Ions, ionisation 681 velocity

Effects arising from the partial decoupling of the flows of the ionised and neutral fluids were subsequently investigated by Mullan (1971) and by Draine (1980). The more general case of a magnetic field which is inclined relative to the shock front has been studied recently by Wardle and Draine (1987). The most important consequence of a magnetic field which is transverse to the flow is ion-neutral velocity drift, or ambipolar diffusion. 

Approximately 5% of the positive ions formed either before or in the ionisation chamber will reach the detector. The ions must first be accelerated through a potential difference, V, which can be as high as lOkV (Fig. 16.4). The process must be conducted under a good vacuum (P < 10 4 Pa) to avoid electrical arcing and to minimise collisions. Under these conditions, all ions with the charge q = ze will acquire the same kinetic energy kin(i) = zeV (equation (16.5)). Thus, their velocity after acceleration will be inversely proportional to the square root of their mass m, (equation (16.6)). 

This type of mass spectrometer, which is not widely used, allows mass determination with a high precision. An ion cyclotron resonance spectrometer is basically an ion trap ions formed by electron impact, for example, are subjected to the orthogonal magnetic field B, which induces cyclotronic movement in the. rv plane (Fig. 16.8). The radius of the circular movement, which depends on kinetic energy, is given by equation (16.2). If the velocity v is small and the magnetic field B is intense, the radius of the trajectory will be small and the ions will be trapped in the ionisation... 

Because of the velocity bunching effect due to initial acceleration the ion beam is nearly monokinetic, and the neutralisation does not effect the velocity distribution The details of the method can be found in [ KAUF 78 ], [ NUE 78] By neutralisation in an alkali vapour, the atomic metastable states are preferentially populated since their energies match the ionisation potential of the corresponding alkali atom Therefore this technic is ideally suited for laser spectroscopy of rare gas, and is recently successfully used to study the heaviest one, radon Fig. 

The total ionisation cross section for hydrogen has been measured by Shah et al. (1987) in a crossed-beam experiment. Slow ions formed as collision products in the interaction region were extracted by a steady transverse electric field. H+ ions were distinguished by time of flight. Relative cross sections were normalised to previously-measured cross sections for hydrogen ionisation by protons of the same velocity. The proton cross sections were normalised to the Born approximation at 1500 keV. 

We now proceed to take up in a httle more detail the two lines of investigation, or modes of treatment referred to above In doing so, it is to be clearly understood that the sections headed—Degree of Dissociation and the numerical value of the equivalent conductivity at infinite dilution, Osmotic Pressure and concentration in solutions of electrolytes and the calculation of the degree of ionisation, Reaction Velocity m relation to the osmotic pressure of ions and molecules, Ionisation in formic acid solutions—-these sections belong to the first mode of treatment of strong electrolytes, whilst the remaining sections are illustrative of the second method... 

In the second method, a laser is used to ionize the atomic fragment C at the same position where the dissociation laser is fired. Because no impulse is transferred to the atom during ionisation, the velocity of the ion is identical to that of the atom. The ions drift away from the point of dissociation with the velocity of the atoms and arrive at different times at the detector. As before, the peak intensities at different arrival times yield the information about the formation of AB in different quantum states. For the photodissociation of water along the B state the application of this technique revealed, for example, that the OH fragment is formed rotationally very hot. It should be emphasized that a direct laser detection of such hot OH radicals by LIF or REMPI is impossible. 

Figure 3.11 shows that the concentration of [Fe(CN)g] decreases more rapidly for solutions containing a higher concentration of NaNOj, that is, the velocity of the reaction increases with the ionic strength. This is what is expected for reactions between two ions of the same charge. One mechanism proposed for the reaction involves initial ionisation of the ascorbic acid (AHj), which rapidly and reversibly forms an ascorbate anion intermediate (AH"), which then subsequently transfers an electron to the hexacyanoferrate(III) in the slowest step in the mechanism ... 

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