Photoionization ionization

Nonsymmetric charge transfer Symmetric (resonance) charge transfer Dissociative charge transfer Associative ionization Photoionization Cumulative ionization... 

TID) or nitrogen/phos (NPD) or alkali flame ionization Photoionization (PID) chemical ionization electric or flame heated UV lamp causes Aromatics 24... 

Table 5. Vibrational Distributions of NO (A) in the HeJ/NO Charge-Transfer, Electron-Impact Ionization, Photoionization, He(2 S)/NO Penning Ionization, Franck-Condon Factors for the NO(X) —> NO (A) Ionization...

The principle of both of these techniques is to excite the atoms of the substance to be analyzed by bombarding the sample with sufficiently energetic X-rays/y-rays or charged particles. The ionization (photoionization for XRF and ionization caused due to Coulomb-interaction in case of PIXE) of inner-shell electrons is produced by the photons and charged particles, respectively. When this interaction removes an electron from a specimen s atom, frequently an electron from an outer shell (or orbital) occupies the vacancy. The distribution of electrons in the ionized atom is then out of equilibrium and within an extremely short time s) returns to the normal state, by transitions... 

In the technique of multiphoton-ionization spectroscopy, two or more photons excite atoms or molecules from the ground state to an excited state which may be ionized by several methods (see Sect.8.2.5), e.g., field ionization, photoionization, collisional, or surface ionization. If the laser is tuned to multiphoton resonances, ionization signals are obtained if the upper level is ionized which can be, for instance, monitored with the setup shown in Fig.8.42. The ionization probe is a thin wire inserted into a pipe containing the atomic vapor. If the probe is negatively biased relative to the walls of the pipe, thermionic emission will lead to space-charge-limited current. Ions produced by the laser excitation partly neutralize the space charge, thereby allowing an increased electron current to flow (see Sect.8.2.4). 

Fragments are ionized by absorption of a single vacuum ultraviolet photon with an energy just above the ionization threshold. Unlike field ionization which may produce significant amounts of both protonated molecules (MH+) and radical ions (M ) during ionization, photoionization results only in the formation of radical cations. 

The temi action spectroscopy refers to those teclmiques that do not directly measure die absorption, but rather the consequence of photoabsorption. That is, there is some measurable change associated with the absorption process. There are several well known examples, such as photoionization spectroscopy [47], multi-photon ionization spectroscopy [48], photoacoustic spectroscopy [49], photoelectron spectroscopy [, 51], vibrational predissociation spectroscopy [ ] and optothemial spectroscopy [53, M]. These teclmiques have all been applied to vibrational spectroscopy, but only the last one will be discussed here. 

A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

Time-of-flight mass spectrometers have been used as detectors in a wider variety of experiments tlian any other mass spectrometer. This is especially true of spectroscopic applications, many of which are discussed in this encyclopedia. Unlike the other instruments described in this chapter, the TOP mass spectrometer is usually used for one purpose, to acquire the mass spectrum of a compound. They caimot generally be used for the kinds of ion-molecule chemistry discussed in this chapter, or structural characterization experiments such as collision-induced dissociation. Plowever, they are easily used as detectors for spectroscopic applications such as multi-photoionization (for the spectroscopy of molecular excited states) [38], zero kinetic energy electron spectroscopy [39] (ZEKE, for the precise measurement of ionization energies) and comcidence measurements (such as photoelectron-photoion coincidence spectroscopy [40] for the measurement of ion fragmentation breakdown diagrams). 

Herrmann A, Leutwyler S, Schumacher E and Woste L 1978 On metal-atom clusters IV. Photoionization thresholds and multiphoton ionization spectra of alkali-metal molecules Hel. Chim. Acta 61 453... 

Yang S and Knickelbein M B 1990 Photoionization studies of transition metal clusters ionization potentials for Fe... 

Koretsky G M and Knickelbein M B 1997 Photoionization studies of manganese oiusters ionization potentiais for Mn-,to Mn, J. Chem. Phys. 106 9810... 

The photoionization process with which we shall be concerned in both UPS and XPS is that in Equation (8.4) in which only the singly charged is produced. The selection mle for such a process is trivial - all ionizations are allowed. 

In Surface Analysis by Laser Ionization (SALI), a probe beam such as an ion beam, electron beam, or laser is directed onto a surfiice to remove a sample of material. An untuned, high-intensity laser beam passes parallel and close to but above the sur-fiice. The laser has sufficient intensity to induce a high degree of nonresonant, and hence nonselective, photoionization of the vaporized sample of material within the laser beam. The nonselectively ionized sample is then subjected to mass spectral analysis to determine the nature of the unknown species. SALI spectra accurately reflect the surface composition, and the use of time-of-flight mass spectrometers provides fast, efficient and extremely sensitive analysis. 

SNMS sensitivity depends on the efficiency of the ionization process. SNs are post-ionized (to SN" ) either hy electron impact (El) with electrons from a hroad electron (e-)heam or a high-frequency (HF-) plasma (i.e. an e-gas), or, most efficiently, hy photons from a laser. In particular, the photoionization process enables adjustment of the fragmentation rate of sputtered molecules by varying the laser intensity, pulse width, and/or wavelength. 

Post-ionization occurs with the probability a (Xf note a photoion) ... 

Since 1960 mass spectrometry has always been an important tool to investigate the molecular composition of sulfur vapor, sulfur melts, and the solid dlotropes [201]. Mostly spectra obtained by electron impact (El) ionization have been reported, except for one study in which the main species present in sulfur vapor (S2-Ss) were studied by photoionization mass spectrometry [202]. The following ionization potentials were reported (in eV) [202] ... 

The ionization energy of gaseous disulfane has been determined by photoionization efficiency spectroscopy as 9.40 0.02 eV [25] and by photoelectron spectroscopy as 9.41 eV [61]. Recently, XANES spectra of H2S and H2S2 have been reported which show distinct differences [62]. 

In this contribution, in order to illustrate tlie importance of shake-up bands for extended systems, we simulate and compare on correlated grounds the ionization spectra of polyethylene and poly acetylene, the most simplest systems one can consider to represent insulating or semi-conducting polymers. Conclusions for the infinite stereoregular chains are drawn by exU apolation of the trends observed with the first terms of the related n-alkane or acene series, CnH2n+2 and CnHn+2. respectively, with n=2, 4, 6 and 8. Our simulations are also compared to X-ray photoionization spectra (7) recorded on gas phase samples of ethylene, butadiene and hexatriene, which provide a clear experimental manisfestation of the construction of correlation bands (8-12). 

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