Collisions, kinetic energy

An electron (or ion) in the gas is accelerated (gains kinetic energy) in the electric field until it collides with a gas molecule (A). In this collision, kinetic energy is transferred to the collision partner and eventually randomized to the rest of the gas. The electron is again accelerated until the next collision, and so on. The average energy attained before each collision is... 

The collision kinetic energy furnishes the potential energy needed to enable the reactants to rearrange to form products... 

Collisions are elastic. This means that, in a collision, kinetic energy is transferred from one particle to another with no overall loss in energy. One particle loses and the other gains an equal amount of kinetic energy. 

Computing the post collision kinetic energy, one finds... 

Solving Equation 6.4 gives the spectrum of bound molecular states with energy Eii = —h kff / 2 x) < 0 and the scattering states ilffCE) with collision kinetic energy... 

There have also been many attempts to reduce the likelihood that there will be a collision involving large amounts of kinetic energy. Obviously, if there is no collision, kinetic energy remains potential and therefore benign. Here are a few examples ... 

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). 

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... 

Figure Bl.23.1. (a) Two-body collision of a projectile of mass and kinetic energy E approaching a...

Classical ion trajectory computer simulations based on the BCA are a series of evaluations of two-body collisions. The parameters involved in each collision are tire type of atoms of the projectile and the target atom, the kinetic energy of the projectile and the impact parameter. The general procedure for implementation of such computer simulations is as follows. All of the parameters involved in tlie calculation are defined the surface structure in tenns of the types of the constituent atoms, their positions in the surface and their themial vibration amplitude the projectile in tenns of the type of ion to be used, the incident beam direction and the initial kinetic energy the detector in tenns of the position, size and detection efficiency the type of potential fiinctions for possible collision pairs. 

When relaxation of the internal motion during the collision is fast compared with the slow collision speed v, or when the relaxation time is short compared with the collision time, the kinetic energy operator... 

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