Collisions, between molecules

The cold plasmas tend to be unstable, are sometimes difficult to maintain, and provide ion yields that are less than those of the hot plasmas. To obviate the difficulties of the interfering isobaric molecular ions from hot plasmas, it has been found highly beneficial to include a collision cell (hexapole see Chapter 22) before the mass analyzer itself. This collision cell contains a low pressure of hydrogen gas. lon/molecule collisions between the hydrogen and, for example, ArO+... 

An ion-molecule collision between two monatomic entities would occur in a plane. The simplest systems on which our computations are based are essentially triatomic (i.e., diatomic rotors and monatomic ions). The trajectories do not lie in planes, as is shown in Fig. 9, which was obtained from calculations for CO -f Ar+. This figure shows a multiple-reflection collision. 

In the case of bunolecular gas-phase reactions, encounters are simply collisions between two molecules in the framework of the general collision theory of gas-phase reactions (section A3,4,5,2 ). For a random thennal distribution of positions and momenta in an ideal gas reaction, the probabilistic reasoning has an exact foundation. Flowever, as noted in the case of unimolecular reactions, in principle one must allow for deviations from this ideal behaviour and, thus, from the simple rate law, although in practice such deviations are rarely taken into account theoretically or established empirically. 

Consider collisions between two molecules A and B. For the moment, ignore the structure of the molecules, so that each is represented as a particle. After separating out the centre of mass motion, the classical Hamiltonian that describes tliis problem is... 

Microwave discharges at pressures below 1 Pa witli low collision frequencies can be generated in tlie presence of a magnetic field B where tlie electrons rotate witli tlie electron cyclotron frequency. In a magnetic field of 875 G tlie rotational motion of tlie electrons is in resonance witli tlie microwaves of 2.45 GHz. In such low-pressure electron cyclotron resonance plasma sources collisions between tlie atoms, molecules and ions are reduced and the fonnation of unwanted particles in tlie plasma volume ( dusty plasma ) is largely avoided. 

Once prepared in S q witli well defined energy E, donor molecules will begin to collide witli batli molecules B at a rate detennined by tire batli-gas pressure. A typical process of tliis type is tire collision between a CgFg molecule witli approximately 5 eV (40 000 cm or 460 kJ mor ) of internal vibrational energy and a CO2 molecule in its ground vibrationless state 00 0 to produce CO2 in tire first asymmetric stretch vibrational level 00 1 [11,12 and 13]. This collision results in tire loss of approximately AE= 2349 cnA of internal energy from tire CgFg,... 

Figure C3.3.8. A typical trajectory for a soft collision between a hot pyrazine molecule and a CO2 bath molecule in which the CO 2 becomes vibrationally excited.

When a pure gas flows through a channel the accompanying fall in pressure is accounted for partly by acceleration of the flowing stream and partly by momentum transfer to the stationary walls. Since a porous medium may be regarded as an assembly of channels, similar considerations apply to flow through porous media, but in the diffusional situations of principal interest here accelerational pressure loss can usually be neglected. If more than one molecular species is present, we are also interested in the relative motions of the different species, so momentum transfers by collisions between different types of molecules are also important. 

Now encounters between molecules, or between a molecule and the wall are accompanied by momentuin transfer. Thus if the wall acts as a diffuse reflector, molecules colliding wlch it lose all their axial momentum on average, so such encounters directly change the axial momentum of each species. In an intermolecuLar collision there is a lateral transfer of momentum to a different location in the cross-section, but there is also a net change in total momentum for species r if the molecule encountered belongs to a different species. Furthermore, chough the total momentum of a particular species is conserved in collisions between pairs of molecules of this same species, the successive lateral transfers of momentum associated with a sequence of collisions may terminate in momentum transfer to the wall. Thus there are three mechanisms by which a given species may lose momentum in the axial direction ... 

Despite the fact Chat there are no analogs of void fraction or pore size in the model, by varying the proportion of dust particles dispersed among the gas molecules it is possible to move from a situation where most momentum transfer occurs in collisions between pairs of gas molecules, Co one where the principal momentum transfer is between gas molecules and the dust. Thus one might hope to obtain at least a physically reasonable form for the flux relations, over the whole range from bulk diffusion to Knudsen streaming. 

After being formed as a spray, many of the droplets contain some excess positive (or negative) electric charge. Solvent (S) evaporates from the droplets to form smaller ones until, eventually, ions (MH+, SH+) from the sample M and solvent begin to evaporate to leave even smaller drops and clusters (S H n = 1, 2, 3, etc.). Later, collisions between ions and molecules (Cl) leave MH+ ions that proceed into the mass analyzer. Negative ions are formed similarly. 

Further explanation of this Cl process can be found in Chapter 1. Briefly, Cl results from collision between sample molecules and specially produced reagent gas ions such that ions are formed from sample molecules by various processes, one of the most important of which is the transfer of a proton (H+, Figure 9.2). 

Additional ionization occurs by collision between the ions and other neutral species (ion/molecule collision see Chapter 1). Unless special steps are taken (see Chapters 8 and 11 ), the ions formed do not fragment, so little or no structural information is obtained. However, the lack of fragmentation does mean that good relative molecular mass data can be obtained. The assembly of ions formed by ion... 

Nonfractionating continuous inlet. An inlet in which gas flows from a gas stream being analyzed to the mass spectrometer ion source without any change in the conditions of flow through the inlet or by the conditions of flow through the ion source. This flow is usually viscous flow, such that the mean free path is very small in comparison with the smallest dimension of a traverse section of the channel. The flow characteristics are determined mainly by collisions between gas molecules, i.e., the viscosity of the gas. The flow can be laminar or turbulent. 

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