Emission-ionization process

To examine a sample by inductively coupled plasma mass spectrometry (ICP/MS) or inductively coupled plasma atomic-emission spectroscopy (ICP/AES) the sample must be transported into the flame of a plasma torch. Once in the flame, sample molecules are literally ripped apart to form ions of their constituent elements. These fragmentation and ionization processes are described in Chapters 6 and 14. To introduce samples into the center of the (plasma) flame, they must be transported there as gases, as finely dispersed droplets of a solution, or as fine particulate matter. The various methods of sample introduction are described here in three parts — A, B, and C Chapters 15, 16, and 17 — to cover gases, solutions (liquids), and solids. Some types of sample inlets are multipurpose and can be used with gases and liquids or with liquids and solids, but others have been designed specifically for only one kind of analysis. However, the principles governing the operation of inlet systems fall into a small number of categories. This chapter discusses specifically substances that are normally liquids at ambient temperatures. This sort of inlet is the commonest in analytical work. 

Thirdly we are also interested in the electron spectroscopy method, which allows investigations on the two-center effects that influence electron emission. In particular, the richness of the ionization process lies in the possibility of measuring the doubly differential cross sections as a function of the electron emission angle and energy. This technique of electron emission spectroscopy is... 

This review illustrates the complementary nature of recoil-ion momentum spectroscopy, projectile scattering measurements, and conventional electron emission spectroscopy in ion-atom ionizing collisions. We have examined recent applications of both the CDW and CDW-EIS approximations from this perspective. We have shown that both models provide a flexible and quite accurate theory of ionization in ion-atom collisions at intermediate and high energies and also allows simple physical analysis of the ionization process from the perspective of these different experimental techniques. 

We must also take into account two further factors. First, the fact that the transmission efficiency of the analyzer is a fimction of the kinetic energy (K.E.) of the photoelectrons in the ESCA-3 Vacumn Generators instrument the transmission is inversely proportional to the K.E. of the electrons (3a). Second, photoelectron yields must refer to total yield from a particular ionization process and this need not, for example, be just the area of the relevant peak. Account must be taken of all processes that divert electrons from the primary peak, e.g., shake-up, shake-oflF, and plasmon peaks. In some cases, e.g., emission from the Cu 2P3/2 level, the contribution of additional processes is small but in others, and emission from the Al(2p) shell is an example, the no-loss peak is substantially less than the true Al(2p) emission. 

Two models can explain the events that take place as the droplets dry. One was proposed by Dole and coworkers and elaborated by Rollgen and coworkers [7] and it is described as the charge residue mechanism (CRM). According to this theory, the ions detected in the MS are the charged species that remain after the complete evaporation of the solvent from the droplet. The ion evaporation model affirms that, as the droplet radius gets lower than approximately 10 nm, the emission of the solvated ions in the gas phase occurs directly from the droplet [8,9]. Neither of the two is fully accepted by the scientific community. It is likely that both mechanisms contribute to the generation of ions in the gas phase. They both take place at atmospheric pressure and room temperature, and this avoids thermal decomposition of the analytes and allows a more efficient desolvation of the droplets, compared to that under vacuum systems. In Figure 8.1, a schematic of the ionization process is described. 

The main aim of this paper is to review the CDW-EIS model used commonly in the decription of heavy particle collisions. A theoretical description of the CDW-EIS model is presented in section 2. In section 3 we discuss the suitablity of the CDW-EIS model to study the characteristics of ultra-low and low energy electrons ejected from fast heavy-ion helium, neon and argon atom collisions. There are some distinct characteristics based on two-centre electron emission that may be identified in this spectrum. This study also allows us to examine the dependence of the cross sections on the initial state wave function of multi-electron targets and as such is important in aiding our understanding of the ionization process. 

When one considers the role of the matrix in the particle-induced emission of secondary ions it is no wonder that it is so difficult to unravel all the processes that take place. The matrix is the medium in which the primary excitation occurs. It must also disperse some of that energy to sites at the surface where secondary ion emission occurs. It must provide the species to be desorbed and at the same time mediate the ionization process. In an attempt to understand these complex coupled processes we have tried to simplify the system by first selecting a homogeneous substrate for the energy deposition and then studying the ionization-emission process for species that are present as a submonolayer on the surface (26). 

One of the most significant uses of LMI sources in connection with SIMS of organic compounds may be as probes in performing measurements of secondary particle yields. Such measurements are important for understanding the processes of secondary ion emission from solid and liquid organic samples. Total particle yields reflect directly the dynamical aspects of emission processes variations in primary beam energy, incident flux density, and primary particle mass, for example, are all manifested in changes in total particle yields. The ratio of secondary ion yield to total particle yield and the ratio of secondary ion yields from two different species can be sensitive, quantitative monitors of the chemistry and kinetics, respectively, of ionization processes. 

Two new fluorine-rich barium fluororochlorides, BaI2Cl5FI9 and Ba7Cl2FI2, have been recently obtained [76,77]. While in BaF2 the Eu2+ 5d - 4f emission is quenched by ionization processes (Sec. 10.5.1), in these fluorochlorides Eu2+ shows an intense emission. Only part of chlorine can be substituted by bromine. The efficiency of the photostimulated luminescence after X-ray irradiation increases with the bromine content. The new fluorohalides have higher chemical stability and are denser than BaCIF Eu and BaBrF Eu. 

Perhaps the most interesting application of electron-electron covariance mapping relates to the question of the major mode of multiple ionization of atoms and molecules. Luk et al. [34] studied the multiple ionization process in Xe using a laser of 193 nm wavelength and 10 ps pulse length and conventional ion TOP spectroscopy they suggested that it was direct (a collective, instantaneous emission of many electrons). Lambropoulos [35] pointed out that, with a laser of such modest rise time, the ionization must proceed sequentially. In fact L Huillier et al. [36] had also studied Xe at 532 nm and observed a knee in the curve of log (ion counts) vs log (laser intensity) for Xe that they attributed to direct double ionization. 

In the tunnelling regime, the total ionization might be expected to depend solely on the instantaneous electric field. However, the Corkum model [39] suggests that when two laser frequencies are applied to an atom, certain aspects of the ionization process depend, in addition, on the relative phases of the two fields. Using a mixture of fundamental and second harmonic of the Nd YAG laser, there is recent experimental evidence that the ATI spectra of Kr depend upon the relative phases of the fields [46]. In particular there is a considerable forward-backward asymmetry in the emission of the higher energy electrons. 

The K(3 IKOi x-ray intensity ratio is an easily measurable quantity with relatively high precision and has been studied extensively for /f-x-ray emission by radioactive decay, photoionization, and charged-particle bombardment (1-3). Except for the case of heavy-ion impact where multiple ionization processes are dominant, it is generally accepted that this ratio is a characteristic quantity for each element. The experimental results are usually compared with the theoretical values for a single isolated atom and good agreement is obtained with the relativistic self-consistent-field calculations by Scofield (4). 

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