Neutral particles

An electron or photon incident on a surface can induce an electroiuc excitation. When the electroiuc excitation decays, an ion or neutral particle can be emitted from the surface as a result of the excitation. Such processes are known as desorption induced by electroiuc transitions (DIET) [82]. The specific teclmiques are known as electron-stimulated desorption (ESD) and photon-stimulated desorption (PSD), depending on the method of excitation. 

A further consequence of the high temperatures is that much of the sample is simply evaporated without producing isolated positive ions. There is a competition between formation of positive ions and the evaporation of neutral particles. Since the mass spectrometer examines only isolated charged species, it is important for maximum sensitivity that the ratio of positive ions to neutrals be as large as possible. Equation 7.1 governing this ratio is given here. 

Schematic diagram showing the development of a dipolar field and ionization on the surface of a metal filament, (a) As a neutral atom or molecule approaches the surface of the metal, the negative electrons and positive nuclei of the neutral and metal attract each other, causing dipoles to be set up in each, (b) When the neutral particle reaches the surface, it is attracted there by the dipolar field with an energy Q,. (c) If the values of 1 and <() are opposite, an electron can leave the neutral completely and produce an ion on the surface, and the heat of adsorption becomes Q,. Similarly, an ion alighting on the surface can produce a neutral, depending on the values of I and <(), On a hot filament the relative numbers of ions and neutrals that desorb are given by Equation 7.1,which includes the difference, I - <(), and the temperature, T,...

Formation of normal ions in an ion source. In this example, some initially formed ions (m,+) dissociate (fragment) to give smaller ions (mj ) and a neutral particle (iio). Unchanged ions (m +) and the fragment ions (mj" ) are drawn out of the source as beams moving with velocities v, V2, respectively. 

In a (B/E)(l - E) -scanning mode, a mass difference is seiected. For exampie, in this case a precursor ion m, is chosen (it is shown as being made up of two parts of mass mj, n,). After fragmentation, the product ion is mj accompanied by a neutral particle of mass n,. The mass difference (n, = m, - mj) can be specified so only pairs of ions connected by this difference are found. 

Talc and Pyrophyllite. Talc (qv) and pyrophjlhte are 2 1 layer clay minerals having no substitution in either the tetrahedral or octahedral layer. These are electrostatically neutral particles (x = 0) and may be considered ideal 2 1 layer hydrous phyUosiHcates. The stmctural formula of talc, the trioctahedral form, is Mg3Si402Q(0H)2 and the stmctural formula of pyrophylUte, the dioctahedral form, is Al2Si402Q (OH)2 (106). Ferripyrophyllite has the same stmcture as pyrophylUte, but has ferric iron instead of aluminum in the octahedral layer. Because these are electrostatically neutral they do not contain interlayer materials. These minerals are important in clay mineralogy because they can be thought of as pure 2 1 layer minerals (106). 

Formal Theory A small neutral particle at equihbrium in a static elecdric field experiences a net force due to DEP that can be written as F = (p V)E, where p is the dipole moment vecdor and E is the external electric field. If the particle is a simple dielectric and is isotropically, linearly, and homogeneously polarizable, then the dipole moment can be written as p = ai E, where a is the (scalar) polarizability, V is the volume of the particle, and E is the external field. The force can then be written as ... 

An example of a practical dielec trofilter which uses both of the features described, namely, sharp electrodes and dielectric field-warping filler materials, is that described in Fig. 22-34 [H. I. Hall and R. F. Brown, Lubric. Eng., 22, 488 (1966)]) It is intended for use with hydrauhc fluids, fuel oils, lubricating oils, transformer oils, lubricants, and various refineiy streams. Performance data are cited in Fig. 22-35. It must be remarked that in the opinion of Hall and Brown the action of the dielec trofilter was electrostatic and due to free charge on the particles dispersed in the hquids. It is the present authors opinion, however, that both elec trophoresis and dielectrophoresis are operative here but that the dominant mechanism is that of DEP, in wdiich neutral particles are polarized and attracted to the regions of highest field intensity. 

By examining the sputtered neutral particles (the majority channel) using nons-elective photoionization and TOFMS, SALI generates a relatively uniform sensitivity with semiquantitative raw data and overcomes many of the problems associated with SIMS. Estimates for sensitivities vary depending on the lateral spatial resolution for a commercial liquid-metal (Ga ) ion gun. Galculated values for SALI... 

Only a portion of the neutral particles present in the ionization volume (IV) of the laser beam is available for ionization. For this description the geometrical yield YjviX ) is introduced (for notation, see Sect. 3.1.3) ... 

Inorganic salts of metals work by two mechanisms in water clarification. The positive charge of the metals serves to neutralize the negative charges on the turbidity particles. The metal salts also form insoluble metal hydroxides which are gelatinous and tend to agglomerate the neutralized particles. The most common coagulation reactions are as follows ... 

Equilibrium in Proton Transfers. In each of the two examples that have been discussed in Sec. 49 the data were derived from a study of the equilibrium between a salt and its saturated solution. Let us next consider the conditions for equilibrium in the transfer of a proton, like that introduced in Sec. 17. In the process (28) four species are involved—two neutral particles and two ions. We may next recognize the fact that in... 

Hereby the initial positive ion becomes a neutral particle and the initial neutral particle becomes a new species of positive ion. The initial dilute solution must have contained sufficient negative ions to neutralize the positive charges these negative ions take no part in the reaction. As far as the ionic fields are concerned, the field of one species of positive molecular ion is replaced by that of another. 

The Electrostatic Energy. In Chapter 2 we drew attention to the fact that, when a proton transfer (117) has been carried out in a solvent, the electrostatic fields of two ions have been created and work must have been done to supply the amount of energy associated with these ionic fields. Let us now compare (117) with the process (123), both in aqueous solution at the same temperature. In both cases an (HaO)+ ion will be formed but in (123), when the proton is removed from the (IIS04)-ion, we have to separate the particles against the mutual attraction of the proton and the doubly charged ion (S04)". Consequently, more work must be done against the electrostatic forces of attraction than in the removal of a proton from a neutral particle. 

A plasma may be defined as a cloud of highly ionised gas, composed of ions, electrons and neutral particles. Typically, in a plasma, over 1 per cent of the total atoms in a gas are ionised. 

The observation that atoms of a single element can have different masses helped scientists refine the nuclear model still further. They realized that an atomic nucleus must contain subatomic particles other than protons and proposed that it also contains electrically neutral particles called neutrons (denoted n). Because neutrons have no electric charge, their presence does not affect the nuclear charge or the number of electrons in the atom. However, they do add substantially to the mass of the nucleus, so different numbers of neutrons in a nucleus give rise to atoms of different masses, even though the atoms belong to the same element. As we can see from Table B.l, neutrons and protons are very similar apart from their charge they are jointly known as nucleons. 

Ashkrn, A. (2000) History of optical trapping and manipulation of small-neutral particle, atoms, and molecules. IEEE J. Select. Topics Quantum Electron., 6, 841-856. 

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