Field ionisation kinetics

The principle underlying time of flight (TOF) mass spectrometers is based on the relationship that exists between mass and velocity at a given kinetic energy. The instrument, which uses pulsed ionisation, measures the time taken by each mass to travel the length L of a field-free analyser tube. The basic equation (I6.l l) used in linear TOF analysers is obtained by eliminating the velocity v from equation (16.5) in conjunction with the relationship L = vt ... 

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... 

Ionisation Potential, Va-—This may he defined as the smallest difference of potential through which an electron must fall in an electric field in order that its kinetic energy, mn2/2, =eVA (e is the electronic charge), may be sufficient to raise an atom after collision from state (1) with energy Er to another possible quantum state (2) with energy E2. In changing back from quantum state (2) to (1) the atom will emit radiation of a frequency vA given by ... 

A Tesla coil is used to create a spark to generate the argon or helium plasma. The electrons generated oscillate in the microwave field and gain sufficient kinetic energy to ionise either gas by rapid and violent collisions. This is achieved by using a microwave frequency of 2500 MHz. Elements such as fluoride, chloride, bromide, iodide, sulphur, phosphorous, and nitrogen, which are not possible to measure by ICP-AES or DCP-AES, can be measured by MIP. 

Far from the ionisation threshold, where the escaping photoelectron has a large kinetic energy, the normal method of calculation for the continuum states is to compute the orbitals in a field determined by using the frozen orbitals of the neutral atom with one electron removed. However, if one is calculating a resonance which lies close to the threshold, this approach may fail. This happens because the escaping photoelectron moves slowly, so that the residual ion has time to relax as it escapes. It is then better to compute the orbitals in the relaxed field of the ion. This approximation is called the GRPAE or the RPAER, and is referred to as the RPAE with relaxation. 

The work that has to be done to remove an electron from a metal into vacuum with zero kinetic energy at zero K is termed the work function, and this is the same as the ionisation potential, but is larger than that of the free atom because of the space charge or surface dipole that exists at the surface, due to the asymmetry of electron density. Work function is greatest at planes having a high concentration of atoms (i.e. generally low-index planes), and decreases with step density at stepped surfaces. Variation of work function with crystal plane underlies the technique of Field-Emission Microscopy (FEM). 

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