Resonance neutralization

A second type of neutralization occurs through a resonance process, in which an electron from the sample tunnels to the empty state of the ion, which should then be at about the same energy. Resonance neutralization becomes likely if the electron affinity of the ion is somewhat larger than the work function of the sample, or if the ion has an unfilled core level with approximately the same energy as an occupied level in the target atom. The latter takes place when He+ ions come near indium, lead or bismuth atoms. The inverse process can lead to reionization. 

The probability that neutralization takes place depends on the energy of the ion, simply because a slow ion is in the vicinity of an atom for a longer time. The maximum distances at which neutralization processes are thought to occur are on the order of 0.2 nm for Auger and 0.5 nm for resonance neutralization. 

In this section, we shaU outline a many-electron treatment of charge transfer, similar in spirit to that of Tully, which enables different charge-exchange mechanisms to be incorporated in the formalism simultaneously. Although we shall concentrate on the TDAN model of resonant neutralization and negative ionization, we shall indicate how other neutralization processes can be included, and the approach for the reverse process of positive ionization will be fairly apparent. 

We shall, for a moment, discuss how to expand the above set of basis functions to include quasi-resonant and Auger phenomena. For quasi-resonant neutralization, the TDAN Hamiltonian (9) must be modifiedso that the ion orbital coq is coupled to a solid core orbital Xd (of energy e ) i.e. to jf t) must be added the terms... 

As a cation approaches a metal or semiconducting surface, an electron from the solid may resonantly tunnel through the vacuum to fill a vacant level on the ion. This process of resonant neutralization becomes facile, when (1) the affinity level of the ion is resonant with the occupied levels of the solid, and (2) the ion is sufficiently proximate to the surface... 

An alternative mechanism to resonant neutralization is Auger neutralization. The latter process involves a two-electron reorganization whereby one electron from the solid tunnels across to the ion while a second electron is excited out of the conduction band of the metal. If the second electron has sufficient energy, it may be ejected from the surface and is referred to as an Auger electron. 

This section is devoted to the analysis of Auger processes, treated independently of resonant processes. Such an analysis is relevant in the case of low-lying atomic energy levels that cannot be resonantly neutralized, like He on Al at distances larger than 1 a.u. 

The most widely used laser photoionization technique is undoubtedly REMPI. This technique yields valuable spectroscopic information on the resonant (neutral) intermediate states involved, but generally yields little or no information on the ionization step itself. The practical details and virtues of the REMPI technique have been described earlier, and the reader is referred to Chapter 9 for further discussion. 

Figure 3.25 Potential energy diagrams representative of (a) resonant charge transfer (RI = Resonant Ionization of a neutral atom and RN = Resonant Neutralization of a positive ion), (b) Qnasi-resonant charge transfer (qRN = quasi-Resonant Neutralization of a positive ion) and Auger charge transfer (AN-Auger Neutralization of a positive ion). The dashed arrows represent electron transfer from populated to vacant electron levels, whereas the horizontal lines represent the allowed electron levels, otherwise referred to as stationary states.

A iiseUfl light source is the helium resonance lamp which produces light of wavelength 58.4 nm or a photon energy of 21.2 eV, enough to ionize any neutral molecule. Often several peaks can be observed in the photoelectron spectnim... 

Time-resolved spectroscopy has become an important field from x-rays to the far-IR. Both IR and Raman spectroscopies have been adapted to time-resolved studies. There have been a large number of studies using time-resolved Raman [39], time-resolved resonance Raman [7] and higher order two-dimensional Raman spectroscopy (which can provide coupling infonuation analogous to two-dimensional NMR studies) [40]. Time-resolved IR has probed neutrals and ions in solution [41, 42], gas phase kmetics [42] and vibrational dynamics of molecules chemisorbed and physisorbed to surfaces [44]- Since vibrational frequencies are very sensitive to the chemical enviromnent, pump-probe studies with IR probe pulses allow stmctiiral changes to... 

As with the quadmpole ion trap, ions with a particular m/z ratio can be selected and stored in tlie FT-ICR cell by the resonant ejection of all other ions. Once isolated, the ions can be stored for variable periods of time (even hours) and allowed to react with neutral reagents that are introduced into the trapping cell. In this maimer, the products of bi-molecular reactions can be monitored and, if done as a fiinction of trapping time, it is possible to derive rate constants for the reactions [47]. Collision-induced dissociation can also be perfomied in the FT-ICR cell by tlie isolation and subsequent excitation of the cyclotron frequency of the ions. The extra translational kinetic energy of the ion packet results in energetic collisions between the ions and background... 

Electron delocalization can be important in ions as well as in neutral molecules Using curved arrows show how an equally stable resonance structure can be generated for each of the following anions... 

The most common modes of operation for ms/ms systems include daughter scan, parent ion scan, neutral loss scan, and selected reaction monitoring. The mode chosen depends on the information required. Stmctural identification is generally obtained using daughter or parent ion scan. The mass analyzers commonly used in tandem systems include quadmpole, magnetic-sector, electric-sector, time-of-flight, and ion cyclotron resonance. Some instmments add a third analyzer such as the triple quadmpole ms (27). 

Proton loss from alkyl groups a or 7 to a cationic center in an azolium ring is often easy. The resulting neutral anhydro bases or methides (cf. 381) can sometimes be isolated they react readily with electrophilic reagents to give products which can often lose another proton to give new resonance-stabilized anhydro bases. Thus the trithione methides are anhydro bases derived from 3-alkyl-l,2-dithiolylium salts (382 383) (66AHC(7)39). These... 

Continuous (i) An isolated neutral system or (ii) A resonant grounded system Between line and ground... 

During faults such as during a ground fault in a resonant grounded system or an isolated neutral grounded system. 

From this equaiion one can determine the required value of neutral circuit impedance for a particular level of ground fault current. The external impedance will be Z, less the ground impedance. In HT systems one c in also delermine the likely value of a ground inductor coil to achieve a near-resonance condition, to eliminate the arcing grounds, on the one hand, and facilitate a strike-free extinction of an arc hy the interrupting device, on the other. 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

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