaced by carefiilly recording the second derivative of the electronic absorption spectrum of the resonant transition [, 59],... 

Fig. 7.12 Experimental and calculated infrared spectra for liquid water. The black dots are the experimental values. The thick curve is the classical profile produced by the molecular dynamics simulation. The thin curve is obtained by applying quantum corrections. (Figure redrawn from Guilbt B 1991. A Molecular Dynamics Study of the Infrared Spectrum of Water. Journal of Chemical Physics 95 1543-1551.)...

A laser pulse strikes the surface of a specimen (a), removing material from the first layer, A. The mass spectrometer records the formation of A+ ions (b). As the laser pulses ablate more material, eventually layer B is reached, at which stage A ions begin to decrease in abundance and ions appear instead. The process is repeated when the B/C boundary is reached so that B+ ions disappear from the spectrum and C+ ions appear instead. This method is useful for depth profiling through a specimen, very little of which is needed. In (c), less power is used and the laser beam is directed at different spots across a specimen. Where there is no surface contamination, only B ions appear, but, where there is surface impurity, ions A from the impurity also appear in the spectrum (d). 

This last m/z value is easy to measure accurately, and, if its relationship to the true mass is known (n = 10), then the true mass can be measured very accurately. The multicharged ions have typical m/z values of <3000 Da, which means that conventional quadrupole or magnetic-sector analyzers can be used for mass measurement. Actually, the spectrum consists of a series of multicharged protonated molecular ions [M + nWY for each component present in the sample. Each ion in the series differs by plus and minus one charge from adjacent ions ([M + uH] + n -an integer series for example, 1, 2, 3,. .., etc.). Mathematical transformation of the spectrum produces a true molecular mass profile of the sample (Figure 40.5). 

Single spectrum, 0.1 s nominal 100-A profile, 30 min Outermost monatomic layer to any sputtered depth 150 pm Yes, limited... 

AES analysis is done in one of four modes of analysis. The simplest, most direct, and most often used mode of operation of an Auger spectrometer is the point analysis mode, in which the primary electron beam is positioned on the area of interest on the sample and an Auger survey spectrum is taken. The next most often used mode of analysis is the depth profiling mode. The additional feature in this mode is that an ion beam is directed onto the same area that is being Auger analyzed. The ion beam sputters material off the surface so that the analysis measures the variation, in depth, of the composition of the new surfaces, which are being continu-... 

Because a FIXE spectrum represents the int al of all the X rays created along the particle s path, a single FIXE measurement does not provide any depth profile information. All attempts to obtain general depth profiles using FIXE have involved multiple measurements that varied either the beam energy or the angle between the beam and the target, and have compared the results to those calculated for assumed elemental distributions. Frofiles measured in a few special cases surest that the depth resolution by nondestructive FIXE is only about 100 nm and that the absolute concentration values can have errors of 10-50%. 

Figure 2 Typical ISS apectral data obtained from routine depth profile of cleaned washed steel. Left spectrum represents surface. Right spectrum represents about 50-A depth. Expansiona ahown top left. Relative atomic compositions plotted top right.

Several features of ISS quantitative analysis should be noted. First of all, the relative sensitivities for the elements increase monotonically with mass. Essentially none of the other surface spectroscopies exhibit this simplicity. Because of this simple relationship, it is possible to mathematically manipulate the entire ISS spectrum such that the signal intensity is a direct quantitative representation of the surface. This is illustrated in Figure 5, which shows a depth profile of clean electrical connector pins. Atomic concentration can be read roughly as atomic percent direcdy from the approximate scale at the left. 

In quadrupole-based SIMS instruments, mass separation is achieved by passing the secondary ions down a path surrounded by four rods excited with various AC and DC voltages. Different sets of AC and DC conditions are used to direct the flight path of the selected secondary ions into the detector. The primary advantage of this kind of spectrometer is the high speed at which they can switch from peak to peak and their ability to perform analysis of dielectric thin films and bulk insulators. The ability of the quadrupole to switch rapidly between mass peaks enables acquisition of depth profiles with more data points per depth, which improves depth resolution. Additionally, most quadrupole-based SIMS instruments are equipped with enhanced vacuum systems, reducing the detrimental contribution of residual atmospheric species to the mass spectrum. 

In nonresonant profiling, the silicon surface barrier detectors that detect the products of the nuclear reaction may also detect signals from incident ions that have been backscattered from the sample. Figure 4 shows an a particle spectrum from the reaction (p, a) along with the signal produced by backscattered... 

Figure 4 Spectrum of diffusion in the mineral olivine ((Mg, Fe)2 SiO ) taken using nonresonant profiling technique with the reaction (p, a) Both the a particles resulting from the nuclear reaction and backscattered protons are collected. Inset shows expanded region of the spectrum, where a yield indicates diffusion of into the material.

State-of-the-art for data evaluation of complex depth profile is the use of factor analysis. The acquired data can be compiled in a two-dimensional data matrix in a manner that the n intensity values N(E) or, in the derivative mode dN( )/d , respectively, of a spectrum recorded in the ith of a total of m sputter cycles are written in the ith column of the data matrix D. For the purpose of factor analysis, it now becomes necessary that the (n X m)-dimensional data matrix D can be expressed as a product of two matrices, i. e. the (n x k)-dimensional spectrum matrix R and the (k x m)-dimensional concentration matrix C, in which R in k columns contains the spectra of k components, and C in k rows contains the concentrations of the respective m sputter cycles, i. e. ... 

Chemical effects are quite commonly observed in Auger spectra, but are difficult to interpret compared with those in XPS, because additional core levels are involved in the Auger process. Some examples of the changes to be seen in the KLL spectrum of carbon in different chemical environments are given in Fig. 2.24 [2.130]. Such spectra are typical components of data matrices (see Sect. 2.1.4.2) derived from AES depth profiles (see below). 

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